Gibson GA20 w/9 pin preamp tubes

[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

[g1]

Here is a GA-20 with 9 pin preamp tubes, and a GA-30 with same:

[davohilts]

Hey g-one!
Thanks for the schematics! These are great.
Dave

[g1]

Dave: The schematic you had drawn up and posted is no longer here, could you repost or alter one of the above so it matches the circuit you had?
I'm looking at a GA-20 (9 pin pre's) right now that doesn't quite match any of the schematics I have seen.

[davohilts]

Sorry, I've been traveling.
Here is the hand drawn preamp circuit that I made from the amp, It isn't drawn properly gut you will get the idea.
Dave

[Audiotexan]

And just for good measure since the original pdfs from 2012 were lost..

This is the one with the 3 inst. level, and one mic level.
Also, shows the tube complement/layout.