Gibson GA20 w/9 pin preamp tubes

[davohilts]

Hello,
I'm taking a break from working on crestline ga19rvt and thought I'd work on a friends GA20. Is is a 2tone tweed with a 12" speaker and 2 vol. and 1 tone. It was found in the back room of a tv repair shop and is in good condition except that the chord was rotted and couldn't be tested. He gave it to me to re-cap and see if I could get it going. It was original inside except that the multi cap tube had been cut and another larger cap can installed. I replaced the caps with 3-10mfd x 450 volt caps and replaced a handful of out of tolerance resistors and have it working. It sounds great on the three instrument inputs but the mic input is weak. The tone knob also doesn't seem to do anything.
I don't have a schematic that matches this amp very much except for a redrawn one I found online but it is missing a few things.
To resolve the volume issue on the mic channel and the lack of tone control what would be a reasonable method to trouble shoot?
As always I'm using known good tubes.
Thanks,
Dave

[52 Bill]

I don't have a schematic that matches this amp very much except for a redrawn one I found online but it is missing a few things.
To resolve the volume issue on the mic channel and the lack of tone control what would be a reasonable method to trouble shoot?
As always I'm using known good tubes.
Thanks,
Dave

Without a schematic, means that you will need to look at the circuit parts and determine what needs to be checked. Look at the tone control, is it similar to a Fender type where it works as a high pass in one direction and a high cut in the other or is it just like a guitar's tone control, high cut only? In either case there is only the pot and one or two caps that need to be checked as well as the ground connection. Generally speaking there really aren't that many tone control circuits that are used in guitar amps, and as far as tone controls go they all are pretty simple designs.

For the mic circuit does it have an extra gain stage? If it does, then test the circuit voltages around that stage to see that the tube is working. Check the mic volume control as well as the coupling caps for that stage.

[davohilts]

Without a schematic, means that you will need to look at the circuit parts and determine what needs to be checked. Look at the tone control, is it similar to a Fender type where it works as a high pass in one direction and a high cut in the other or is it just like a guitar's tone control, high cut only? In either case there is only the pot and one or two caps that need to be checked as well as the ground connection. Generally speaking there really aren't that many tone control circuits that are used in guitar amps, and as far as tone controls go they all are pretty simple designs.

Thanks for the advice.
For the mic circuit does it have an extra gain stage? If it does, then test the circuit voltages around that stage to see that the tube is working. Check the mic volume control as well as the coupling caps for that stage.

Here is how the mic input is wired:
from the input there is a .05 cap which connects the tip to pin 2 of the 1st 12AX7, pin 2 is connected to pin 3 with a 1M resistor and 3 shorted to 8 and 8 is connected to 7 with a 470K resistor. Pin 1 is connected to the tag board and that point is grounded through a 10M resistor. the feed from pin 1 continues with a .05 cap to the middle connection of the mic vol control which has its right hand tab connected to pin 2 of V3 (12AX7). The Mic Vol right hand tab in connected to the right hand tab of the instrument Vol. control and then connects to the Tone middle tab using a .01 cap.
Voltages are as follows:
12AX7 Preamp tubes:
V1P1 204v, p2 (-.5v) p3&8 go to ground p6 66v p7 (-.5v)
V2P1 66v p2 (-.4v) p3&8 go to ground p6 66v p7 (-.35v)
V3P1 72.4 p2 .001v p3&8 1.2v tied together and grounded thru a 1K resistor p6 101v p7 .01v
6V6 Power tubes:
Pin 1: ground
Pin 2: 14.2V
Pin 3: 340V
Pin 4: 245V
Pin 5: .5V
Pin 6: n/c
Pin 7 & 8: 13.5V

Here are some pics.

[davohilts]

Here is a link to a schematic I found that is sort of close it think.
GA20 (9pin preamp).pdf

[52 Bill]

Here is a link to a schematic I found that is sort of close it think.
GA20 (9pin preamp).pdf

The values of the two caps on the tone control in this schematic have been reversed. The 500pf cap should be across the volume control and the .005 should go to ground.

The voltages on the first tube seem wrong as the two plates I think are connected together, so should have the same voltage on both pin 1 and pin 6. Check this to see if they are in fact both connected together.

Have you tried a different tube in the mic preamp circuit?

[davohilts]

The values of the two caps on the tone control in this schematic have been reversed. The 500pf cap should be across the volume control and the .005 should go to ground.

The voltages on the first tube seem wrong as the two plates I think are connected together, so should have the same voltage on both pin 1 and pin 6. Check this to see if they are in fact both connected together.

Have you tried a different tube in the mic preamp circuit?

The plates are not connected together on the first tube, I know the schematic shows them so but the pics that I posted above show the plates are not connected. I don't trust the schematic because of the tone circuit issue and the 10M resistors shown at the inputs seem strange.
I've rolled several known good tubes thru the first 12AX7 without any change.
this is such a simple circuit but just will not settle down.
Do you think its a good idea to try wiring the plates together as shown on the schematic?
I'm willing to try anything. I just wish I could find a good schematic.
On another note, in the fist pic above the 3rd 12AX7 (far right on the pic) has some kink of resistor or something tying the plates together, do you have any idea what this is, it says AP, 100, 20% on it. I've never seen anything like it, is it a diode?? There is another one that says 10, 10%, NPO. Any ideas?? When I measure them I get infinite resistance and no capacitance measurement so I'm stumped.
Dave

[Diablo]

The 10M resistors at the inputs are grid leak bias, and those values aren't unusual. Notice there aren't any cathode bias resistors.

[davohilts]

The 10M resistors at the inputs are grid leak bias, and those values aren't unusual. Notice there aren't any cathode bias resistors.

The frustrating issue I have is that the amp seemed to be original and doesn't match the schematic in many ways. The preamp tubes have cathode resistors which connect to the grids and the cathodes are directly grounded. V1 has a 1M on pin 2&3 and a 470K on pin 7&8 and pins 3 & 8 are tied together, V2 has same except both cathode resistors are 470K, V3 has the cathodes tied together and being grounded using a 1K resistor. Nowhere is the amp is there a 10M resistor. The plate of V1 (pin 3) runs to the circuit board where is was connected to a ground using a small white component. It is labeled 10, 10%, NPO. I removed this and installed a 10M resistor but had no effect on my issue. The mystery component has an infinite resistance and no capacitance.
thanks for any help in this matter. I just don't have enough know how to really proceed without some advice.
Dave

[Jazz P Bass]

10,10%, NPO seems to indicate that it is a capacitor.
10pf. 10% tolerance.
My understanding is that NPO (Negative-Positive-Zero) refers to the capacitor's ability to avoid a change in value with regard to temperature.

[davohilts]

As they say a picture is worth a thousand words, here is a pic of the mystery component in the amp, there are two of them. The first ties the plates of V3 and is labeled AP, 100, 20%, the other was used to tie the plate of V1P3 to ground and is labeled 10, 10%, NPO. Does anyone have any idea what these are??

[davohilts]

10,10%, NPO seems to indicate that it is a capacitor.
10pf. 10% tolerance.
My understanding is that NPO (Negative-Positive-Zero) refers to the capacitor's ability to avoid a change in value with regard to temperature.

Thanks for the help. The component doesn't measure on my meter so it must be bad. So I need to use a new one of some sort. Silver mica? The other one is labeled 100, 20% AP so this must be 100 pf?, 20% tolerance?, ??

[52 Bill]

The frustrating issue I have is that the amp seemed to be original and doesn't match the schematic in many ways. The preamp tubes have cathode resistors which connect to the grids and the cathodes are directly grounded.

OK, the schematic is not correct for your amp. Maybe there is one out there, maybe not. Because the cathodes are grounded there are no cathode resistors, those are input grid resistors.

The plate of V1 (pin 3) runs to the circuit board where is was connected to a ground using a small white component. It is labeled 10, 10%, NPO. I removed this and installed a 10M resistor but had no effect on my issue. The mystery component has an infinite resistance and no capacitance.
thanks for any help in this matter. I just don't have enough know how to really proceed without some advice.
Dave

V1 is a 12AX7 correct? The plates are located at pins 1 and 6. Please refrain from removing and replacing parts until you know that they may be involved in your problems.

If the original problem was that the mic input didn't work, then there are only a few components that can cause this. Follow the signal from the mic input jack to the mic volume control. Check each component as you go. When you turn up the mic volume control can you hear an increase in hiss or noise?

[davohilts]

The story with the amp is that it was given to me for repair (for some reason I do this for fun) and it had not been tested because its power chord was rotted. I installed a 3 wire chord and removed the death cap. The amp had the original filter caps clipped off but not removed and some way off caps put in their place. I installed new proper filter caps and fired the amp up and the mic channel had very low gain and the tone control didn't work. The original tone control was a .01 mfd cap which connected from grid of V3P2 thru the volume pots to the middle connection on tone pot. That was it for the tone circuit and I don't know how it worked or IF it ever worked but it sure looked original. Besides the misguided cap job the whole amp looks original.
I have checked all the .05 caps in the amp and they all have drifted from the .05 level (some more than others) and the ones that were off quite a bit I tried substituting new mallory 150 caps. Still nothing doing on the mic channel. This circuit had so few parts it is a mystery why I can't get it to work. The second preamp tube (all preamp tubes are 12AX7) P6 is connected to the V1P6. The 4 grids of the first 2 preamp tubes are connected to the tips on the 4 inputs (one mic and 3 instrument). The third preamp tube's two plates are routed thru .05 caps and connect to each 6V6 pin 5.
Seems like everything should work but the mic channel has very low gain. When I put in the caps on the tone circuit like a fender deluxe then the tone circuit works on the instrument channels but not on the mic channel.
I'm going to go back online and see if I can find another schematic that is a closer match to the amp. This is frustrating but I am having fun after a fashion.
Thanks for the help and advice, I will stop myself from taking the amp apart without having a plan. I have replaced the two mystery components which I have found out were 10pf and 100pf caps with new silver mica caps.
I'm hopeful that I will finally see the light!

[davohilts]

I have sat down and drawn up a schematic for this amp's preamp and will include it here.
If anyone can help out that would be fantastic!
I haven't drawn a schematic before so please excuse the quality.
Dave

[52 Bill]

I have sat down and drawn up a schematic for this amp's preamp and will include it here.
If anyone can help out that would be fantastic!
I haven't drawn a schematic before so please excuse the quality.
Dave

There are a few errors in your drawing, but it looks good for a first try.

There are 8 components in the mic circuit, if you haven't already tested them, test them. If they all check out, then you need to check for problems beyond the norm. When you insert a plug into the jack, does the shorting contact open fully up? Check the tube socket to be sure that there is continuity from the component side to the tube side. Are the tube socket contacts clean and tight? Are any of the component leads or any of the related wiring frayed or shorting to ground?

[davohilts]

There are a few errors in your drawing, but it looks good for a first try.

There are 8 components in the mic circuit, if you haven't already tested them, test them. If they all check out, then you need to check for problems beyond the norm. When you insert a plug into the jack, does the shorting contact open fully up? Check the tube socket to be sure that there is continuity from the component side to the tube side. Are the tube socket contacts clean and tight? Are any of the component leads or any of the related wiring frayed or shorting to ground?

will do, and thanks!

[Bruce / Mission Amps]

Yes there are a number of goofs there.
Also, I don't think I ever seen grid leak bias preamp tubes with 470K grid leak resistors... maybe those are actually 4m7... we used to call this contact bias. Is there a difference?

[davohilts]

Yes there are a number of goofs there.
Also, I don't think I ever seen grid leak bias preamp tubes with 470K grid leak resistors... maybe those are actually 4m7... we used to call this contact bias. Is there a difference?

Hey Bruce,
Yes I'm sure the goof level is off the charts but I will test the grid resistors and report back. The amp looked original except for a hack cap job but who knows. I've got access to another ga20 that is similar, i'm travelling this week so stay tuned

[J M Fahey]

The component doesn't measure on my meter so it must be bad.

Probably your meter is unsuitable for the kind of measurement you are trying to make

[J M Fahey]

The component doesn't measure on my meter so it must be bad.

Probably your meter is unsuitable for the kind of measurement you are trying to make

[davohilts]

Well, I've been trying to post but for some reason the site has been down.
I have good news to report. The amp now works perfectly. The schematic with the goofs (it would be nice to know what they are so I can make the changes) is correct. This is an undocumented gibson ga20 schematic which works and sounds great. My mistake was when I installed the 100K resistors at the voltage divider to the first 12AX7 I grabbed some carbon comp resistors out of the drawer and I must have measured them because I always do but somehow I installed a 100K and a 10K as the brown stripe looked yellow but was light brownish and even looking at them installed they matched visually. As I was doggedly going thru measuring the resistance of the grid leak resistors (470k) to see if they were possibly 4.7M I decided to check all of my resistors since there weren't that many. Well the first pair I tested were the 100K pair and there was the mistake, my mistake, which was the root of the whole problem. the minute I put in a matching 100K resistor and fired up the amp the tone control and the volume on both channels was fully operational. I had a chance to look inside another amp like this one and the tone control has a .01 and a .1 mfd capacitor going from the common lugs on the volume controls to the middle lug on the tone control, I added a .1 mfd cap and bingo it sounds really good. So there you go, I've learned a bunch working on this simple amp, things like don't start taking apart things you don't understand, you can't measure very low value caps, stop working when you can't solve a problem and come back later with fresh eyes, and If you can find an old 50's GA20 with 12AX7 preamp tubes wire it up using grid leak bias, you won't be disappointed.
I would also like to thank everyone who helped out on this project, you guys are the best.
Dave

[J M Fahey]

Glad you could solve it.
Must be my tired eyes but under room light I often can't distinguish between 4K7 and 47K.
One seems to end in orangish red, the other one in reddish orange. Oh well.
Easy either with a jeweler's loupe (where I see perfect colors) and/or under a powerful CFL lamp which lives by my left ear.

[Jack Hester]

The amp now works perfectly. The schematic with the goofs (it would be nice to know what they are so I can make the changes) is correct. This is an undocumented gibson ga20 schematic which works and sounds great.

Dave -

Can you post your corrected schematic? As time permits, I can CAD it and save as a PDF file. I'm having to do this with some old PA amps that I have. I did the same for a Les Paul GA-40, because the hand drawn schematic was a very poor copy. I now have a corrected (from hand-tracing) schematic for anyone who needs such ('52-'54 LP GA-40).

Jack

[davohilts]

Dave -

Can you post your corrected schematic? As time permits, I can CAD it and save as a PDF file. I'm having to do this with some old PA amps that I have. I did the same for a Les Paul GA-40, because the hand drawn schematic was a very poor copy. I now have a corrected (from hand-tracing) schematic for anyone who needs such ('52-'54 LP GA-40).

Jack

Hi Jack, Thanks for the offer. The schematic that I drew up is shown above and is correct as far a values for the resistors and capacitors. My mistake was reading the wrong value on the resistor so the schematic is correct but I have made some mistakes drawing it up because I don't really know what I'm doing in that regard. A few posters above mentioned some 'goofs' that I made but the information is correct, I guess I just didn't draw in up in the correct fashion. Does this make sense?
Thanks,
Dave