Gibson GA-19RVT (Crest version) rebuild to "Tweed" specs.

[Bill Thomas]

Hello "Friends On the Quest for 'TONE'!"

Rather than "muddy the waters" by tacking this on to the previous "Tweed" vs. "Crest" thread, I decided to start a new one in the hopes that this might be helpful to someone who is trying to "Tweedify" a "Crest" version of this venerable amplifier. First, a little background is in order.

A very dear friend asked me to take a look at his "new" acquisition. It was a Gibson GA-19RVT "Crest" version amplifier. It was found in a garage and was in INCREDIBLY poor condition. The foot pedal was missing and the cable to it had simply been cut. The connections to the reverb tank had also been cut.

But, this all *paled* in comparison with what I found inside. It was painfully obvious that this unit had been "modded" previously, but the work had been done VERY poorly. Upon examination, I found that the Volume Control had been mis-wired. The reverb was hard-wired to be inoperable and there was scorched wiring everywhere!

The four filter capacitors had been replaced, but they were held in place by a combination of electrical tape and a couple of twist-ties from a couple of loaves of bread! (Sorry, no pictures, but BELIEVE me, it was positively SCARY!) The wiring to the speaker had been spliced and (of course) insulated with electrical tape

The Volume Control wiring had been "modded," but it didn't correspond to ANYTHING remotely useful or proper.

It quickly became clear to me that this unit had been "modded to death." Yet, there DID appear to be *some* hope. The tubes all tested VERY well. Most of the "unobtainable" items appeared to be here. While the controls were ANYTHING but clean or smooth, (two of them would barely turn at all!) they just *might* be salvageable. So, the first order of business was to remove them all and THOROUGHLY clean, lubricate and check them. By de-mounting the "tone stack board," I was able to remove the potentiometers, as well as get a better look at the board and the wiring underneath it.

Working on this particular unit is made more difficult by the "wedge shape" of the chassis. It is physically impossible to solder anything to the far side of the tone stack board when it is installed in the chassis. It *can* be lifted and moved to an accessible position, but with the unit in such poor condition, this is MORE than risky. The solution? Start over!

Now realize, tearing ANY electronic unit down to a bare chassis and "starting over" can be a daunting task, but if you take things slowly and work carefully, it can yield the BEST, most RELIABLE results. After all, we don't want to wind up with a unit that we have to "baby." We want a unit that we can plug in, turn on and just PLAY - day in, day out! With that in mind, let's discuss "The Philosophy of Electronic Salvation." (Well, *something* like that anyway!) Here's a general guideline that will help you *properly* tackle a complete rebuild.

Generally, there are three basic steps to this sort of rebuild.

Step One: Thoroughly evaluate what you are starting out with! In the case of our Gibson GA-19RVT, there are MANY "variations on the theme." While I had almost NO exposure to Gibson amplifiers, I was able to find out that the GA-19RVT has been divided into two "basic" versions: The "Tweed" version and the "Crest" version. But things are not *quite* that simple. (They NEVER are!) The "Tweed" version was produced in fairly small numbers, but the "Crest" versions(s) number in the MANY thousands. "Purists" appear to like the tonal qualities of the original "Tweed" version. The actual circuit variations between the two models are "relatively" minor.

In the original "Tweed" version, the tube complement called for three 6EU7's, one 7199, two 6V6's and one 5Y3. The unit we are dealing with uses three 6EU7's, one 6C4, two 6V6's and a 5Y3. In the original "Tweed" version, one-half of one of the 6EU7's is not used! That's right, it is a waste of one-half of a 6EU7! Swapping a 6C4 for a 6EU7 eliminates this waste, PLUS, 6C4's are a LOT cheaper and easier to find than 6EU7's these days!

If you are contemplating a rebuild like this, you *might* have a totally different tube lineup. Much later examples of the "Crest" version used different Output tubes and had other changes. (We won't be covering them here.)

Part of Step One involves doing LOTS of research! Here, the Internet is your friend! You will quickly find that there are two easily available schematics on the web. But be prepared to discover that your unit will be somewhat different from either schematic! Gibson apparently made "running changes" without fully documenting those changes. (Or else, they didn't make this documentation available while they still had it.)

Take LOTS of pictures! I cannot stress this enough. But, when your unit has been stripped "to the bone," it will be IMPOSSIBLE to remember how each and every wire was originally routed. And you definitely DO want to route the wires as they were originally routed - with a few exceptions that we'll cover in just a bit.

Thoroughly check the parts you MUST re-use. In the case of the GA-19RVT, ALL of the potentiometers are either VERY difficult, or IMPOSSIBLE to source. It would make ZERO sense to attempt to rebuild this amplifier if any of the potentiometers were bad and couldn't be sourced. Most transformers CAN be sourced (or can be rewound at a high cost).

Decide EXACTLY what changes you plan to make, if any. Do you REALLY want to keep the "Death Capacitor?" (If you don't know what the "Death Capacitor" is, this would be a good time to do a little more research!) In our little GA-19RVT, the "Death Capacitor" is a tubular capacitor that connects between one side of the Power Cord and the chassis of the amplifier. While it is EXTREMELY rare, if this capacitor shorts, it can put full Power Line Voltage on the metal parts of your guitar! Touch them and something in your "playing zone" that is grounded and YOU light up like a Christmas Tree! For safety's sake, replace the 2-wire Power Cord and Plug with a 3-wire cord and plug. If you leave the "Death Capacitor" in-circuit and it fails, it will cause the branch circuit breaker to trip, rather than kill you. I prefer to remove the capacitor completely.

Part of the research you NEED to do involves making out a FULL parts list and ordering the replacement parts. You don't want to come up "short" when you are rebuilding your amplifier. Have EVERY part "in-house" BEFORE you start your rebuild. While it is VERY rare, even a NEW part *could* be defective. It's a GREAT idea to test the NEW parts you will be using. There are some areas in the amplifier that will benefit from carefully selecting parts that are very closely matched. Take the time to do the job RIGHT and match those parts so they are within 1% of each other, or better. Once you have ALL the parts "in-house" and tested, you can proceed to...

Step Two: "Un-build" your existing amplifier. Take LOTS of pictures along the way. Some parts and wires will be exposed when you remove the parts that are on top of them. You want LOTS of pictures to show you where EVERYTHING really belongs. there is no such thing as too many photos!

We're lucky! We have reasonably good documentation covering our amplifier, but if you are not lucky enough to find a schematic, you will need to MAKE one as you dis-assemble your amplifier! This is NOT an "optional" step! Without an accurate schematic, you will have NO idea where that rat's nest of wires eventually connects together. Even though we have a schematic, there were some changes that are not covered by our available schematic. Make SURE you modify the "stock" schematic, or make a small "snippet" of a schematic wherever you discover a deviation from "stock." In a unit that has been heavily "modded" this schematic is the ONLY way you can find out WHAT was done, so you can try to determine WHY it was done! (And if you want to keep the change, or discard it.)

With all that in mind, it's time to "un-build" your unit. This brings us to the art of de-soldering. There are LOTS of tools available that are designed to help you remove solder from a terminal. MOST of them require you to grow a third arm and hand. If you do a LOT of service work, it will be WELL worth your time to invest in a de-soldering gun. Hakko makes a unit that will suck the solder from a terminal with the pull of a trigger. At $170.00 from Amazon.com, it ain't exactly cheap! But it will save you SO much time and aggravation that I cannot recommend it highly enough. Using de-soldering braid or squeeze bulbs, it will take you several nights to "un-build" your amplifier. Using the powered de-soldering gun, you can "un-build" it in a couple of hours! (And the de-soldered terminal points will be clean as a whistle!)

Once you have removed the parts and wiring you will be replacing, take a moment to clean the metal parts that remain. Clean and de-oxidize the tube sockets and connectors. (DeoxIT is available at your nearby Radio Shack and is HIGHLY recommended!) And with all that in mind, let's proceed to...

Step Three: BUILD it! And we'll cover THAT in our next post, coming up a bit later!

Sincerely,

Bill Thomas

 

[Bill Thomas]

OK, let's get "down to bizness!"

You'll remember that one of the *first* things we did was to carefully clean, condition and check each control BEFORE we committed to the rebuild. There's a LOT of "folklore" out there about the "proper" way to do this. I can only pass on the method that works best for me.

We'll need a few "supplies" in order to get the job done. We need: naphtha, (otherwise known as common lighter fluid), DeoxIT DN5, DeoxIT GN5 (available from Radio Shack) and an Ohm meter.

Begin by thoroughly dousing a control with lighter fluid. (Best to do this over a sink.) Go ahead and bathe the element. Allow the naphtha to leak from all opening. Turn the control back and forth throughout its range. Shake out the excess. Then... Douse it again! Rotate the control and again, shake out the excess. Allow the control to dry. Now spray a VERY light mist of DeoxIT DN5 at the junction of the wiper contact and the resistive element. Again, operate the control back and forth. Give it another little spritz and work the control again. You really don't need a lot of DeoxIT, just a tiny little "spritz." Wait for at least two minutes and then douse the control with naphtha again. Work the control back and forth again and shake the excess naphtha from the control. Allow it to dry thoroughly. Our final step is to apply a REALLY tiny mist of DeoxIT GN5 contact conditioner and work the control again. Allow the control a little more time to dry.

To see if the control is still operating properly, connect one Ohm meter probe to the wiper of the control and the other probe to one leg. Rotate the control VERY slowly and watch the resistance change. If the control is ok, you will see a gradual increase or decrease to the resistance reading. If it is defective, you will see the reading jump all over the place, or worse yet, the control will show an open reading at some positions of the wiper. Be aware that some digital meters require a bit of "settling time" before the readings can be accepted. Don't confuse this with an open control.

As it turned out, all five of our controls checked out nicely. This REALLY surprised me, considering the condition of the bodies of the potentiometers! (They were pretty rusty!) Of course, we won't REALLY know how quiet they are until we get a chance to use them in a working amp. Let's just keep our fingers crossed!

Since three of these controls (Volume, Tone and Reverb) are closely wired together, let's insert them in their holes on TOP of the panel. This gives us a stable platform with the correct spacing between the controls. Orient the controls so their terminals match the position in the picture you took. Now, install the parts on the Tone control and wire the audio feed wire to all three controls. The other end of this wire will connect to the Tone Stack board later.

I have chosen to make a deviation from the "stock" configuration. In a "stock" amplifier, the feed from the center terminal of the Reverb control is unshielded. But this wire runs several inches away, to the grid of V1. In order to minimize hum pickup, I have decided to shield this wire. Since Gibson DID shield a similar wire from the Volume Control, shielding the Reverb send wire makes a LOT of sense. The shield of the wire is connected at one end only - soldered to the body of the Reverb control.

Now, remove the three controls from the top of the main panel and mount them properly to the underside of the panel. Remember to use a lock washer between the control and the panel, as well as a flat washer between the mounting nut and the main panel's top surface. Here's what that looks like:

Nice and neat! You can see the wire for the reverb send has been routed to the surface of the chassis and routes to the left and connects to V1. The audio feed wire from the Tone Stack board is on the left. We'll connect it when we install the Tone Stack board.

Now, we clean the three Switchcraft Phone Jacks and pre-wire them for installation in the Main Panel. Use the same method you used with the three potentiometers. Mount them on TOP of the main panel and install the resistors.

In the original "Tweed" version of the amplifier, the wire from the two 100K resistors is shielded. In the "Crest" version, it is unshielded. Considering the very low level of many pickups, let's go with shielded wiring! Again, the shield is connected at one end only. We've grounded the shield to a convenient ground at the Input Jack.

In the original "Tweed" version, the wire leading to the Monitor jack is also shielded. They even went so far as to install a metal "cup shield" around the Monitor jack. We won't go quite *that* far, but let's go ahead and shield the audio feed wire. Again, the shield is connected at one end only. We're grounding it at the terminal strip next to the Output Transformer. Here's how these jacks look after installation:

The shielded wire from the Input Jacks is routed to V1. The shielded wire from the Monitor jack routes through the grommet to the terminal strip area next to the Output Transformer. We'll cut the excess wire and solder it a little later. Here's a look at the under-chassis wiring:

The white wire from the Input Jacks is connected to pin 5 of V1. The red wire from the Reverb control will connect to pin 8 next.

Now, let's shift our focus to the Power Supply. I mentioned that the filter capacitors had been replaced, but the job left a LOT to be desired. It is NOT considered "good practice" to use twist ties to hold a "blob" of electrical tape and capacitors to the chassis. Instead, I installed two 5 lug terminal strips. One is located to the right of the Tone Stack board, while the other is located to the left of the tone stack board, in front of the Input Jacks. This gives us the ability to securely mount two radial lead capacitors to each terminal strip. Here is one area where we will make another *slight* deviation from "factory stock."

In a stock GA-19RVT, the common ground of the dual tubular capacitor located to the left of the Tone Stack board is grounded to a grounded terminal on an Input jack. While this will certainly *work*, I prefer the Power Supply grounds to all terminate to the same location, namely; the point where the center-tap of the Power Transformer is grounded to the chassis. This is the central ground point for the entire amplifier. From this point, a wire connects to the terminal strip with two capacitors located to the right of the Tone Stack board and a wire leads from this terminal strip, directly to the other added terminal strip to the left of the Tone Stack board. This ties the ground connections for the Power Supply together. It should be noted that modern electrolytic capacitors are MUCH smaller than the originals. There is plenty of room for them at each terminal strip. Just be sure they are mounted in a way to keep them away from the Input jacks and to avoid interference with the bottom cover to the chassis.

Now, let's replace the resistors mounted on the chassis itself. This works out two just three resistors, actually. Two of the resistors are "grid-stopper" resistors. They are 10K 1/2 Watt resistors. The original parts were 20% tolerance resistors. We are using 5% resistors that are matched within better than 1%. Is this *crucial* to amplifier performance? No, but in a push-pull Output stage, anything we can do to match the two halves as closely as possible is a GOOD thing. As it turns out, our 10K resistors were both measured at 10.1K each exactly. This is better than a 1% match and within 1% of the nominal value of 10K each. The other resistor we need to replace is the 470 Ohm 1 Watt resistor in the Power Supply circuit. Again, the original resistor was a 20% tolerance part, but we are replacing it with a 5% tolerance part. It's checked value was 471 Ohms; better than a 1% deviation from the "nominal" value.

Now we need to add a couple of cables to our chassis. The first is the multi-conductor cable that goes to the foot switch. The second is the "return" cable from the reverb tank. The reason we need to add them now is that they are going to be impossible to add after the tone stack board is installed.

The multi-conductor cable to the foot switch has three wires plus an overall shield. This shield is soldered to the body of the Volume Control. This secures the cable to the chassis and provides a good ground for the reverb muting circuit.

The reverb return cable was obtained from a pair of turntable interconnect cables. These cables use low-capacitance wire and gold-plated RCA connectors. We measured the distance to the output RCA socket on the reverb tank and cut the cable accordingly. The return cable is identified by a red ring on the RCA plug. The reverb send cable will be mounted later, but it will have a white ring on the RCA plug. At the chassis end, the reverb return wire's center conductor connects to pin 5 of V2, but other components will also connect to that pin so don't solder pin 5 just yet. The shield connects to the ground lug adjacent to the tube socket.

OK, so now we have the chassis fairly well pre-wired for the installation of the Tone Stack board. in our next installment, we'll cover the buildup of the Tone Stack board and finish up our amplifier.

It should be mentioned that I didn't actually strip this particular chassis down to bare metal. The wiring around the Power Supply and the Output tubes was in fine, factory-connected condition, so I left it alone. I *did* clean up the connections of the filament wiring to the small tubes in order to minimize any possibility of a short circuit.

We'll finish the buildup of this amplifier in the next installment. The Tone Stack board is where we will really make the changes that are required to "Tweedify" our "Crest" model.

Sincerely,

Bill Thomas

 

[capnjuan]

Hi Bill; interesting project! Link to the other Tweed/Crest LTG thread for reference here. It's hard to understand what the previous owner was trying to do with it but I don't think it was to take it back to the prior version. All that would have taken, assuming the same transformers, would be to change out the preamp cathode and voltage divider resistors, bump up a coupling cap, and jump out that Crest filter.

All of that could be done on the circuit board and wouldn't have required messing with tube socket connections which is presumably how all those wires got skinned up. I wonder if they were trying to convert it to 12AX7 operation which would mean jerking socket connections around; was the heater supply on the correct-for-6EU7 pins? Agree the folded chassis is a pain. Good stuff; thanks!

 

[Bill Thomas]

Hi, capnjuan,

It was a curious mess, but he had definitely changed the resistors on the Tone Stack board to the "Tweed" values. It's the *other* changes that leave us in "head scratching mode." he had lowered the reverb return resistor from 680K to 220K, but he had eliminated the 100K resistor from the wiper of the Volume Control to the grid of V1. This means the reverb send and return were shorted out! No doubt that sent the reverb circuit into feedback of some sort, but there was NO way the reverb would work at all.

I don't believe he had converted V1 to a 12AX7, but a prior owner *might* have. As you can see, the wiring around V1 and V2 was in VERY poor shape. Also, he added a series R/C network across V2 pin 8 (the gain stage after the main Volume Control) and ground. I assume he didn't like how bright the amplifier turned out and tried to roll off the highs a little. It certainly doesn't belong, so it's a goner!

There is no question that this amp had suffered terribly from "mad modder's disease." (The cure is worse than the original ailment!) But we're setting things right with the rebuild. By the way, just for giggles, I checked the existing resistors in the Tone Stack. Most of them had drifted high, but not far enough that the amp would stop working. Doesn't matter. They are all being replaced with NOS Allen-Bradleys that are within 2% of nominal values (and in *most* cases, better than 1% of the nominal values.) We should be makin' good tone soon!

Sincerely,

Bill Thomas

 

[capnjuan]

I'm a big fan of the model. Despite getting dissed in the 2007 Tonequest article about Victoria amps (article no longer available on-line), it was maybe Gibson's best-selling model; with either 6V6s or the later 7591s, they sold a ton of them. Their prices pushed up a couple of years ago and are still holding ... holding better than the GA40s which, up until this year, couldn't be touched for anything under $1,500. Good luck; pics when they're ready.

 

[Bill Thomas]

Let's "Stack some Tone!"

Without further ado, let's take a look at the Tone Stack board. The following picture shows the board at about the 90% completion point. The rest of the components will be added when we install the board into the chassis. Here's the picture:

As we have mentioned, Gibson buried several disc capacitors underneath the resistors. My goal is to UN-bury all capacitors in order to make any "capacitor rolling" simpler. Of course, that is the *secondary* goal. The PRIMARY goal is to replace the old components with all new (NOS) parts that will allow this amplifier to live another lifetime with a minimum of down time.

The Tone Stack board has 30 pairs of terminals for component mounting. Most of the components mount from the top terminal to the bottom terminal (with a few exceptions). Let's go down the component list from left-to-right.

At the far left, we have R34. The top terminal has not been soldered in this photo because a ground wire will be added to this terminal. In the "Crest" version, this resistor is 4.7K. The earlier "Tweed" version uses a 2.7K resistor here. To "Tweedify" your amp, replace this resistor with a 2.7K as shown here.

Next, we see C11. This part is the same in both the "Tweed" and the "Crest" versions. It was originally a 20 uF 6 Volt capacitor. Today's capacitors use 22 uF as a "standard value." The new capacitor is rated at 35 Volts. It's absolutely fine to use a capacitor rated at a higher Voltage than the original. The additional 2 uF of capacitance is of no consequence at all. You will never hear a difference.

R34 and C11 are in parallel and are connected between the Cathode (Pin 4) of V1 and ground. The resistor develops the negative bias for the grid of the tube. The capacitor acts to decouple the cathode, signal-wise.

In the third position, we will be adding an "Orange Drop" capacitor (listed as C12 on the schematic). This is the Output of the first gain stage and feeds the Volume Control. (The green wire from our potentiometers). In the "Crest" version, this capacitor is a .001 ceramic disc capacitor. To "Tweedify" this amplifier, we bump the value up to .02 uF. The capacitor I'm using is rated at 600 Volts. "Purists" may object to this change, preferring to keep a ceramic disc here, but film capacitors are MUCH better suited to audio coupling. Gibson built their amplifiers to sell at a low price-point. Ceramic discs are a LOT cheaper than film or foil capacitors. You make the call, but I STRONGLY recommend swapping out the ceramic discs for better coupling capacitors.

The fourth position on the Tone Stack board is R23. This is the Plate load resistor for the first half of V1 (Pin 6). In the "Crest" version, this resistor is 470K. The "Tweed" version uses as 220K resistor, as shown here.

The fifth position on the Tone Stack board shows R25. This resistor is the same in both the "Tweed" and the "Crest" versions of the amplifier. This is a 100K 1/2 Watt resistor. (The additional yellow band on the resistor indicates a Mil-Spec part.) This is the Plate load resistor for the second half of V1 (Pin 7). This is the first gain-stage after the reverb level control. No changes here.

The next two positions' resistors have been swapped. This is a "deviation" from the "stock" layout in order to allow more convenient mounting of the coupling capacitor. Let's review what is "stock" first.

In a "stock" "Crest" or "Tweed" version, the sixth position is for R34, a 470K resistor grid resistor for the reverb tank driver tube. In the "Crest" version, this is tube V3, the 6C4. There is a .005 ceramic disc capacitor (C21) buried underneath these resistors in a stock amplifier. The resistor values are the same in a normal "Tweed" or "Crest" amplifier. We have moved R26, the 3.9K cathode resistor for the second half of V1 to this location. This gives us a wider terminal spacing for the new "Orange Drop" coupling capacitor that replaces the buried ceramic disc. R34 is then moved to position seven on the Tone Stack board. The new "Orange Drop" capacitor connects between the bottom terminals at positions 4 and 6. If we hadn't exchanged those two resistors, the Orange Drop would have to connect to the bottom terminals at positions 4 and 5. With the wider spacing, we have a MUCH more secure mounting for the "Orange Drop" capacitor. By using the original color codes for the wires, we will make CERTAIN that we connect the correct terminals to the tube sockets. The yellow wire at terminal 6 connects to the cathode of the second half of V1 (pin 9) and the green wire from terminal 7 connects to the grid of V3 (pin 6).

Position 8 on the Tone Stack board is for R27. This is the cathode resistor for V3 (in the "Crest" version.) It is a 1K 1/2 Watt resistor in both versions. Nothing to change here.

Position 9 on the Tone Stack board is for a 2.2K 1/2 Watt resistor (R28). This is the cathode resistor for the reverb return gain stage. This is the same value in both the "Tweed" and the "Crest" versions, but we will have more to say about this particular position later. For now, notice that there is a yellow wire coming from the bottom terminal at position 9. In a "Tweed" version and in MANY "Crest" versions, this wire connects to the cathode of the reverb return amplifier directly. In later versions of the "Crest" amplifier, this wire is replaced with a 470 Ohm resistor. This resistor lowers the gain of the reverb return stage somewhat and it also provides a small amount of negative feedback for reduced distortion. This additional resistor was discovered after this photo was taken. We will be adding the resistor to our amplifier. It is not shown on any of the two available schematics, so there is no part number for it.

Position 10 on the Tone Stack Board is for C14, originally a 20 uF 6 Volt cathode bypass capacitor across resistor R28 (at position 9 on the Tone Stack board). We are using a modern 22 uF 35 Volt electrolytic here.

Position 11 is specific to the "Crest" version of the amplifier. In a stock unit, this is a .01 uF ceramic disc capacitor. We have replaced it with a .01 uF 630 Volt film capacitor. It is listed as C23 on the "Crest" version of the schematic. This is the Output capacitor from the reverb return gain stage. You'll notice that we still have to solder the top terminal at position 11 because we will be adding a 680K resistor from the top terminal to run off the board to the Volume Control area. We'll do this when we install the board in the chassis.

Position 12 is the plate load resistor for the reverb return gain stage. In the "Crest" version of the schematic, this is listed as R29. It is a 100K 1/2 Watt resistor. Since the "Tweed" version uses a different tube for this function (The pentode section of a 7199), the resistor is changed to a 470K resistor. We will keep the 100K resistor here.

Position 13 is an open position in this amplifier. In the "Tweed" version, I *think* a 1.5 Meg resistor is located here. (Since I don't have one to compare handy, this is, at best, just a guess.) We don't need this resistor because there is no screen grid in a 6EU7, so there is no need for a screen resistor here.

Position 14 is R24, the plate load resistor for the first half of V2. In the "Tweed" version, this is a 220K resistor. In the "Crest" version, this is a 470K resistor. We will use a 220K resistor here. This is the second gain stage of the amplifier, located after the main Volume control.

Once again, we have swapped the resistors at positions 15 and 16 in order to allow a wider terminal spacing for the capacitor (C22) mounted (now) above these resistors. The resistor at position 15 is now the cathode resistor (R32) for V4 pin 4. In a "Tweed" version, this is a 1.5K resistor. In the "Crest" version, this is increased to a 3.3K resistor. We are using a 1.5K resistor here. This half of V4 is used as the phase inverter stage for the Output tubes. The signal at the cathode is in-phase with the grid signal, while the signal from the plate is 180 degrees out of phase from the grid signal.

At position 16, we now have resistor R30, a 470K grid resistor for V4 pin 5. Capacitor C22 also connects here. Capacitor C22 is the coupling capacitor from the second gain stage section. In a stock amplifier, this is a ceramic disc. We have replaced it with a 510 pF dipped silver-mica capacitor. These are EXTREMELY stable capacitors and are a HUGE improvement over a ceramic disc. The extra 10 pF of capacitance will make no discernible difference in the sound of the amplifier. These values are the same in both the "Tweed" and "Crest" versions of the amplifier.

At position 17, we have R42. This is a 100K resistor in the "Crest" version and a 47K resistor in the "Tweed" version. We will use a 47K resistor here.

At position 18, we have R33. This is the cathode resistor that connects to pin 9 of V2, the second gain stage after the main Volume control. In a "Tweed" version, this is a 2.7K resistor. In the "Crest" version, this is a 4.7K resistor. We are using a 2.7K resistor here.

At position 19, we have C13, the cathode decoupling capacitor for V2 pin 9. In the "Tweed" version, this is a .02 uF ceramic disc capacitor. In the "Crest" version, this is increased to a .1 uF ceramic disc capacitor. We are replacing this with a .02 uF "Orange Drop" capacitor, as per "Tweed" specs.

At position 20, we have C17, the coupling capacitor for the "in-phase" output from the phase inverter tube (V4). This was originally a .02 uF ceramic disc capacitor, but we are using an "Orange Drop" film capacitor instead. This capacitance value is the same in both the "Tweed" and the "Crest" versions.

At positions 21 and 22, we have two closely-matched 220K resistors (R35 and R36). These are the resistors that allow the tremolo output signal to "modulate" the audio to the Output tubes. These resistors were matched to be EXACTLY the same value of 220K for MAXIMUM effect of the tremolo signal. These values are the same in both the "Tweed" and the "Crest" versions of the amplifier.

Position 23 is shown as vacant in the photo, but we have yet to add another 47K resistor (R43). We will add this resistor when we mount the Tone Stack board to the chassis. In the "Crest" version, this is a 100K resistor. This resistor is EXACTLY the same measured value as R42 (discussed earlier) for minimum distortion. The closer the match, the lower the distortion. Simple. We will also be adding another .02 uF capacitor to the bottom terminal at position 23. The other end of this capacitor will connect directly to the tube socket below the terminal. In a "stock" amplifier, this would be a .02 uF ceramic disc capacitor that is "buried" underneath the resistors. We have un-buried it to allow future "capacitor rolling." There will also be additional wiring added to the top terminal, so we have not soldered it yet.

Position 24 is resistor R37. This is the plate load resistor for the Tremolo oscillator tube section; the second half of V4. This is a 270K resistor in both versions of the amplifier.

Positions 25 and 26 are 2 of the three .047 uF capacitors used in the feedback network for the Tremolo oscillator. The third .047 uF capacitor is shown connected to the bottom of position 26 and the bottom of position 28. These are C18, C19 and C20 in the schematic. The same values are used in both versions of the amplifier.

Position 27 is resistor R38, a 240K resistor. No difference between the "Tweed" and the "Crest" versions.

Position 28 is resistor R39, a 510K resistor. Again, no difference between the "Tweed" and the "Crest" versions.

At position 29, we have C20, originally a 20 uF 6 Volt capacitor. Again, we are using a 22 uF 35 Volt capacitor to replace it. The two versions of the amplifier use the same value here.

At position 30, we have the final cathode resistor R40, a 2.7K resistor. This is the cathode resistor for the tremolo oscillator and it is bypassed by the capacitor at position 29. No change between the two versions of the amplifier.

The "Tone Stack" is literally the heart and soul of the amplifier. "Purists" may argue with my choice of film or mica capacitors over ceramic discs. Since we have "un-buried" all of these capacitors, it is a simple matter to replace them, if desired. Note: we have soldered connecting wires that are significantly longer than required for installation of the tone stack board. These wires will be cut-to-length when the board is installed, in order to keep our construction neat and tidy.

Let's add one final "installment" to show the installation of the "Tone Stack" and the final wiring of the amplifier. We'll get to THAT, shortly!

Sincerely,

Bill Thomas

 

[Default]

<applause>

 

[Bill Thomas]

Let's put it all together!

Before we add the "Tone Stack" board, we've replaced the 47K resistor from the leg of the Tremolo "frequency" control to the case of the control. We've replaced the 47K and the 470 Ohm resistors on the terminal strip next to the Output Transformer and wired a black and green twisted pair of wires for the speaker, as well as the shielded monitor feed wire to the terminal strip. We've installed the Tremolo "frequency" and "Depth" controls to the Main Panel.

Next, we've replaced the old 2-wire Power Cord with a grounded 3-wire cord. The green ground wire is soldered to the ground lug at one of the Power Transformer mounting studs. The .02 uF (so-called) "Death Capacitor" has been removed from the amplifier. it was a tough fit to get the thicker Power Cord through the rubber grommet. We used a drop of dishwashing detergent to "lubricate" things and then cleaned the cord with a damp paper towel. To keep the new cord from "pulling through" the grommet, we have added a cable tie behind the grommet. Works like a hose!

And with all that out of the way, it's time to install the "Tone Stack" board. The connections to the Top of the board are made first. The two missing components are added and the final wiring from the top terminals is connected and soldered.

There are three ground wires that connect to the top terminals. Make SURE these are solid! A fourth ground wire connects to the bottom terminals, but then routes underneath the board and grounds to the body of the Tremolo "Depth" control. This one needs to be solid too! (Of course, it can be argued that ALL of the connections must be solid, but it is all too easy to fail to verify the solidity of the connections to the top terminals of the board due to the restricted "working room" around the top of the board.)

The twisted Orange wires lead to the two filter capacitors in front of the Input Jacks. It doesn't really matter which wire connects to which capacitor. Both capacitors are the same value so either wire will work with either capacitor. The red wire from the foot switch cable connects to the junction of C18 and C19. NOW is the time to add the 680K resistor to the terminal where C23 was unsoldered. The free end of the resistor should point to the center terminal of the Volume Control. The white wire in from the foot switch cable also connects to this point and the terminal can then be soldered. Now, add a 100K resistor to the center terminal of the Volume Control. Connect the other end of the 100K resistor to the free end of the 680K resistor. The green wire from the pair of 220K resistors (R35 and R36) on the Tone Stack board connects to the center terminal of the "Depth" potentiometer. Solder it.

Next, we mount the board to the chassis. Don't forget to add the terminal strip on the right side of the board's mounting screw. We'll add the .22 uF capacitor and the 100K resistor after the board is securely mounted.

Now, one-by-one from left-to-right, cut each wire to the proper length and connect it to the proper terminal at each tube socket. Route each wire according to your pictures, or use our next picture to help you out. When you get to the yellow wire that connects to pin 4 of V2, remember that we are actually connecting a 470 Ohm resistor in place of that wire. (By the way, in the interest of full disclosure, in the following picture you'll see this 470 Ohm resistor connected to pin 3 of V2. DON'T connect it to this terminal or your reverb will be dead! Make SURE it is connected to pin 4 or you will be dis-assembling the completed unit to FIX the problem - as *I* did!) Hey! We ALL make mistakes!

Once all the wiring is done at V2, it's time to add a 100K resistor and a .001 capacitor from Pin 5 of V2 (where the reverb return wire is already connected but not soldered) to the mounting rivet at the bottom of the tube socket. Make SURE there is a wire from this rivet to the ground lug to the left of the tube socket! Then, continue making all the connections for the rest of the terminals at the bottom of the Tone Stack board. We have one additional part to add - the .02 uF "Orange Drop" coupling capacitor C16. This capacitor connects from terminal 22 on the Tone Stack board to pin 6 of V4.

Now, add the .22 uF film capacitor. It connects between terminal 24 and the added terminal strip on the right side of the Tone Stack board. Next connect a 100K resistor between the same terminal of the added terminal strip and the ungrounded leg of the "Depth" potentiometer. Connect the black wire from the foot switch cable to the center terminal of the "frequency" potentiometer. Solder all connections.

Now, replace the 270 Ohm 2 Watt resistor that connects to the cathodes (pin 8) of the two Output Tubes. The other end is grounded to the body of the "Depth" potentiometer. Replace the 20 uF capacitor that bypasses this resistor. it is mounted between the two 6V6 Output tube sockets. Connect the free end of the 4.7K 1 Watt resistor to pin 4 of the left-hand 6V6. Connect a shielded cable from the center pin of the Volume Control to pin 8 of V2. The shield is grounded to the body of the Volume control and there is no connection to the shield at the other end. And FINALLY, add a .002 uF 1000 Volt or better capacitor across the Output Transformer primary. This is shown in the schematic for the "Crest" model. This capacitor helps to smooth out any non-linearities in the Output Transformer windings. It also helps to prevent arc-overs in the Output tubes during clipping of the Output waveform. While it wasn't used in the "Tweed" version, it is good practice to use it, so we are.

That's it! We're done! Here's a picture:

But make SURE you take a LOT of time to check your wiring, to make SURE you have soldered every connection and to shake out the solder balls and splatter that inevitably inhabit EVERY finished project.

There is only one thing left - we need to replace the missing foot switch. From a cost standpoint, nothing beats a Johnson 2-button switch. At $20.95 this metal-bodied foot switch is truly easy to take! Due to the wiring of the foot switch cable, we need to remove the supplied 3 wire cable and connect the four conductor cable (3 wires plus a shield) in its place. The shield and white wires connect to the right-hand switch and the black and red wires connect to the left-hand switch.

Here's the finished "guts" prior to mounting in the cabinet:

Notice that I have installed the 5Y3 rectifier and the two 6V6 Output tubes. If you have a Variac, it is a good idea to GRADUALLY apply Voltage to the unit and measure the High Voltage Power Supply Voltages at each main filter capacitor. (The new terminal strips make that EASY to do.) NEVER plug the unit in without the Output tubes installed. You will exceed the Voltage rating of three of the four filter capacitors almost instantly! The 6V6 tubes provide the proper load to drop the Voltage to safe levels for these capacitors. If everything checks out, Power Down, add the rest of the tubes, connect the reverb tank and a speaker and Power Up! Plug in your guitar and give it a try. Check the reverb and tremolo functions. Make SURE the foot switch can cancel either function. If everything sounds ok, take one final look at the Output tubes to make SURE there are no red plates. Power down, allow it to cool off and install it back into the cabinet. Solder the Output wires to the speaker terminals. The Green wire goes to the "+" or Positive terminal of the speaker. If your speaker isn't marked, you can determine the positive terminal by touching a single cell 1.5 Volt battery across the terminals. If the speaker cone moves outward, the terminal connected to the positive terminal of the battery is the "+" terminal of the speaker. If the cone moves backwards toward the basket, the OTHER terminal is the positive terminal. Simple, right?

I'll save my "impressions" for one final "update." (Along with another "confession of incompetence.") Needless to say, the unit is now complete and functioning properly. I only had to dis-assemble the completed unit once to fix my earlier mistake.

In case you're curious, the total cost for our rebuild was $120.91! This includes the cost of the new foot switch, a new #47 pilot lamp and the necessary replacement screws, washers and spacers that we needed to pick up at the local Ace hardware store. (Really! Drywall screws are NOT the way to attach the back panel to the cabinet!)

Happy rebuilding!

Sincerely,

Bill Thomas

 

[Bill Thomas]

Thoughts, musings and impressions!

Many people say this amplifier can't play LOUD enough. Are you deaf??? No, this won't fill a concert hall or stadium. it wasn't designed to. But it will MORE than fill up a huge living room or practice area and will do a DANDY job in a recording studio!

This is a fairly "bright" amplifier so selecting the right guitar and pickup may prove important to you. (It all depends upon the "tone" you are seeking.) I am NOT a "player," so I returned the amplifier to the owner and asked him to put it through its paces. it was night time so he was unable to really hit it hard, but he reported that it will break up nicely and controllably when "pushed." The Reverb is solid and strong - and the tremolo can be made to nearly mute the audio at high settings of the "depth" control. By turning the main Volume Control down, the "wet" reverb signal only is heard. this can be useful as an "effect" for some tunes. The foot switch operates quietly and positively cancels the reverb or tremolo functions. At low volume, the amp is AMAZINGLY clean and "tasty" to my ears. There is no trace of hum. None! Even the reverb signal has no hum at all! Needless to say, I am EXTREMELY happy with the final result.

When I receive more information from the current owner, I'll post his impressions to the thread. All I can say is, this is one VERY "sweet" amplifier! We can chalk that up to Gibson's engineers.

In case you're curious, this amplifier was originally made sometime in 1962 (according to the date codes on the transformers). The 6EU7 tubes are RCA's also made in 1962. The 6C4 is branded as a Raytheon from 1962, but was made by Sylvania and the 6V6 tubes are from 1966. They are RCA's and test nearly new (as do the rest of the tubes). The 5Y3 is branded as a Raytheon, but it appears to be made by Brimar in England. (No idea on the date code.) Except for the Power Switch, the Pilot Lamp holder, the tube sockets, the reverb tank, the potentiometers and the transformers, all passive parts have been replaced. This unit *should* have another entire lifetime of use restored to it.

Now, for my final "confession"! When I first tested the amplifier, there was NO audio except for a *slight* amount of "bleed" or "crosstalk" heard from the speaker. The problem turned out to be the ground wire that connects to the top terminal at position #1 on the Tone Stack board. When I soldered the terminal, I had apparently slid the ground wire too far into the terminal so only the insulation was actually touching the terminal. One good "tug" and the wire pulled right out. Well, without this ground, tube V1 has NO ability to operate at all. It didn't until I made a "proper" solder joint. That's when I discovered my mistake at pin 3 (4) of V2. Once these two "goofs" were finally corrected (and I THOROUGHLY checked EVERY joint in the entire amplifier) we were truly "cookin'"!

Let's see... two mistakes in a few hundred connections. Well, I *could* have done better, I suppose. Fortunately, a job like this is a lot like finding a good life partner. It's not important to be the "first" - It's important to really be the LAST! Thankfully, this appears to be one amplifier that will truly last!

I hope you've enjoyed this little odyssey. I have learned a LOT about the differences between "Hi-Fi" amplifiers and "Instrument" amplifiers. I want to extend a sincere "Thank You" to LeonC who provided some truly invaluable pictures, and to capnjuan who first posted the comparison between the "Tweed" and "Crest" versions of this amplifier.

It should be noted that while this amplifier HAS been rebuilt to "Tweed Specs," there are still a few differences. The reverb section is quite different since we don't have the gain of the pentode section of a 7199. To make up for that gain, the reverb is added one stage ahead of the location in a "Tweed" amplifier. This also has the advantage of inverting the reverb's "wet" signal, thus subtracting the "dry" signal from it. This allows MUCH deeper reverb. I consider this a distinct advantage, so I kept this arrangement, rather than try to "Tweedify" the reverb design completely. I *think* I made the right choice.

Ok. We're done now. Comments are certainly welcome. Questions too. I'll do my best to answer them all honestly. I DO hope you've enjoyed the thread.

Sincerely,

Bill Thomas

 

[BluesDan]

I DO hope you've enjoyed the thread.

Sincerely,

Bill Thomas

Hell Yeah Bill! Thanks.

 

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<more applause>

 

[capnjuan]

<the sound of two hands clapping> great stuff Bill.

 

[davohilts]

I've followed Bill Thomas' advice on two crest ga19rvt's now and both have turned out absolutely fantastic. Compared to the stock crest 19rvt falcon the tweed mod turns it into an completely different animal and about 5 on the volume dial, well you have to hear it to believe it.
I can't thank you enough Bill!!!!
Dave

 

[drc]

I want to do the mod.. but I am just to stupid when it comes to that kinda thing. If I pay someone to do it, I'm afraid it will cost me a fortune. I also have 2 of these amps. I want mod one and keep one original.