1959 GA-20T

[52 Bill]

I'm pulling what is left of my scalp hair out trying to make heads or tails of this schematics table the heading is all blurry and faded. Is there a better table of proper voltages out there?

I don't know which schematic you are using, but the one I found online is okay. The top header read from left to right: Tube Number - Tube Type - Ep (Plate Voltage) - Esg (Screen Grid Voltage) - Eg (Grid Voltage) - Ek (Cathode Voltage).

Try cleaning the pots with Deoxit they may not need to be replaced. The 10K resistors should be 1 watt, so they will get warm. Is the trem working?

[soundmasterg]

I don't know which schematic you are using, but the one I found online is okay. The top header read from left to right: Tube Number - Tube Type - Ep (Plate Voltage) - Esg (Screen Grid Voltage) - Eg (Grid Voltage) - Ek (Cathode Voltage).

Try cleaning the pots with Deoxit they may not need to be replaced. The 10K resistors should be 1 watt, so they will get warm. Is the trem working?

I second what 52 Bill said. The old pots were better quality than what is generally found now. All the pots were frozen on my uncle's amp but DeOxit made them all work again just fine. Keep in mind that when going through the current limiter, since it limits current the voltages will be lower than normal operation. Use the current limiter for when you are troubleshooting failure problems like you were, then once you have fixed those, run it normally without going through the limiter and see how it works. If it doesn't blow fuses or otherwise get hot or malfunction, then there is no need for the current limiter anymore.

Sounds like you are making progress! You may have it all fixed by the time I can locate the layout and send it your way. I hope to have a chance next week...but I will let you know.

Greg

[Thndr]

You just cleared that up I'll take the readings tonight. What tolerance should I be looking for say +/- 10%? Also I have what looks like "EB" as one of the table entries.. would that be B+ voltage or??

I'll get some Deoxit and clean the pots.

The foot switch was not with the amp, I attached wires at the proper locations (schematic is in the othe room) and ran them through a gromet in the chassis. A temporary splice engaged it, it's real weak, but might be due to the pots.

What a journey

T

[52 Bill]

You just cleared that up I'll take the readings tonight. What tolerance should I be looking for say +/- 10%? Also I have what looks like "EB" as one of the table entries.. would that be B+ voltage or??

The chart on my schematic only has the 6 columns that I described. I don't know what the EB would be.

The foot switch was not with the amp, I attached wires at the proper locations (schematic is in the othe room) and ran them through a gromet in the chassis. A temporary splice engaged it, it's real weak, but might be due to the pots.

Yes, you must have the footswitch for that to work, so jumpering the wires will do for now. The usual causes for weak trem are the phase shift caps and the cathode cap. The caps to check are the 3-0.047 uF caps that connect between the grid and the plate of the 6SQ7 and the 20uF cathode bypass cap. Well a weak tube can also cause this.

What a journey

Welcome to my world.

[Thndr]

Im having a blast.

I'll get the pots functioning properly then dive in to the caps u mentioned and switch out the 6SQ7 with the older one to see if that helps.

back to work...
T

[Thndr]

Didn't have a lot of time but here it goes:
Tremolo off, Depth at a minimum
5y5
Plate 338-340
heated cathode 363-363

6v6
Plate 355
sgv 268
cathode 15.3

12ax7
Plate 39.6
0 on everything else ??

6sq7
Triode Plate 86
cathode 1.22
0 on everything else ??

5879
Plate 110
Grid 2 = 24
cathode 1.2

12ay7
plate 48.5

oh I had a cable plugged into channel 1 during measurements

I havent fixed the pots yet maybe tomorrow

I played on it for 10 or so minutes, distorts pretty quickly, clean is real tinty. I suspect the pots are partly responsible for this.

The controls are not labeled anymore, printed out a top shot I found, will sharpy label them tomorrow.

There is nothing like the smell of tubes

Thanks again!

T

[soundmasterg]

The 6V6 plate voltage sounds closer to what it should be than before. I'll see if I can look for the schematic tonight when I get home from work to compare.
The 12AX7 voltage seems off as does the 6SQ7...you shouldn't have 0 v on cathodes. Unsolder the cathode resistors for those stages and make sure they don't read open. Also unsolder one end and check the plate resistors for value. They should be within whatever their tolerance is of their value. Measurements should be taken with no input plugged in and all controls at zero btw.

I'll post correct voltages if I find my schematic later.

Greg

[52 Bill]

Agree about the 12AX7 voltages being off. The pots probably have little to do with the tinny sound and early distortion, I'd guess coupling caps.

[Thndr]

Thanks you two,

Before I start pulling legs, i'll take measurements with no input, to insure a good baseline of data.

I'll check the resistors and caps hopefully tonight. the 680 ohm is good, pulled a leg off and checked it this past weekend, in fact there was not a wire from the cathode to the 680 before this weekend. I'll double check it as well as my earlier workmanship.

Thanks again,
Bill

EB voltage in the schematic table from Gibson.com site is apparently Eb = Plate to cathode potential (P to K), Volts I found this (if someone else was wondering too..) http://www.daheiser.info/VTT/section1.pdf

[Thndr]

New Numbers!

New numbers:

5y3: EP 340 | EK 363

6v6: EP 355 | ESCP | 268 | EK 15.3

12AX7 EP 101 | EK .8 <--Big improvement ... Wire to 680 ohm was on the wrong side of resistor..

6SQ7 EP 90 | EK 1.2 <-- Not sure why this improved yet..

12AY7 EP 48.5

5879 EP 110 | ESCP 24 | EK 1.2

Termolo greater than 20% from specification, I'll work on that one.

5879 is slightly greater than 10% from specification.

The rest of the readings fall within +/- 10% of specification! I hope that's good

I used the schematic from Gibson.com GA20T.

I'm darn close. OH the distortion/Tinny from yesterday is gone, it plays very clean in both channels! At least so far, I'll break it in a bit better this weekend.

Termolo is still an issue, in the coming days I'll work on that.

I sooo appreciate all the help, all I had going into this was a big desire, some old relay injection molding electrical experience and I reached out to this forum needing direction and it was given.

Thank you Thank you.
Bill

[52 Bill]

Congrats! Feels great to acheive a successful repair, doesn't it? No matter how badly screwed up something is, if you spend enough time and energy you can undo the stupid and bring things back to life. It just depends on how much time and energy you want to spend on something.

Congrats again.

[soundmasterg]

Don't worry too much about the 5879 voltages being off from spec....in fact most of the tubes can be off a bit from the schematics and still be just fine. Part of that is due to the AC line voltage difference these days, but part is just due to the variance and tolerance in parts and tubes. I built an amp several years back that used a 5879 and was all worried about the voltages being different than what I thought they should be from looking at other schematics etc., but in the end I just started playing around with cathode resistor, screen resistor, plate resistor, and dropping resistor values and it didn't take long to get it sounding good, and once it was sounding good, all the voltages were different than any schematic I could find.

It sounds like you aren't going to need a copy of my layout/schematic after all....you're almost there! Which is good because I haven't had a chance to look for it....

Greg

[Thndr]

wow, I buttoned it last night, put new chicken heads on and drilled/installed an input for the tremolo pedal. Cleaned the pots, they are nice now. Set the amp on the floor for the first time in months, played through it clean, nice tone in both channels, I'll test out the tremolo this weekend. Borrowed/installed a reverb pedal from my buddy Wade, OH MY.... Nice does not do it justice. I am so impressed.... People have told me, blues tones are best from a tube amp.. I now believe them.

I am very pleased.

I may get the camera out and post some pictures this weekend.

This is the two tone cab, the tolex is pretty beat up what are your thoughts on replacement? Not much is left of the control printing. Like the insides this amp's been beat to heck. Any thoughts on replacement of tolex or the control printing? I could just leave it a sleeper.

I plan to do some more testing this weekend for science purposes... ( yeah right, Im going the play the heck out of it...)

It may be a bit early, but I'm planning on building a modified Bassman this summer... Ya'll going to be around?

Again thanks so much for your help, for without it, it would not have happened.

T

[52 Bill]

I'd love to see photos. I'm not a big fan of recovering unless it's so bad that it can't survive without it. It's kinda like people, the outer covering shows its' history, scars and all.

[soundmasterg]

Glad you've got it all together Thndr! The Gibsons have a small OT than the equvilant Fenders, and are harder to work on, but they still sound great!

Greg