Gibson GA19RVT Tweed v. Crest ... compare/contrast

This is, in part, to fix a boo-boo in the other GA19RVT thread and a closer look at these two versions of the same model. One of our BBers owns a 'Crest' version and I have a standing offer to do general maintenance on his. He also said that his didn't break up as early as a Tweed. I recently offered to 'degrade' his 6V6/5Y3 model back to a Tweed so, in case he takes me up, I guess knowing what the differences are would be helpful .... If you're singing along at home; schematics Tweed and Crest. What they look like; the Tweed version takes its name from the patterned, cotton twill-backed, 'table-cloth'-style vinyl tolex and the Crest version from the dookie little coat of arms 'crest' on the chassis (the red spot):



The orange/brown stains commonly seen on the Tweeds is a mold caused by the water-based tolex glue and the cotton tolex-backing; easily 60% of the remaining tweed amps have the stains in varying degrees. Roughly, the tweed version was in production '60-'62 and the Crest '63-'65. I don't believe the model was re-done in Gibson's 'white face/doghouse' cosmetics but was done over later in black tolex, stick-em-on faux wood-grain paper on the control panel (for more transparent highs :wink: ) and with the cab sides sticking above the top so that there would be something to break off if the cabinet were dropped or tipped over. For more on Gibson amps, see Mr. Marx' fine book Gibson Amplifiers, 1933-2008.

Preamp: The cathode resistors (magenta box) are different although the value of the bypass cap is the same (also applies to the other tube section shown below). This resistor sets the bias of the tube section. The Crest model has the larger value resistor inducing a larger voltage on the cathode giving, all things being equal, a somewhat hotter tone. The volume/tone controls (green box) are too similar to be worth messing with. The coupling caps (red circle) indicate a larger value in the same position for the Tweed model. Finally, the blue box is a filter in the Crest model not found in the Tweed version. To Tweedify this section, the cathode resistors would be matched, the coupling cap upsized, and that Crest filter would be disconnected at one end and jumped out of the circuit.




Phase Inverter / Output: The magenta boxes show the voltage divider; the source of the AC signal when it swings to the common side; design is the same, only the resistor values are different. The Crest model has a small value cap (blue box) from one plate supply leg to the other. Not really certain what it does but maybe acts to filter out high frequency oscillation or maybe absorb cosmic rays ... who knows. To Tweediate the Crest model, those voltage divider resistors would be matched to the tweed values and that cap discarded.




Reverb: Except for the 7199 reverb operator in the Tweed and the 6C4 in the Crest, these are identical. The 7199, sometimes used in Ampegs and always used in the workhorse Dynaco ST70 35 wpc stereo amp, is a split design - two different tubes in the same bottle. One side is a typical triode; plate/grid/cathode ... like one half of a 12A_7, but the other side is a pentode ... more in common with a power output tube than a preamp tube. The blue box shows the more elaborate 7199 circuit in the Tweed model but, with a twin-triode 6C4, no need for that stuff there. Finally, the footswitch got moved; in the Tweed it's across the output of the reverb can; in the Crest, it's in the reverb return leg. Tweedification? Nothing to do.




Tremolo: Gibson tremolo, with fresh parts, is organic and pleasing and Gibson had the (rare) good sense to stick with something that worked well. The modern ear seems to like slower 'Speed'; can be done by replacing some of the caps and resistors ... but will leave that for another day. The Crest model has an additional 100K resistor (not highlighted; just right of C2) whose function I don't understand; since it's otherwise the same as the Tweed ... nothing to do here.




Power Supply:
Dropping resistors: 470/1W, 4.7K/1W, 10K/.5W: same for both models
Filter capacitors: Tweed: 20uf/10uf/10uf ... Crest: 20uf/20uf/10uf
The difference in filter caps is the section for the 6V6 screens; the screen supply is commonly regulated in higher power/better grade amps like the Guild Quantum and others; the increased uf in the Crest model might provide a slightly tighter sound. I'd be more inclined to bump the Tweed up than change out an otherwise good cap in a Crest.

There you have it and assuming the same output transformers - to degrade a Crest back to a Tweed; change out the preamp cathode and voltage divider resistors, bump up a coupling cap, jump out that Crest filter and ... when your hairs comb right and your pants are tight it's gonna be all right. John

Excellent breakdown El Capitan :!: :!: :!:

I think you are now going to be responsible for marked increase in traffic to the LTG by budding Gibbie valve literati. Really, I'm not kidding. This may be the only Falcon GA-19RVT mod analysis currently on the net. Let the linking begin!

I'm also itching to hear the results. Any willing test subjects? I know of at least three LTGers holding the Falcon card in their hand. It's also making me wish I still had my Falcon, though I doubt my '65 circuitry was moddable to Tweed specs.

Hi JP; thanks. Part of the reason for going to so much trouble was to get my Falcon-owning friend to tug his chassis out and ship it down here ... we'll see. For every one tweed Falcon, there has to be 100 or more Crest models ... the one in the other thread was maybe the first tweed in nearly a year and maybe the strongest cosmetics in the last two. As you've instructed, it's all about the cheese and my guess is that the 6V6 Crest amps could be made to get closer to a typical Deluxe tone and be more successful and happier at it than it would be spending its life trying to be a cleanish Ampeg. For all their pluses and minuses, it's easy to lose sight of the fact that these were designed and manufactured to hit a price point in Gibson's product line .... if they happened to sound good ... well ... I guess that would've been okay too! :wink: John

Just thought I'd check in to say I read the post, Obi Juan, and as Vader says in Star Wars: "Impressive. Very impressive."

Anyway, you've been goosing me about this with the point of your light saber for many moons, now, although this is the most complete explanation to date. And I better understand now why you want to tinker with my Crest amp.

My reluctance is built on liking it the way it is. Would it be possible to tweedify only one of the two channels (without changing the simplicity of the modifications)?

JP, as a former Falcon owner, what am I missing here?

Guildzilla said:

Would it be possible to tweedify only one of the two channels

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Hi 'Zil; not really ... if I have this doped out correctly, I'm talking about 4 resistors, 1 coupling cap, a nip and a tuck around that filter, and one wild card ... the output transformer. Without PNs stamped on the frames from both models, can't be sure they used the same OT in the Crest as they did in the Tweed.

It's also possible that there's something I'm missing and that for a lot of shipping but just a little twiddling, it comes out not too much different. JP once said that in France, there are 246 registered types of cheese. If your amp does swiss and I give you back gruyere .... :? :wink: John

guildzilla said:

My reluctance is built on liking it the way it is. Would it be possible to tweedify only one of the two channels (without changing the simplicity of the modifications)?

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That question is best left to the capn. Despite how much amp lit I've consumed, my technical skills are sorely lacking.

guildzilla said:

JP, as a former Falcon owner, what am I missing here?

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The best way is to hear the alternative.Through my cheapie computer speakers, the Tweed is markedly warmer than my Crestline ever sounded. The trademark, over-the-top Gibson reverb is still inherent in the Crestline DNA, but the Tweed seems to have a better defined tone--a little more lush, less harsh. The biggest difference is that charming bit of grit the Tweed can achieve. I could never get that with my Crestline. I play very dynamically. I like to be able to set tone and volume on both amp and guitar, so that I can get a nice clean warm sound at a specific volume. Then with just a little more playing attack, and a bump on the guitar volume, I can get more bite, grind, and some of the fuzzies. That's just me, though.

capnjuan said:

JP once said that in France, there are 246 registered types of cheese.

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Quote swiped from Charles de Gaulle. More than 50 years later, they boast more than 350 registered types, though they probably produce more than twice that many.

capnjuan said:

If your amp does swiss and I give you back gruyere .... :? :wink: John

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But what if you find out that you like Gruyere better than Swiss? :wink: Although, I do understand the "if it ain't broke" approach."

capnjuan said:

... I don't believe the model was re-done in Gibson's 'white face/doghouse' cosmetics but was done over later in black tolex, stick-em-on faux wood-grain paper on the control panel (for more transparent highs :wink: ) and with the cab sides sticking above the top so that there would be something to break off if the cabinet were dropped or tipped over.

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.... like this Skylark on eBay Here.

jp said:

... Through my cheapie computer speakers, the Tweed is markedly warmer than my Crestline ever sounded. The trademark, over-the-top Gibson reverb is still inherent in the Crestline DNA, but the Tweed seems to have a better defined tone--a little more lush, less harsh. The biggest difference is that charming bit of grit the Tweed can achieve.

Click to expand...

Well ... haven't heard 'Zilla's but that's what I've been assuming the difference would be.

jp said:

Quote swiped from Charles de Gaulle. More than 50 years later, they boast more than 350 registered types, though they probably produce more than twice that many.

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I don't respect any person or country that lies about cheese :evil: :wink: I used to beat myself up for letting that tweed go but that stopped when the GA30RV got to the house. Essentially the same circuit with 12" and 8" speakers but no trem ... but I held the GA18 and sold the GA19 because (.........was ) for the GA18. :wink: John

As ever...it can be the small stuff; same pic as above but with the plate resistors highlighted; lower values in the Tweed, higher in the Crest. So what? Since the power supply dropping resistors and the voltages to-be-reduced are the same, then the Crest is running at a lower plate voltage ... maybe in the 105 - 115 volt range ... but, thanks to higher value cathode resistors, at a higher current flow. (No; the voltage drop across resistors isn't linear ... the angle of the dangle rule equalling ...... doesn't apply)

Note that the tweed schematic conveniently puts the 135V plate voltage on the schematic (Hi Gibson; back in the day, didja ever wonder where the love was :evil: ?) I don't know the exact tone implications but if I was degrading a Crest, I'd change those highlighted resistors; .25/ea, takes 5 mins to change them, life's too short and the downside not self-evident. That filter though, in any case, I'd take that thing right outta here

Risk Management Dept.

Hi, Friends of the "Tone Monster!"

I'm a newbie here, but I am knee-deep in bringing a GA19RVT Crest back "from the dead." This one had been "modded" before I ever got to it. Unfortunately, the "mods" insured that the reverb would NEVER work and the Tone Control is also DOA. As it turns out, the "mad modder" attempted to "Tweedify" this unit. He got relatively close, but didn't really have a basic understanding of how electronic circuits operate. However, the quality of his work was simply abysmal! Terminals that were originally joined have been cut for no real reason, wire insulation is burned and melted all OVER the place. He replaced the filter capacitors with discrete parts wrapped up in electrical tape and attached to the chassis using a couple of twist-ties from loaves of bread! Needless to say, there was only one solution - Start OVER! But in tearing this thing down, I discovered what I *think* is a "running change" in the circuit design that isn't covered by either of the two available schematics. This has to do with the wiring of the foot switch for the "tremolo" function.

In both available schematics, the tremolo is activated by grounding one leg of the "rate" potentiometer via the foot switch. However, in *this* unit, the foot switch has been moved (electrically speaking) to break the connection from the wiper of the "rate" potentiometer and the junction of capacitors C18 and C19. The 47K resistor has been moved to the leg of the potentiometer to ground. This means that the tremolo function requires TWO wires that are totally independent of the reverb function. The foot switch cable has three wires and an outside shield. Connecting the shield to the white wire mutes the reverb return. To activate the tremolo function, the foot switch must short the red and black wires together.

At first, I thought this was a "mod" performed by the previous tech guy, but some photos I found online hinted that another unit was wired this way as well. Does ANYONE know why Gibson made this change? Is it possible that crosstalk from the higher audio level of the reverb section affected the tremolo - even when it was switched off? Can anyone actually CONFIRM this as a "factory modification?" Since this unit was not operational when I received it, I am "flying blind" here, but I want to "put things right" before returning it to its current owner.

Any help would be deeply appreciated.

Thanks in advance,

Bill Thomas

Bill Thomas said:

... But in tearing this thing down, I discovered what I *think* is a "running change" in the circuit design that isn't covered by either of the two available schematics ... wiring of the foot switch for the "tremolo" function. ... In both available schematics, the tremolo is activated by grounding one leg of the "rate" potentiometer via the foot switch ... The foot switch cable has three wires and an outside shield ...

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Hi Bill; not sure I have anything useful to offer or not. Below is for the benefit of anyone else reading this; I know you recognize this as the original trem circuit applying to the 6V6/tweed/7199 (V2) and 6V6/crest/6C4 (V3) versions.



I had a tweed model and a friend in Columbus has a crest model ... both 6V6 versions ... and neither had/has cross-talk problems between the reverb and trem functions. Gibson changed this model again to 7591s/ss rectifier but I don't have a schematic for that version and don't know whether, in those later models, they starting jacking the FS in with a multi-conductor plug instead of hard-wiring it. Not that it's news to you but the way yours is wired now doesn't make much sense.

The pic below is the termination of the FS cable in the tweed version. Can't be certain but I think the red wire is headed to the trem Speed control. If you have 3 wires; one is reverb, one is trem, and the third is a common ground and could be jumpered from one FS lug to the other. If it were my amp, I'd ground the chassis end of the shield but the only suggestion I have is to put it back in a known-good arrangement.




Btw: this is how I solved the cap replacement problem in my GA19; the caps are suspended between two wiring strips; had I used F&Ts in their smaller package instead of Spragues, the fit would have been that much better. Good luck with your amp.

Hi, capnjuan!

(I had hoped you would chime in here!) The version I'm working on is a "Crest" model with the standard complement of 3 6EU7's, 1 6C4, 2 6V6's and a 5Y3 rectifier. Your photo of the "Tweed" model clearly shows the 47K resistor connected to the junction of the two .047 uF capacitors. This is exactly as the schematic shows it.

In the one I am working on, the 47K resistor has been moved to the leg of the "frequency" potentiometer and the red wire is connected directly to the junction of those two .047 uF capacitors. The black wire connects to the wiper of the "frequency" potentiometer. Electrically, this provides the same function as the original wiring scheme, but it requires two isolated wires to activate the tremolo. Doesn't make a great deal of sense to me either, but according to the photos sent to me by LeonC, his amplifier was also (apparently) wired this way.

Also, Leon's amplifier has an extra 470 Ohm resistor connected between the top of the 2.2K resistor and the cathode of V2 (Pin 4). This would reduce the gain (and improve distortion somewhat) of the Reverb return amplifier. Since there is an extra gain stage in the "Crest" version of the Reverb circuitry, this *might* make sense. The amp I am working on does not have this resistor, but again, someone had *attempted* to "convert" this one to "Tweed" specs so it *might* have had this resistor originally. (I have decided to add it to this amplifier. We'll see what happens!)

Sometimes, working on these old gems can be frustrating when they are a little like snowflakes: "No two are alike." This can be made even MORE complicated when a unit has been "modded" in search of that "glorious tone." Since I'm not "married" to it in ANY way (other than my attempt to "set things right"), I just hope it makes music (without any internally generated hum) when I get done with it.

Gibson does NOT make it easy to do any "capacitor rolling" in ANY of these amplifiers, since they truly BURY a few of the capacitors UNDER the resistors on the tone stack board. In this rebuild, I decided to "un-bury" them and move them ABOVE the resistors for easier replacement later on. We'll see if this was a bad idea (or not) VERY soon. I *hope * to have this thing fired up within the next 24 hours.

Thanks again for your help.

Sincerely,

Bill Thomas

Bill Thomas said:

... "Crest" model with the standard complement of 3 6EU7's, 1 6C4, 2 6V6's and a 5Y3 rectifier ... Doesn't make a great deal of sense to me either, but according to the photos sent to me by LeonC, his amplifier was also (apparently) wired this way.

Also, Leon's amplifier has an extra 470 Ohm resistor connected between the top of the 2.2K resistor and the cathode of V2 (Pin 4). This would reduce the gain (and improve distortion somewhat) of the Reverb return amplifier. Since there is an extra gain stage in the "Crest" version of the Reverb circuitry, this *might* make sense.

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Hi Bill; it looks like you have two things going for you: you know how to get the trem running with what's in the amp and you have LeonC on the other line - he knows as much or more about these models as anyone. Had forgotten about the Crest model's mixing amp V2A acting as another gain stage. Agree that the 'extra' 470 ohm R bypassing the V2 2.2K cathode resistor would drop the gain ... who knows ... mellow it out a little? It might not have anything to do with the additional gain stage but in any event, you can't break it by putting it in.

Bill Thomas said:

... Sometimes, working on these old gems can be frustrating when they are a little like snowflakes: "No two are alike."

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Yes ... been there ... a Forrest Gump amp: "You don't know whatcha got until you run yer thumb into the bottom of it ..."

Bill Thomas said:

... since they truly BURY a few of the capacitors UNDER the resistors on the tone stack board.

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Is this the Gibson 'feature' ( :wink: ) you're referring to - a stock GA18 board with the the caps [and jumpers :evil: ] on the bottom ?




Tweed GA19 cb with tone, coupling, and trem caps excavated and updated ....




If you get a moment, post either pics of your amp or Leon's. J

It's quite interesting how very similar our approach to these amps is. When I get a spare moment, I'll post some pics of this particular amplifier. I'm on my "supper break" right now, but I finished the chassis wiring about an hour ago. (I'm resisting the "urge" to just plug it in, turn it on and "let the smoke out.") Actually, I find it VERY helpful to take a fairly big break after doing a complete, ground-up rebuild. Then, when I come back to it, I have a fresher pair of eyes in order to check the wiring more carefully and shake out the "gremlins" with the little solder balls.

Apparently, we have the same opinions regarding capacitors. Ceramic discs shouldn't pass anything but RF to ground. Mica is our friend! (You'll see what I mean when I post my pics.)

Thanks for the consult, Doctor. More knowledge is ALWAYS a good thing!

Sincerely,

Bill Thomas

Hi Bill; pics when you find the time. This is my GA19 from a couple of years ago; good not great cosmetics - common brown/orange stains on the other side .... control panel chrome in good shape.




I don't think the seller ever pulled the tube shields off; if he had, he would have mentioned in his auction that the preamp was still loaded with all the original Gibson-branded tubes.

Hi,

First post here. Great thread.

I'm curious if anybody knows what the specs are (and/or where to find a replacement) for T3 in the Crest schematic. This is the reverb transformer. I'm getting extremely high resistance on the secondaries, which climbs up to infinity in a few seconds. So, it appears that I have an open coil.

I guess I should ask the same question for T2, the transformer before the speaker. It appears that a beetle ate down into T3, which is probably the cause of the open coil. I found another beetle carcass dug down into T2 as well, though it still seems to be working alright.

???

Hi hj and welcome to LTG.
T3/reverb transformer: I'm pretty sure that any transformer sold for use in a vintage Fender will work ... the Gibson and Fender circuits are nearly identical ... Weber's reverb transformer specs (out of stock) but same as Mojotone's.
T2/output transformer: does you amp have 6V6s or 7591s? Tube or ss rectifier?

Well ... it sounds like your amp is already set up for beatle music ... hehehe ... yes, crappy pun. :wink: Welcome!

Cool. Thanks CapnJuan

I have a '65 Crest. From digging around in the circuit, I've found, so far anyway, that it is exactly like the Crest schematic you posted in the first post of this thread, which has been a BIG help. Thanks.
So, to answer your question: 6V6's, tube rectifier.

I've actually already done the modifications to turn it back into a Tweed, as per this thread, and I think it sounds a lot better.

I don't know how many people have actually done it thus far?
Anyway, the reason I bought a Falcon was because I played a Tweed model ('61 I believe?) and thought it was the best sounding amp I'd ever heard.

And this is just going from memory of that one time I actually played an original tweed version, but I can confirm that the set of mods you described in this thread DOES seem to get it sounding like a Tweed in terms of breakup. However, I'm going to need to get the reverb working to really make this one sing.

BTW, just a couple more things to mention. All I had on hand was 1/4 watt resistors for these mods (metal film) which I'm going to replace with 1/2 watt asap. I know 1/4 watt is probably not a good thing, but they'll hopefully be alright until I get some appropriate carbon comp. Should I go even higher than 1/2 watt? Or will that be fine?

Also, I went ahead and did one more mod that wasn't mentioned. On the Tweed schemo, there is another 2.7K resistor, but with a .02uF cap bypassing between V3's (pin 9?) and ground. On the crest schematic at the equivalent spot (V2), it has a 4.7K resistor and a .1uF cap.
I went ahead and changed these back to tweed specs whether or not it made a difference I don't know.

Just thought I'd mention that and see if you guys had any comments.

honkyjonk said:

... Anyway, the reason I bought a Falcon was because I played a Tweed model ('61 I believe?) and thought it was the best sounding amp I'd ever heard ... And this is just going from memory of that one time I actually played an original tweed version, but I can confirm that the set of mods you described in this thread DOES seem to get it sounding like a Tweed in terms of breakup.
Hi HJ: all things being equal, if you build it back like its predecessor model, it ought to sound more like the tweed version ... I had one ... I really liked it.

However, I'm going to need to get the reverb working to really make this one sing.
Do you know why it isn't working?

All I had on hand was 1/4 watt resistors for these mods (metal film) ... Should I go even higher than 1/2 watt? Or will that be fine?
No; except for the dropping resistors in the power supply, everything else will be fine at 1/2 watt.

On the Tweed schemo, there is another 2.7K resistor, but with a .02uF cap bypassing between V3's (pin 9?) and ground.
The tweed and crest schematics don't exactly match up because the tweed doesn't use both halves of one 6EU7 and they subbed in the 6C4 ... a single triode ... for simplicity. They also re-numbered the tubes from L to R to R to L. The 2.7K resistor runs from pin 8 / cathode (or pin 3 depending) to ground and, together with the bypass cap, sets the idle current / bias for the (tweed) tube.

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About a replacement OT; I contacted Mercury Magnetics about upgrading the output transformer in my GA20T, another 6V6 amp and their suggestion was any good-grade OT designed for a Fender 6V6 Deluxe (6.6K primary) would be fine. Weber's Deluxe OT would do it if you decide to make a move.

Good going on your mods; the gentleman who followed Seth Lover at Gibson to be the big amp cheese ... he just jerked a few things around and called it an update ... update schmupdate :evil: :wink:

Ah, I see,

So, because the other side of that particular 6EU7 is being used in the reverb circuit, the tube is/should be biased a little differently? In that case I'll sub back in the 4.7K and .1uF cap.

I'm positive though, in my amp, this resistor and cap are between Pin 9 and ground, rather than pin 8 or 3. Hmmm. Wierd.
If you zoom in on the crest schematic really far, there appears to be a 9 there also. At first I thought it was a 3, but zooming in really far, it actually looks more like a 9.

Thanks so much for all the info so far.

Jon