Gibson GA-15 RVT repair log
- Thread starter Jewellworks
- Start date
Lowerleftcoast
Poster Extraordinaire
I would try to talk the owner out of it. The resistors will use power and make a lot of heat in the chassis.
I understand PT price can be a problem. Consider a toroid PT. Make a plate to cover the PT opening and drill a hole in the center. Easy to install and they are quieter by design. Lighter too.
The info below shows the AnTek AS-05T280 should be OK for this. AnTek price $29 + shipping.
I have no affiliation with AnTek.
I understand PT price can be a problem. Consider a toroid PT. Make a plate to cover the PT opening and drill a hole in the center. Easy to install and they are quieter by design. Lighter too.
The info below shows the AnTek AS-05T280 should be OK for this. AnTek price $29 + shipping.
I have no affiliation with AnTek.
Jewellworks
Tele-Holic
Im on the AnTek page looking at this, but im not understanding what im seeing in the spec's. whats the current rating on the HT? the heater looks fine, but the HT? 149mA?
i was looking at the Hammond 261G6
250CT @ 130mA
6.3 @ 2A
$60 w shipping from HawkUSA. id have to use a full wave bridge rectifier, but it looks like id have to do that w the Toroid as well.
is there any advantage or downside to Toroid PTs? i know nothing about them
Jewellworks
Tele-Holic
i posted the schematic im using earlier in this post, but here is the link:
Lowerleftcoast
Poster Extraordinaire
This Gibson is drawing near 3A on the 6.3 secondary. The Hammond 261G6 doesn't have enough.
The *Load Test* section gives the mA capability. The PT is given a rating of 50mA but this is how much amperage draw it takes to make the HT 280V. The *Load Test* section shows the voltage at different loads... *290V @ 0.0A, 250V @ 160mA, and 232V @ 250mA*. Iow, the PT is not at it's maximum capability at 50mA. According to the *recommended uses* in the screenshot provided in post #22, the AS-50T280 is capable of powering 5E3 and 18Watt style builds. This Gibson draws about the same HT amperage as those.
The advantages of toroid PTs is weight and they don't throw magnetic fields around. Usually an OT can be placed right next to a toroid PT with no hum.
The downside is if the circuit uses a fixed bias. Ideally, the fixed bias should sag when the HT sags. Most toroid PTs do not have a dedicated bias tap on the HT winding, so the bias sags at a different rate. Simply adding a cap in the bias circuit alleviates this problem.
Good news for the Gibson. It is cathode biased, so no problem.
Right. It would need a bridge rectifier. 4 diodes instead of two.
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My brother and I got that exact GA-15RVT amp as part of a guitar purchase in the 80s. Back then I didn't rate anything smaller than a Twin Reverb, but even still I liked the swampy reverb and tremolo in our GA-15RVT. I used it to great effect on the intro to one of our best songs.
Years later my brother performed some key mods on the amp and changed out the speaker--changing the speaker is important in this instance.
I was very impressed with the result and very much wanted something that only one Fender amp ever used (and even there, for only about 5 seconds) - EL84s. Also I wanted something I could change the power transformer to 230v guilt free for use in Europe where I live now.
So I decided to get my own mid-60s GA-15RVT.
The next one to come along wasn't a GA-15RVT, however, but the Epiphone version, the Comet EA-32RVT. My brother did all the mods and the amp is actually pretty good, but it wasn't a mirror image of our GA-15RVT as it turns out the Gibson has (or at least the one we have has) a separate reverb drive transformer.
My brother made a video of him doing the mods here:
Here is a discussion he had about the differences between my Epiphone and the Gibson of the same year elsewhere.
Years later my brother performed some key mods on the amp and changed out the speaker--changing the speaker is important in this instance.
I was very impressed with the result and very much wanted something that only one Fender amp ever used (and even there, for only about 5 seconds) - EL84s. Also I wanted something I could change the power transformer to 230v guilt free for use in Europe where I live now.
So I decided to get my own mid-60s GA-15RVT.
The next one to come along wasn't a GA-15RVT, however, but the Epiphone version, the Comet EA-32RVT. My brother did all the mods and the amp is actually pretty good, but it wasn't a mirror image of our GA-15RVT as it turns out the Gibson has (or at least the one we have has) a separate reverb drive transformer.
My brother made a video of him doing the mods here:
Here is a discussion he had about the differences between my Epiphone and the Gibson of the same year elsewhere.
Epiphone Comet EA-32RVT Reverb
Question: I have a late 60's Epiphone Comet EA-32RVT. The reverb does not work well, but i can hear it....barely!!! There have been many posts regarding the various Epi/Gibson mid to late 60's circuit varieties. My Comet does not have a separate Reverb Drive Transformer...just as the circuit...
www.thegearpage.net
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Jewellworks
Tele-Holic
This is HUGELY informative.
So the 2, 6.3v heater feeds are 2A each, and I would use them both in this case?
I'd like to hear how to add that sag cap... What value and where...
You've talked ME into it, but now I've got to convince the owner. I don't think he'll put up much of a fight tho...
This is gonna be a super modified Gibson GA-15
Bob Womack
Friend of Leo's
What I found with the Ranger was that the bass needed to be reduced, the mids up all the way and treble up to taste. The amp didn't come alive until three o'clock on the volume and really woke up if you jumpered the two channels and opened both all the way. That gave a smooth distortion with lots of sag and compression and NO ice pick. Also understand that the tone stack is pre-gain, so you can actually dial in which band enters distortion first.
Bob
Lowerleftcoast
Poster Extraordinaire
Yes. Probably one winding for the power tubes and the other for the rest.
It is not a sag cap. It is a capacitive coupled bias supply. The Valve Wizard bias article will explain sizes to consider etc. This is just for info because this Gibson is cathode biased and doesn't need a fixed bias supply.
https://www.valvewizard.co.uk/bias.html
Good luck.
Jewellworks
Tele-Holic
Update:
going by the values shown on the schematic (earlier in this post) i sourced a Hammond 290XXA to replace the oversized Mojo PT. the same used in the Fender Pro Jr. : 260VAC @ 170mA, 6.3VAC @ 3A. no CT for either. seemed like a good match. much better than the one that was in here, for sure.
all the pieces parts and tubes and transformer finally trickled in, and i removed the oversized PT, and the extra long leads, and the original filter caps, and all the crap that was hanging off the Tag strip, and suddenly, i had a lot of room to work with.
i had to make a plate to mount the new PT, because they cut the chassis hole bigger to fit the Mojo PT, and this new PT is a lot smaller. then i made a Bassman style filter cap board, and mounted it where the filter caps were, using the same holes, so i didnt need to drill anything.
since the new PT doesnt have a CT, i built up a full wave bridge rectifier using 3W, 1000v diodes on the tag strip and ran the B+ to the filter board. i also removed a grounding wire coming off the filimant leads (from the factory!) and replaced it with 2, 100ohm resistors for a VCT for the heaters. (theyre within 1 ohm of eachother). i also landed the choke leads on the tag strip and used solid core 18 gauge back to the filter board. it also got a new 3 wire power cord.
before and after pics:
the bottom pic is still a works in progress, but you can clearly see how much of a massive improovalance it is. im really digging my filter board.
then, last night, i figured out all the leads coming from the circuit board, and shortened and landed them where they needed to be.
i removed the shared cathode lead from the power tubes (the yellow wire) back to the tag strip, and instead, just ran the 130ohm and 2uf cap directly off the pins to the negative side of the filter board. so much cleaner and neater. and it should be a lot quieter.
plugged it into the light bulb current limiter, left the tubes out and checked voltages. everything powers up well and with higher than normal voltages, but thats to be expected w no load. verified that the correct voltage is going to the correct pins. So far so good!
then, before i put the tubes in, I tested them with my newly acquired Eico 667 tube tester. both power tubes were completely dead (no surprise there) but the 3 preamp tubes (2, 6EU7's and a 12AU7) still tested good, -which surprised me. put the tubes in, including the new JJ EL84s, and turned it on, still with the current limiter.
its ALIVE! then i measured voltages again, and im awfully close to where they say they should be on the Gibson Schematic! slightly low, but im also still on the current limiter.
the low voltage readings also include the heaters running at just under 6v. 5.8ish... im guessing thats a current limiter issue as well?
now that i can safely turn it on and run it for a while without fear of nuking the tubes, i can hear scratchy pots and the like... and i wasnt able to test the reverb or tremolo yet. ill check all that tonight.
one thing i noticed right away is how clean this amp sounds, and how the bass pot seems to control not just the bass, but also the mids. if i turn it up, i can get some nice overdrive.
and i think someone already mentioned that the build quality of this Gibson amp sure isnt what Lupe' and her coworkers would have allowed to leave the Fender factory. wire dress wasnt a thing at Gibson apparently. i did what i could without rewiring the entire thing, although, im still tempted...
all the resistors are still within spec. ill check for leaky coupling caps after i clean the pots.
and thats the story so far....
more pics and maybe a VIDEO to come when its all DONE done.
EDIT: one thing i forgot to mention is the half-broken tab on the reverb driver transformer. i figured out a way to secure it by taking apart an old dead filter cap (i wanted to see what was in there) and its the same paper that is wrapped around the transformer. so im going to use a foot or 2 of that to wrap around it, then use wax to hold it all together, including the half-broken tab. wish me luck!
going by the values shown on the schematic (earlier in this post) i sourced a Hammond 290XXA to replace the oversized Mojo PT. the same used in the Fender Pro Jr. : 260VAC @ 170mA, 6.3VAC @ 3A. no CT for either. seemed like a good match. much better than the one that was in here, for sure.
all the pieces parts and tubes and transformer finally trickled in, and i removed the oversized PT, and the extra long leads, and the original filter caps, and all the crap that was hanging off the Tag strip, and suddenly, i had a lot of room to work with.
i had to make a plate to mount the new PT, because they cut the chassis hole bigger to fit the Mojo PT, and this new PT is a lot smaller. then i made a Bassman style filter cap board, and mounted it where the filter caps were, using the same holes, so i didnt need to drill anything.
since the new PT doesnt have a CT, i built up a full wave bridge rectifier using 3W, 1000v diodes on the tag strip and ran the B+ to the filter board. i also removed a grounding wire coming off the filimant leads (from the factory!) and replaced it with 2, 100ohm resistors for a VCT for the heaters. (theyre within 1 ohm of eachother). i also landed the choke leads on the tag strip and used solid core 18 gauge back to the filter board. it also got a new 3 wire power cord.
before and after pics:
the bottom pic is still a works in progress, but you can clearly see how much of a massive improovalance it is. im really digging my filter board.
then, last night, i figured out all the leads coming from the circuit board, and shortened and landed them where they needed to be.
i removed the shared cathode lead from the power tubes (the yellow wire) back to the tag strip, and instead, just ran the 130ohm and 2uf cap directly off the pins to the negative side of the filter board. so much cleaner and neater. and it should be a lot quieter.
plugged it into the light bulb current limiter, left the tubes out and checked voltages. everything powers up well and with higher than normal voltages, but thats to be expected w no load. verified that the correct voltage is going to the correct pins. So far so good!
then, before i put the tubes in, I tested them with my newly acquired Eico 667 tube tester. both power tubes were completely dead (no surprise there) but the 3 preamp tubes (2, 6EU7's and a 12AU7) still tested good, -which surprised me. put the tubes in, including the new JJ EL84s, and turned it on, still with the current limiter.
its ALIVE! then i measured voltages again, and im awfully close to where they say they should be on the Gibson Schematic! slightly low, but im also still on the current limiter.
the low voltage readings also include the heaters running at just under 6v. 5.8ish... im guessing thats a current limiter issue as well?
now that i can safely turn it on and run it for a while without fear of nuking the tubes, i can hear scratchy pots and the like... and i wasnt able to test the reverb or tremolo yet. ill check all that tonight.
one thing i noticed right away is how clean this amp sounds, and how the bass pot seems to control not just the bass, but also the mids. if i turn it up, i can get some nice overdrive.
and i think someone already mentioned that the build quality of this Gibson amp sure isnt what Lupe' and her coworkers would have allowed to leave the Fender factory. wire dress wasnt a thing at Gibson apparently. i did what i could without rewiring the entire thing, although, im still tempted...
all the resistors are still within spec. ill check for leaky coupling caps after i clean the pots.
and thats the story so far....
more pics and maybe a VIDEO to come when its all DONE done.
EDIT: one thing i forgot to mention is the half-broken tab on the reverb driver transformer. i figured out a way to secure it by taking apart an old dead filter cap (i wanted to see what was in there) and its the same paper that is wrapped around the transformer. so im going to use a foot or 2 of that to wrap around it, then use wax to hold it all together, including the half-broken tab. wish me luck!
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Jewellworks
Tele-Holic
since the last post, i took it off the light bulb limiter, and the voltages went a lot higher. ive been struggling to get the bias adjusted so the PD is around 10W (out of 12) and still not cook the plates. ive heard that 350v is close to the edge for EL84s, so my target is 325 @ 10W PD. adjusting the bias resistor alone was raising the plates voltage too close to the 350 mark, so i decided to drop the voltage w/a pair of matched 100 ohm, 5W resistors off the B+/- to the first filter cap for a balanced drop, and with a 200 ohm bias resistor, im at 328 @ 10.25W PD. thatll'doo!
i also rewired the input power to include the light this time. i left that out the first time around...
and removed the death cap. its mortal remains are at the bottom of the tab rail.
now that i have the power where i want it, its time for tone.
as many have observed on this post, and many others involving this particular amp, it sounds awful. thin and weak, with no "body". the Bass and Treble controls are bizarre... especially the Bass control. the schematic shows a 2M pot, with a tap in the middle that has a .01 cap to ground. i dont know what the intent was, but what it actually does is thin the begeeziz out of it up to about 4, then there is a very strange scoop in the mids right at 4, but when you get to 5, it fills back in again and starts adding mids. lots of mids until you get to 10. the treble rolls off highs, but not enough to mute it by much. the only way this sounds any good at all is with the gain and both tone pots up all the way. anything other than that sounds awful and unusable.
there are many places in the circuit where Gibson was cutting lows. RC network right at the input cuts below 75hz. thin 2uf bypass caps cut around 175hz. that happens several times in this circuit. 1st thing i did was eliminate the RC at the input and install a 1M grid leak, then beef up the preamp bypass caps from a 2 to a 15u. still not as deep as Leo, so im controlling the lows, but still, it warmed up the amp with more lows, but it still sounded like hammered crap with that awful tone stack. so i decided to do something radical (for a vintage amp) and remove the tone stack and replace it with a Fender style B/T blackface tone stack, with modifications...
at first, my thought was to keep the tone "flat" when the tone knobs are at 5. then adjust highs and lows from there. lots of mids but overall, the sound was very buzzy and the controls didnt really respond like i was expecting. too many mids for the controls to be heard. so i went back to the drawing board (the TSC on the web) and redesigned it 3 times, then tweaked the 3rd design until i got it sounding REALLY good and responding well with the controls. i ended up with a slightly thinner mid scoop than Fender, but with about 2db more gain @ 100 and 1k. it cleaned up the buzzyness as well. plenty of juicy grit, not just a square wave. im really happy with the way it sounds.
Orange line is stock Fender, im the Green. values in the calc are measured values. the 420pf is actually labeled a 390pf
and an observation: the buzz im hearing sounds a lot like grid distortion, but i dont think it is. i think im actually overdriving the transformers. which one (or both?) i dont know. i havent gotten out my O-Scope to see it. but as observed earlier in this post, the transformers are REALY small. plus, this amp uses an interstage transformer for the PI, also very small. with the mid-scoop tone stack, the buzz isnt overwhelming. its a Throaty Grit at B:5 , T:6 , V:8
i also rewired the input power to include the light this time. i left that out the first time around...
and removed the death cap. its mortal remains are at the bottom of the tab rail.
now that i have the power where i want it, its time for tone.
as many have observed on this post, and many others involving this particular amp, it sounds awful. thin and weak, with no "body". the Bass and Treble controls are bizarre... especially the Bass control. the schematic shows a 2M pot, with a tap in the middle that has a .01 cap to ground. i dont know what the intent was, but what it actually does is thin the begeeziz out of it up to about 4, then there is a very strange scoop in the mids right at 4, but when you get to 5, it fills back in again and starts adding mids. lots of mids until you get to 10. the treble rolls off highs, but not enough to mute it by much. the only way this sounds any good at all is with the gain and both tone pots up all the way. anything other than that sounds awful and unusable.
there are many places in the circuit where Gibson was cutting lows. RC network right at the input cuts below 75hz. thin 2uf bypass caps cut around 175hz. that happens several times in this circuit. 1st thing i did was eliminate the RC at the input and install a 1M grid leak, then beef up the preamp bypass caps from a 2 to a 15u. still not as deep as Leo, so im controlling the lows, but still, it warmed up the amp with more lows, but it still sounded like hammered crap with that awful tone stack. so i decided to do something radical (for a vintage amp) and remove the tone stack and replace it with a Fender style B/T blackface tone stack, with modifications...
at first, my thought was to keep the tone "flat" when the tone knobs are at 5. then adjust highs and lows from there. lots of mids but overall, the sound was very buzzy and the controls didnt really respond like i was expecting. too many mids for the controls to be heard. so i went back to the drawing board (the TSC on the web) and redesigned it 3 times, then tweaked the 3rd design until i got it sounding REALLY good and responding well with the controls. i ended up with a slightly thinner mid scoop than Fender, but with about 2db more gain @ 100 and 1k. it cleaned up the buzzyness as well. plenty of juicy grit, not just a square wave. im really happy with the way it sounds.
Orange line is stock Fender, im the Green. values in the calc are measured values. the 420pf is actually labeled a 390pf
and an observation: the buzz im hearing sounds a lot like grid distortion, but i dont think it is. i think im actually overdriving the transformers. which one (or both?) i dont know. i havent gotten out my O-Scope to see it. but as observed earlier in this post, the transformers are REALY small. plus, this amp uses an interstage transformer for the PI, also very small. with the mid-scoop tone stack, the buzz isnt overwhelming. its a Throaty Grit at B:5 , T:6 , V:8
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Jewellworks
Tele-Holic
the next thing i need to look at is the Tremolo. it sounds great, but you have to turn it to 5 before you hear anything at all. they're using a 250k-A pot. at 5, there is 50k of resistance to ground. change that to a 250k-L , and the 50k mark should be around 2.
then the reverb...
im having a hard time getting the half-broken tab on the driver transformer to stay solid. i wrapped it 2 times w more paper and dipped it in wax, but its still weak. what a terrible idea to use the tabs on the transformer... the reverb return goes to an RCA jack. they couldnt spring for an RCA jack for the driver?
anyway...
the reverb also has a LOT of bass cutting circuitry. .0047u coupling caps, 2u bypass caps... but maybe thats how it works w reverb tanks? not sure how warm i can make it. the reverb on these amps has generally been a good thing, described as "spacy". i dont want to ruin that, nor change every single aspect of what makes this amp NOT a Fender...
i will add this tho, in both the original circuit, and with my modifications, im taking the feed to the reverb BEFORE the Volume control. the original schematic had the reverb fed BEFORE the Tone Stack! so the reverb was raw guitar signal, the rest of the amp was tone stack. blended at the PI driver. 2 different signal tones. my circuit moved the feed after the tone stack, but before the volume. im curious what the reverb sounds like. havent gone there yet. in theory, you should be able to play through the reverb alone with the volume turned all the way down. itll be quiet, but itll be raw reverb only.
then the reverb...
im having a hard time getting the half-broken tab on the driver transformer to stay solid. i wrapped it 2 times w more paper and dipped it in wax, but its still weak. what a terrible idea to use the tabs on the transformer... the reverb return goes to an RCA jack. they couldnt spring for an RCA jack for the driver?
anyway...
the reverb also has a LOT of bass cutting circuitry. .0047u coupling caps, 2u bypass caps... but maybe thats how it works w reverb tanks? not sure how warm i can make it. the reverb on these amps has generally been a good thing, described as "spacy". i dont want to ruin that, nor change every single aspect of what makes this amp NOT a Fender...
i will add this tho, in both the original circuit, and with my modifications, im taking the feed to the reverb BEFORE the Volume control. the original schematic had the reverb fed BEFORE the Tone Stack! so the reverb was raw guitar signal, the rest of the amp was tone stack. blended at the PI driver. 2 different signal tones. my circuit moved the feed after the tone stack, but before the volume. im curious what the reverb sounds like. havent gone there yet. in theory, you should be able to play through the reverb alone with the volume turned all the way down. itll be quiet, but itll be raw reverb only.
bottlenecker
Poster Extraordinaire
For whatever it's worth, the reverb and tremolo are wonderful on my GA-15RVT. Spacy is a good word for the reverb. Dark with a long decay. The tremolo is my favorite. It gets out of the way of a strong input better than any other tremolo I've tried, and has the perfect feel and range of speed and depth.
The complete lack of bass and overall thinness is why I'm following this thread. I would love to learn how to fill in all the missing frequencies and keep what I like. Right now it only sounds good to me with one oddball guitar.
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