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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
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