Gibson GA-15 RVT repair log

[Jewellworks]

a friend of a friend brought me 2 amps to "look over". 1 is a Blues Jr, the other is a vintage Gibson GA-15 RVT "Explorer" from 1966, according to the date code on the original filter caps. pretty cool. i was able to download a schematic, and with just a quick look through, its fairly accurate, although i understand that Gibson wasnt very good at keeping accurate schematics, so well see how that goes as i get deeper into the amp.

2, 6EU7 tubes, 1, 12AU7, and 2 EL84's in PP w/an interstitial transformer for the PI.

i want to share my thought process as i go through this amp, and get you guy's' opinions/ideas. so lets start here:

at some point in its past, the Power Transformer was replaced with an older Mojo Tone 26v6P transformer. its out of production, so i called Mojo and then emailed them with pictures. they quickly sent me a schematic of the PT, and its dated May 22, 1997. more on this later...

also, at some point, ... i dont know what happened, but both the OT and the Reverb Driver transformer "came off" and both were re-mounted with all the wrong hardware. the Gibson schematic has a parts list with all the transformers (and voltages on the tubes!) and i verified the transformers are the originals, except for the PT, obviously.

the Reverb driver has tabs right on the transformer that feed the reverb tank, and again, at some point, they were ripped off, and now the Transformer windings are exposed and one of the wires w/the tab is dangling around. i havent measured for continuity or resistance yet, so i dont know what state its in, if its still operational or not.
the OT, in my opinion, is surprisingly small. its half the size of the 5W OT i use in my SE builds. im not too experienced in PP amps, so maybe this is normal? but geez... its supposed to be around 12W

there are 2 multi-cap filter caps (2, 20uf each w/a shared ground). 1 of them has been cut away and replaced w 2, 33uf caps, they left the other original multi-cap in place. when they changed out the PT, they left the original 2 wire power cord. oooooo K. they also left "most" of the leads at their original LONG-ASS length. uuugh.

the rest of the components on the board all look original. i did a quick resistor measurement, and most are still within spec. measured the cap values, and they also seem close enough. i didnt measure anything for DC leakage yet.

now the fun part:
i put it on a Variac and slowly brought up the voltage. i dont know the last time this was powered on. then i moved it to the current limiter and turned it on. slight glow, but nothing crazy. i plugged in a guitar, and it works, but weak sounding. then i started measuring voltages. i didnt have the Mojo PT schematic at the time, so i just assumed the wiring was correct.
-never assume anything.
i was getting 4.7v on 1 leg of the heaters and 0 on the other, referenced to (what i assumed was) the heater CT ground. well that aint right...
then i measured the HT and what was on the plates of the EL84's. over 350v HT and 373 on the plates after rectification (solid state, and the original diodes were also replaced).
the Gibson schematic calls for the PT HT to be 250v and 300v on the plates. so this is WAY high. i did a quick buncha other tests and calculations, and its running 68mA at 27W! supposed to be 12! i shut it off after that.

so were under powering the heaters, and nuking the plates.
im also not seeing any screen grid resistors. not on the schematic either.

now that i have the MOJO PT schematic for whats in here, whoever did this is using the 5v rectifier leads for the heaters, and not using the 7v leads at all, there is no CT for either one, and also no virtual CT anywhere that i can see. plus the aforementioned super high HT.

i can correct the heaters easily enough, and add a VCT, and im guessing i can also lower the B+ by using larger dropping resistors and Filter Caps.

im also seeing that the grounding scheme really isnt one. it sure looks like "ground is ground" in here.

i havent talked to the owner yet, but assuming hes not interested in originality, i plan on removing all the filter caps, installing a set of Tag strips where the filter caps were, and mounting new Filter caps and dropping resistors on the new tag strips. this will also give me the opportunity to come up with a much better grounding scheme. id also like to add a screen resistor.

then the only thing is the tubes. it looks like the 6EU7s are similar to a 12AX7, but more expensive. when i did the guitar test, i wasnt getting any tremolo. im guessing thats a tube. i didnt try the reverb, what with the broken transformer and all... and i dont have a tube tester, nor a spare 6EU7.

pictures attached





















thoughts? advice? experience??

 

[Lowerleftcoast]

the OT, in my opinion, is surprisingly small. its half the size of the 5W OT i use in my SE builds. im not too experienced in PP amps, so maybe this is normal?

The PP OT can be smaller. The SE OT should have an air gap that a PP OT does not need. This can make the SE OT appear more formidable. Guitar amps were not trying to be HiFi. They don't need to have a substantial OT for more bottom end. These models probably were also considered *student* grade like the Princeton. Smaller transformers cost less.

i have the MOJO PT schematic for whats in here, whoever did this is using the 5v rectifier leads for the heaters, and not using the 7v leads at all, there is no CT for either one, and also no virtual CT anywhere that i can see. plus the aforementioned super high HT.

i can correct the heaters easily enough, and add a VCT, and im guessing i can also lower the B+ by using larger dropping resistors and Filter Caps.

Chances are the heaters were originally wired like other early amps, 6.3v to one leg and the other leg to ground. No heater CT needed. Imo, you can make a case for making the heater wiring modern with a virtual CT.

I see some PT wires that are taped off. Might there be some lower voltage HT taps on that Mojo PT?

Is there a choke on/in this chassis as well? I am trying to decipher what I am seeing.

I would need to see a schematic and a diagram of the PT to kibitz any further.

 

[corliss1]

Any Gibson amp with the two slanted faces on the top of the cabinet, usually isn't worth fixing. They don't sound good even after service, they were wired by blindfolded children in a dark basement 50 years ago, and are expensive to repair. Okay, no, that's not fair to blindfolded children.......................they could do better work.

That being said, I wouldn't have powered it up at all with those caps. That top one in the 6th image seems to have already spilled it's guts.

I was surprised when you said 6EU7s were more expensive, but sure enough, I checked some sites and even those have went up a ton.

I've gotten to the point where I say no to most mid-60s Gibson amps, and there aren't very many things I say no too.

 

[SnidelyWhiplash]

60s Gibson amps sound their best when hitting the bottom of a dumpster. All jokes aside, seems like a lot of work for an amp that isn't very well thought of from a tonal/workmanship point of view.

 

[bottlenecker]

I love mine. It does a few things better than most amps and with the right guitar it can sound cool in a way mainstream amps just can't. I'm following the thread hoping people who don't like them won't talk you out of learning more (so I can too).

 

[Jewellworks]

I see some PT wires that are taped off. Might there be some lower voltage HT taps on that Mojo PT?

Is there a choke on/in this chassis as well? I am trying to decipher what I am seeing.

I would need to see a schematic and a diagram of the PT to kibitz any further.

The wires that are taped off are the 7.1v heater wires they should have used, and a 42v bias tap. If "old school" heater wiring has 1 "hot" lead and the other to ground, then that's not how it's wired, and I would assume the other leg off the PT would go to ground? They have both leads going to the heater pins, like "normal".

The choke is in the 6th picture, inside the chassis, right next to the PT and the filter caps .



Below is the diagram for the MOJO tone PT. -Don't know what O. C. V. means...?


And the Gibson schematic I'm using:


You can see that Gibson used a PT w 250VHT, and this new one is 354v. Over 100v higher. Not a good match. That said, it's what's in here, and in order to use it, I need to drop the voltage. I calculated 1K at 10W (50w) but do I insert it before the OT? Or take it to ground and sink away 100v?

I would agree the wiring is sloppy, and not just from the last tech that was in here.
But I disagree that it's not worth the trouble. I'm still new at this and I think It's a cool old amp and I hope to bring it back for years of service. It won't be a modern day gigging amp, but it's definitely different than most amps you see these days and a conversation piece, like an old Oahu or something. You wouldn't throw that in a dumpster, would you?
it's in good shape overall and it'll be a fun project. I'm happy to have it on my bench.

 

[Tony65x55]

I have one of these amps. I have dropped it into a 1x12" cab and have tried it with a number of speakers , several of which sounded terrific. It is a great little grab and go amp and is loud enough to play with most drummers, much like a Princeton.

Enjoy the little beast.

 

[zook]

I have the Epiphone version here from a friend. It is a mess. After recapping is still has distortion and terrible volume and tone. I've checked everything I could think of but I'm still not finding the problem. I'm sending it back to its owner, he can take it to Tucson's guru John Markovich.

 

[Jewellworks]

I have one of these amps. I have dropped it into a 1x12" cab and have tried it with a number of speakers , several of which sounded terrific. It is a great little grab and go amp and is loud enough to play with most drummers, much like a Princeton.

Enjoy the little beast.

someone changed out the speaker for a 10" Fender blue label. looks funny to see a Fender speaker in a Gibson amp.
ive got a few speaker cabs i intend to try out w/this, once i get it healthy again, and stop nuking the tubes. one is a 12" Celestion EVH, (although its 4 ohms, so maybe not) and the other is a Celestion Ten 30. ive got a Jensen P10R i can use too, just to try it.

 

[Powdog]

I have the Epiphone version here from a friend. It is a mess. After recapping is still has distortion and terrible volume and tone. I've checked everything I could think of but I'm still not finding the problem. I'm sending it back to its owner, he can take it to Tucson's guru John Markovich.

I have the same amp, in a box on the floor for about a year now. Recapped, new tubes and still sounds like crap. Haven’t felt the itch to revisit it…..yet.

 

[Jewellworks]

so the few open questions i have for this is:
whats the best way to drop the voltage by 100v? off the rectifier to 1k (50W) to the OT? or off the rectifier to the 1k and sink it to ground? will that affect anything else? i wouldnt think so...

Chances are the heaters were originally wired like other early amps, 6.3v to one leg and the other leg to ground. No heater CT needed. Imo, you can make a case for making the heater wiring modern with a virtual CT.

is there an advantage to this? i was able to find 1 picture (from someone selling one of these amps on Reverb) that has a gut shot, and they have the green heater wires going to pins 4&5 just like any other heater wires ive seen. but im certainly NOT seeing a virtual CT. is this part of how the original PT would have been wired internally? with one leg of the heater lines going to ground? i dont think the MOJOtone PT is wired like that.
it seems easy enough to add a VCT. and id like to add a 1k screen resistor too.

 

[bottlenecker]

someone changed out the speaker for a 10" Fender blue label. looks funny to see a Fender speaker in a Gibson amp.
ive got a few speaker cabs i intend to try out w/this, once i get it healthy again, and stop nuking the tubes. one is a 12" Celestion EVH, (although its 4 ohms, so maybe not) and the other is a Celestion Ten 30. ive got a Jensen P10R i can use too, just to try it.

I've tried mine with a number of speakers and cabs, and the amp has no low end. That's it's main drawback, and why it's kind of a special character amp for me and not a main amp. It is fantastic for spy theme twang, or in a dense mix, but if I could find a way to get some bass out of it, it could be a great all around amp. It's strengths are very reactive touch controlled tremolo and breakup, and a spooky reverb.
The best speaker for it has been an eminence legend 15" in a deep ported cab, because it squeezes out the last drop of bass the amp has.
Mine is pretty stock, so no telling how different yours will sound, but don't be surprised if the low end is thin.

 

[Jewellworks]

I've tried mine with a number of speakers and cabs, and the amp has no low end. That's it's main drawback, and why it's kind of a special character amp for me and not a main amp. It is fantastic for spy theme twang, or in a dense mix, but if I could find a way to get some bass out of it, it could be a great all around amp. It's strengths are very reactive touch controlled tremolo and breakup, and a spooky reverb.
The best speaker for it has been an eminence legend 15" in a deep ported cab, because it squeezes out the last drop of bass the amp has.
Mine is pretty stock, so no telling how different yours will sound, but don't be surprised if the low end is thin.

most (if not all) the low end cut is coming from the 1st preamp stage. there is an RC network right at the input that cuts the signal at about 47hz. eliminating that .0047uf cap and bumping the 470k grid leak to 1M might help w overall gain and tone. the bypass cap is 2uf, whereas Fender uses a 22uf, and the coupling cap is .01uf, and .02uf is a little deeper. but by then, its not a Gibson anymore, but much more Fender-like.
also, the way the tone stack is wired, you have to have the bass up all the way or it cuts a lot of signal. the schematic shows more tone-sucking networks around the tone stack, but its unclear how much of that is actually in the circuit. i havent gotten that deep into it yet.

 

[Bob Womack]

I hear all the crap talk about the Gibson missile series and wonder if the badmouthers ever played one of these the right way? Back in the '70s I had a '66 Gibson GA55RVT Ranger, 50 watt, 4x10 combo that I played in a band with another guitarist who had a Fender black face Super. The consistent opinion from listeners was that the Ranger was a much nicer amp which broke up sweetly at a lower level. I jumpered the channels and ran an EH LPB-2 preamp into it and it sounded lovely. Now, I recently bought the 2x10 version, the GA45RVT Saturn, same year, and found it thin sounding. I perused the schematics of the two and discovered the difference: The Ranger had midrange controls on both channels that I ran wide open at all times. The Saturn didn't have the midrange control but DID have the midrange circuit with a fixed resistance, and the resistance values seem to place the virtual "knob" in the all-the-way-counter-clockwise "off" position. My intention is to go in and alter the resistor values to virtually turn it to the all-the-way-clockwise "full" position and see what happens. There are a few other little oddities about this animal that I'm probably going to have checked by a tech.

Bob

 

[Powdog]

I hear all the crap talk about the Gibson missile series and wonder if the badmouthers ever played one of these the right way? Back in the '70s I had a '66 Gibson GA55RVT Ranger, 50 watt, 4x10 combo that I played in a band with another guitarist who had a Fender black face Super. The consistent opinion from listeners was that the Ranger was a much nicer amp which broke up sweetly at a lower level. I jumpered the channels and ran an EH LPB-2 preamp into it and it sounded lovely. Now, I recently bought the 2x10 version, the GA45RVT Saturn, same year, and found it thin sounding. I perused the schematics of the two and discovered the difference: The Ranger had midrange controls on both channels that I ran wide open at all times. The Saturn didn't have the midrange control but DID have the midrange circuit with a fixed resistance, and the resistance values seem to place the virtual "knob" in the all-the-way-counter-clockwise "off" position. My intention is to go in and alter the resistor values to virtually turn it to the all-the-way-clockwise "full" position and see what happens. There are a few other little oddities about this animal that I'm probably going to have checked by a tech.

I have a few Gibson amps. Two GA series from the 50’s, two Crestlines from the 60’s and the GA-15 Explorer. The first four SOUND way better than the Explorer. How can I play it “the right way” so it sounds better? Can I make it sound like a 50 watt GA-55?

 

[Lowerleftcoast]

whats the best way to drop the voltage by 100v?

First off, the O.C.V. on the PT datasheet probably stands for *open circuit voltage*. So the voltages in circuit will be lower. This PT is probably around 325-0-325 when judging it with other companies PT specs that include an estimated load.

Imo, the owner should get a PT better suited to the circuit. Barring that I would install a 5Y3 rectifier to drop the HT. My guess using a 5Y3... this PT would end with a B+ of around 360VDC. I wouldn't bother with dropping resistors to lose this much voltage.

im certainly NOT seeing a virtual CT.

I would wire the heaters like modern amps with an artificial CT. Imo there is less chance of heater hum using a heater CT.
I can't see two wires running to each tube on your pictures. I am assuming *one side* of the heater secondary is attached to ground. Simply use your multimeter set to ohms... one probe on ground the other probe on a heater pin. A low ohm reading, close to continuity, would mean *one side* of the heater secondary is attached to ground.

id like to add a 1k screen resistor too

No problem adding screen resistors.

 

[Jewellworks]

Imo, the owner should get a PT better suited to the circuit. Barring that I would install a 5Y3 rectifier to drop the HT. My guess using a 5Y3... this PT would end with a B+ of around 360VDC. I wouldn't bother with dropping resistors to lose this much voltage.

i just talked to the owner, and hes pretty much given me free reign to do whatever needs doing, including circuit changes to get more tone and low end out of it.
he wants to see if i can use the MOJO PT and lower the voltage. adding a 5Y3 would be a pretty drastic move, and as un-original as this amp has become, im thinking drilling another hole and adding another tube is really over the top. and 360v is still pretty high and would need to drop a little more. id like to be in the 325 range at the plates. so with that, i'd prefer (or, i should say, "id like to try") a large dropping resistor, just to see if itll work right, and remain reliable. if its just too wonky, then hes up for getting the correct PT. but he wants me to try it first. -i can understand that.

I would wire the heaters like modern amps with an artificial CT. Imo there is less chance of heater hum using a heater CT.
I can't see two wires running to each tube on your pictures. I am assuming *one side* of the heater secondary is attached to ground. Simply use your multimeter set to ohms... one probe on ground the other probe on a heater pin. Low ohms, close to continuity, would mean *one side* of the heater secondary is attached to ground.

ill do the continuity test tonight, but im guessing this is a "normal" heater 2ndary, and ill do the V-CT.

 

[bottlenecker]

I have a few Gibson amps. Two GA series from the 50’s, two Crestlines from the 60’s and the GA-15 Explorer. The first four SOUND way better than the Explorer. How can I play it “the right way” so it sounds better? Can I make it sound like a 50 watt GA-55?

In my opinion, no way an explorer will ever sound like a 50s gibson, which are all around great, or like a crestline falcon, which is moving in the trashier garagey/explorer direction, but still fuller sounding. The value of the explorer to me is that it's weird, and it can sound kind of old fashioned and classic at the same time it sounds strange. It's an odd amp. I tend to use it with odd guitars.

most (if not all) the low end cut is coming from the 1st preamp stage. there is an RC network right at the input that cuts the signal at about 47hz. eliminating that .0047uf cap and bumping the 470k grid leak to 1M might help w overall gain and tone. the bypass cap is 2uf, whereas Fender uses a 22uf, and the coupling cap is .01uf, and .02uf is a little deeper. but by then, its not a Gibson anymore, but much more Fender-like.
also, the way the tone stack is wired, you have to have the bass up all the way or it cuts a lot of signal. the schematic shows more tone-sucking networks around the tone stack, but its unclear how much of that is actually in the circuit. i havent gotten that deep into it yet.

Mine will never sound like a fender. It's character throughout the frequency range is just entirely different. I kind of figured the low end is being dumped early in the circuit, but it's missing lows that are much higher than 50hz.
As I learn more, I might try some things, but I don't really work on amps yet.

 

[D'tar]

Do we ha e a proper schematic for this? Link please....

Does this circuit have the three legged tone sukka?

 

[Powdog]

Do we ha e a proper schematic for this? Link please....

Does this circuit have the three legged tone sukka?

https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga15rvt.pdf