Just bought a Gibson GA-100 and need some help

BoomTexan

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So, just bought this this afternoon for $500. This is one of like 400 GA-100 amps that Gibson ever shipped. It was their top of the line bass amp and was produced in extremely limited numbers, especially because it was only 30-50 watts, and for half the price, you could get a Bassman or high-power Twin. I was very surprised to find a lot of info on it proportional to the amount of amps that exist, but I do need help with one issue.

This amp has an 0C2 voltage regulator tube that seems to be choking the amp's power output under certain circumstances. If I play very hard with a higher-output guitar, the tube will enter a shutdown state and for around .5 seconds it'll get very dim. Normally, it's glowing a lot, but it'll go almost completely dim under these circumstances. It'll also do this randomly about every 20 seconds. This causes the volume to instantly drop a lot, by around 20-30%.

For the issues with playing very hard, I'm guessing that it's trying to compensate for an increased voltage pull when the tubes are really really pushed. Should I just use this with single coils and no boost? Then, could the second cause be due to failing filter caps that are randomly discharging or not properly filtering voltage? Wall voltage is also running on close to 125V, so could it also be that the wall voltage is too high and I need to use this hooked up to a variac? Is the 0C2 failing?

Sorry for the questions, but this is the first amp that I've ever played through that has a voltage regulator and I'm really not sure how to approach this.
 

slider313

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If you haven't replaced the power supply electrolytics, start there. What voltages do you see on the first node (rectifier)? Does it match the schematic?
 

PhoenixBill

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Absolutely replace all those old electrolytics. They will bring the power supply voltage way down and introduce ripple. The 0C2 should not be an issue. No problem with a slightly higher wall voltage either, it’s not a big deal despite some folks who seem obsessed with that notion IMHO.
 

Wally

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There is a tube compression circuit in that amp…ime, it is subtle but I never tried to overdrive the GA100 I had. Overdriven signal is not what that amp does well, imho….and I did not want to ruin the OEM speaker that was in it.
The GA100 yielded the best sonics from an electric-acoustic that I have ever heard. Clean amp….rich harmonic content….an electric-acoustic through that amp sounded like a huge electric-acoustic! Mine was in excellent condition when I bought it and was better when I sold it as it had been properly serviced.
fwiw, I did not think there were even 400 of those GA-100s built. The number in my mind was 139??? 189?….no matter. They are rare.
Congrats on nabbing that one. Good price, imho.
 

BoomTexan

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Absolutely replace all those old electrolytics. They will bring the power supply voltage way down and introduce ripple. The 0C2 should not be an issue. No problem with a slightly higher wall voltage either, it’s not a big deal despite some folks who seem obsessed with that notion IMHO.
I was planning on doing that immediately, but it is still definitely playable as-is, and none of them are actually leaking, weirdly enough. I'm still gonna change them out, but I've played amps with old electrolytic parts and none of them had this issue, other than a Stage UB252 which I suspect was on its last legs regardless of electrolytic condition. The main thing I'm trying to solve is the sound shutdown issue.

I'm not really ever concerned with wall voltage either, but I was just wondering because I didn't know if there was a sort of shutdown point where the 0C2 is having far too much voltage pass through it and just shuts down for a second. This is the ONLY amp I've ever seen with a voltage regulator tube, and I'm really still not entirely sure what it does and how it does it, even after looking at schematics and datasheets.
 

BoomTexan

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There is a tube compression circuit in that amp…ime, it is subtle but I never tried to overdrive the GA100 I had. Overdriven signal is not what that amp does well, imho….and I did not want to ruin the OEM speaker that was in it.
The GA100 yielded the best sonics from an electric-acoustic that I have ever heard. Clean amp….rich harmonic content….an electric-acoustic through that amp sounded like a huge electric-acoustic! Mine was in excellent condition when I bought it and was better when I sold it as it had been properly serviced.
fwiw, I did not think there were even 400 of those GA-100s built. The number in my mind was 139??? 189?….no matter. They are rare.
Congrats on nabbing that one. Good price, imho.
This were the specs that the guy I bought it from found somehow. I don't know how he got shipping numbers for this, but he apparently had them.

35W 1x12" Bass Combo Speaker: 1x12" Inputs: 2 Channels: 1 Volume Controls: 1 Tone Controls on Each Channel: Bass, treble Tremolo: No Tubes: 9 (6EU7, 2x6BD6, 2x6FM8, 2x6L6, GZ34, 0A2 (correspond to Epiphone EA-70 Bass-Amp)) Extension Speaker Jack: Yes Monitor Jack: No Watts Output: 35
Shipping Totals: 1960: 113, 1961: 246

That would make about 350-360 all together if his numbers are accurate.

Mine has the original speaker long gone, and there's some weird brand replacement speaker in there, so not too worried about getting the full 35-50 watts of power this thing can output. There's a football game tonight and most of the people in my dorm are gone so I've been cranking this amp sporadically for the past couple hours.

Definitely gonna try my acoustic electric through there now that you mention it, and I'll see what a bass sounds like through there as well.

Do you have any idea about the function of a 0C2 voltage regulator tube? I'm completely stumped as to what that thing actually does, how it does it, and what Gibson was thinking/their reasoning behind putting it in the amp.
 

Lowerleftcoast

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Do you have any idea about the function of a 0C2 voltage regulator tube? I'm completely stumped as to what that thing actually does, how it does it, and what Gibson was thinking/their reasoning behind putting it in the amp.
The OC2 lowers the voltage to the screens of the power tubes. Ideally this is a good idea.

(In practice it can be not so ideal.
The OC2 will reduce the voltage by a constant amount when the plate voltage may vary quite a bit.
The current through the screen may not always be enough to allow the OC2 to work properly.
The OC2 should have a warm up period of near 20 minutes.)
 

BoomTexan

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We are sorry to have to inform you that if you do not post detailed pics of the amp immediately you will be BANNED!
Fine fine fine. These are pics taken from the parking lot where I bought it. Still need to clean it a little, and no gutshots for now, but here y'all go.
IMG_20220910_152446_656.png
IMG_20220910_152446_776.png
IMG_20220910_152446_859.png
IMG_20220910_152446_701.png
 

andrewRneumann

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A little more info on that 0C2 tube. In this circuit (if the schematic I goggled is correct), the 0C2 is not regulating voltage. It is supplying a constant voltage drop for the power tube screens and the rest of the pre-amp/compressor. If you interpret the schematic, you will find B+ in the range of 425V and the 6L6 screens at 345V. That's an 80V drop--but there's only a 470R resistor there which is not dropping 80V. The majority of this 80V drop is caused by the 0C2, which is designed to drop 75V (+/- 10%) even if the current varies. This is exactly the same function as a Zener diode. If I wanted to make this amp more reliable and I didn't care about the "light show", I would consider replacing that 0C2 with a 75V power Zener or a string of Zeners that add up to 75V. (You could pull the tube and solder the Zener directly across the socket pins... easy to undo.) But that would be extremely uncharming... I suspect you probably want to leave the 0C2.

The 0C2 has some limitations that you need to observe, the main being the minimum and maximum current. The minimum is 5mA and the maximum is 30mA. I don't know anything about the current draw from the preamp and screens, but if you were to overdrive the power tubes hard, I'm guessing the maximum current could be exceeded.

The other issue is that gas-discharge tubes do not handle ripple or AC very well, so it is critical that you replace those power supply capacitors. You can use a true RMS DMM to measure AC at various points in the power supply to get an idea of how much ripple is there. The 2x20uF on either side of the choke and 40uF after the 0C2 would be critical to its function.

Your symptom of the 0C2 losing conduction every 20 seconds or so randomly is definitely a sign that something isn't right. As @Lowerleftcoast mentioned--the tube data sheet does recommend a 20 minute warm up time. I'm skeptical that is super-important for a crude guitar amp, but you should probably leave it on for 20 minutes and observe the performance to see if there is any change between minute 1 and minute 20. It is possible that the 0C2 has been damaged by abuse (over-current), so can't rule that out, but do the cap job first.
 

Wally

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I need to look into my stash of oddball tubes just to see if I have these. I know I have a slew of OC2s.
 

BoomTexan

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I need to look into my stash of oddball tubes just to see if I have these. I know I have a slew of OC2s.
If you have any, I'd love to buy one from you just to see if there is something wrong with mine/to have a spare.
 

BoomTexan

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A little more info on that 0C2 tube. In this circuit (if the schematic I goggled is correct), the 0C2 is not regulating voltage. It is supplying a constant voltage drop for the power tube screens and the rest of the pre-amp/compressor. If you interpret the schematic, you will find B+ in the range of 425V and the 6L6 screens at 345V. That's an 80V drop--but there's only a 470R resistor there which is not dropping 80V. The majority of this 80V drop is caused by the 0C2, which is designed to drop 75V (+/- 10%) even if the current varies. This is exactly the same function as a Zener diode. If I wanted to make this amp more reliable and I didn't care about the "light show", I would consider replacing that 0C2 with a 75V power Zener or a string of Zeners that add up to 75V. (You could pull the tube and solder the Zener directly across the socket pins... easy to undo.) But that would be extremely uncharming... I suspect you probably want to leave the 0C2.

The 0C2 has some limitations that you need to observe, the main being the minimum and maximum current. The minimum is 5mA and the maximum is 30mA. I don't know anything about the current draw from the preamp and screens, but if you were to overdrive the power tubes hard, I'm guessing the maximum current could be exceeded.

The other issue is that gas-discharge tubes do not handle ripple or AC very well, so it is critical that you replace those power supply capacitors. You can use a true RMS DMM to measure AC at various points in the power supply to get an idea of how much ripple is there. The 2x20uF on either side of the choke and 40uF after the 0C2 would be critical to its function.

Your symptom of the 0C2 losing conduction every 20 seconds or so randomly is definitely a sign that something isn't right. As @Lowerleftcoast mentioned--the tube data sheet does recommend a 20 minute warm up time. I'm skeptical that is super-important for a crude guitar amp, but you should probably leave it on for 20 minutes and observe the performance to see if there is any change between minute 1 and minute 20. It is possible that the 0C2 has been damaged by abuse (over-current), so can't rule that out, but do the cap job first.
Wow, thanks for the depth of info, I really appreciate it. I woke up this morning and the 0C2 made sense (probably sleep deprivation was getting to me last night), but thanks for the info on gas tubes, didn't know that and that'll definitely come in handy. Not gonna take out the 0C2, because it's so so cool and the light show is fun to watch.

I'll try the 20 min warm up time and see if the loss of conduction is caused by that, but I'm gonna take it apart and check the caps as well.
 

andrewRneumann

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Wow, thanks for the depth of info, I really appreciate it. I woke up this morning and the 0C2 made sense (probably sleep deprivation was getting to me last night), but thanks for the info on gas tubes, didn't know that and that'll definitely come in handy. Not gonna take out the 0C2, because it's so so cool and the light show is fun to watch.

I'll try the 20 min warm up time and see if the loss of conduction is caused by that, but I'm gonna take it apart and check the caps as well.

I read through this thread here https://www.tdpri.com/threads/gibson-ga-100.926507/ .
OP built one of these from scratch and sounds like he had similar problems with the 0C2. Based on what he wrote, it sounds like the B+ voltage sags too much when the power tubes are working hard. If the B+ voltage sags, but the pre-amp/compressor voltage doesn't sag very much (it has a 40uF capacitor), then the voltage differential across the 0C2 will be less than 75V and the gas light will go out. Then the preamp has to sag to about 115V less than B+ to cause the gas to strike again and resume operation.

What could be done about this? I would start with that reservoir cap + choke filter combination. How big can you make those capacitors without risking damage to GZ34 or PT?

Could you switch to a solid state rectifier? Your B+ will be higher--could you live with that or would you need to install a Zener(s) in the HT C.T. to tame it?

Screen stoppers on the power tubes would limit the maximum plate current and therefore limit sag.

Possibly reducing the 40uF preamp power supply cap to 20uF would allow the preamp to sag more in unison with the B+ and reduce the chances of losing gas light.

Just some ideas to explore. You didn't say you were interested in modifying the amp, so feel free to ignore. :)
 

PhoenixBill

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On several professional amp repair forums, there’s a guy named Enzo. Experienced, knowledgeable and very helpful, he has repaired amps for a living for many years. Not just some Internet hack! Anyways, he has a favorite saying: If you see hoof prints, think horses, not zebras! In other words, go with the simplest, most obvious stuff as someone troubleshoots. I suspect he would be pretty blunt here: why immediately focus on the OC2 when there’s a very real bunch of tired old filter caps that are half a century past their “best by…” date. Get all of those old caps changed which will ensure the backbone of the amp—the power supply—is strong, NOT jump into thinking about redesigning the voltage regulator circuit. Once we know the caps are good, then we see if there’s a tired OC2 or perhaps think about replacing it with zeners. (But on a screen supply, won’t zeners be noisy? Isn’t one advantage of a gas regulator it’s quiet operation?)
 

muscmp

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cool amp.
there were two versions of that amp. the first, like yours has the tweed cab and 409 were made in 60-61. this book is by wallace marx about gibson amps. note that there is also a gibson tube amp group on FB.
play music!

ga100.jpeg
 

GlideOn-Designs

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I'll upload some pics later, but I have a GA-200 amp and matching GA100 cab converted to 2x12 Kendrick Brownframes and matching GA200 cab with Kendrick Blackframes. The amp is weird flimsy AC panel material but the cabs alone are worth the price of entry. They are all pine but that old yellow pine and absolutely built like a **** brickhouse. Kind of a convertible open back due to the "piggyback" amp compartment.

I had it converted to a sort of Fender Bandmaster circuit 14 years ago, the tech who did recommended gutting it instead of bringing up to quirky original specs which wasn't terribly well suited for bass to begin with. Two channels; Volume, Treble and Bass controls. If I remember, the transformers are rather wimpy but that contributes to it's saggy, somewhat rich midrange sound. Cathode biased too, which sounds like it has a tube rectifier even though it's solid state rectified.

A very quirky era for Gibson but IMHO cooler and better than anything Silverstone was making.
 




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