1959 GA-20T

[Thndr]

First off I did not wire this monster, I am picking up the pieces from the person that gave me the amp. With that said, came in this evening put the ohm meter on the second side of 10k to gnd now it's readings 1.16 Mohms... something changed..

The 220k resistors measured ok,

While started my trace...at the 20uf side of the 2nd 10k there is a wire that goes to the 12AX7, pin 8, cathode, that's not on my drawing.. (refer to first sentence) I snipped it at the resistor, did not have an effect on the 1.16 Mohms.. Kinda difficult to visually trace this 2 sided board. I removed it and all the pots and inputs. when the board released from the stand off the meter went to 000. There was a splat of loose solder on the chassis, and was the only visual inconsistency found.

That all the time I have this evening, I'll continue tracing tomorrow night.

does that cathode wire make sense?

Thndr

[soundmasterg]

If I have time Thndr, I will take a look for my layout and schematic that I drew up for my uncle's and see if I can get you some help. Where are you located? I may be able to send you a copy...or maybe I can scan it in. I drew it by hand on a large piece of paper so it is awkward to scan...but I think it could help you out quite a bit. Those older Gibsons with the caps on the bottom of the board are very annoying to work on!

Greg

[52 Bill]

First off I did not wire this monster, I am picking up the pieces from the person that gave me the amp. With that said, came in this evening put the ohm meter on the second side of 10k to gnd now it's readings 1.16 Mohms... something changed..

Is the 10uF cap connected or not? The fact that it is no longer reading as a short either means that there is something loose and intermitently connecting to ground, or you have been reading a stored voltage from one of the caps.

While started my trace...at the 20uf side of the 2nd 10k there is a wire that goes to the 12AX7, pin 8, cathode, that's not on my drawing.. (refer to first sentence) I snipped it at the resistor, did not have an effect on the 1.16 Mohms..

The 12AX7 is the driver phase inverter. Both cathodes pin 3 and pin 8 should be connected together and grounded through a 680 ohm resistor. If the power supply was connected to that point, that could be the cause of the overheating first 10K resistor. Check the value of the 680 ohm resistor.

While started my trace...at the 20uf side of the 2nd 10k there is a wire that goes to the 12AX7, pin 8, cathode, that's not on my drawing.. (refer to first sentence) I snipped it at the resistor, did not have an effect on the 1.16 Mohms.. Kinda difficult to visually trace this 2 sided board. I removed it and all the pots and inputs. when the board released from the stand off the meter went to 000. There was a splat of loose solder on the chassis, and was the only visual inconsistency found.

I don't understand the 000 reading. Please explain. Do you have the schematic for this amp?

[Thndr]

If I have time Thndr, I will take a look for my layout and schematic that I drew up for my uncle's and see if I can get you some help. Where are you located? I may be able to send you a copy...or maybe I can scan it in. I drew it by hand on a large piece of paper so it is awkward to scan...but I think it could help you out quite a bit. Those older Gibsons with the caps on the bottom of the board are very annoying to work on!

Greg

Greg,

That would be great, I'm fighting 3 things here, the inexperienced person before me, my own inexperiece and attempting to trace components one can't see. So a layout and schematic would be a great help. I live in East Texas near Athens, I'll message you my address if you want.

Thanks
Thndr

[Thndr]

Remember I had already disconnected the 10uf the night before.

I was correct on the 12ax7 wire being wrong.. good

000 = open from 10k 6v6 side to chassis, so there is something loose, exposed causing intermediate shorts.

Schematic -- In the beginning I found 3 or 4 different versions on the net... I choose the one that matched the 12ay7 1meg input resistors.

I spent too much time on the 12ax7 wire last night, attempting to make sense of it, i'll correct this and move on.

The 680ohm is fine, there was not a gnd wire to it until a week or so ago.

This amp is a mess, but I will not give up.

I appreciate the help

T

[52 Bill]

000 = open from 10k 6v6 side to chassis, so there is something loose, exposed causing intermediate shorts.

Well, you haven't proven that yet but it is possible. For clarity sake, when there is no resistance reading on your meter please call it open or infinity, every meter has a different way of showing open circuits and "000" can easily be mistaken for zero ohms.

This amp is a mess, but I will not give up.

And you shouldn't. You should be able to revive it and get it working 100%.

I'd suggest that you start comparing the amp with the schematic. I understand that it is not that easy with the way that the amp is constructed, but you really need to try and figure out exactly what the previous person did in there. Start by looking at all of the connections in the amp. It usually is fairly easy to see what was done originally when the amp was manufactured and what has been changed or altered.

Don't trust anything that looks like it has been changed. Double check that if something was replaced, it was replaced with the correct part and was connected correctly. So far you have found reverse wired filter caps and a wire added shorting out the power supply. There may be other errors that you have yet to find. Check for bad soldering and bridged connections.

Get the power supply working and then you can start to work on the signal circuits.

[Thndr]

Ok... Ive gone through this Amp, followed everything to the schematic. The Wife is a bit PO'd from my extended time consumption

Powered it up gently (i.e. variac, light bulb current limiter) with only the 5Y3 installed, the light is dim at 115vac, the following are my readings:

Pin 2 filament = -450 dc
Pin 4 Plate = 328 ac
Pin 6 Plate = 329 ac
Pin 8 filament = -450 dc

The first 10K resistor appears to be holding up, I touched it barely seems warm.

From what I am reading from the schematic -450 dc is way high, should I proceed with 6v6's? Also I was getting some fluttering on plate voltage Ive never seen before.. a variance of 2 volts down then up to the peak above.

Also I am receiving -440 dc volts a the ground side of the second 10K resistor.

Oh all the pots were turned all the way down.

A cord was plugged into channel 1 during these readings.. apparently if a plug is not in it's in standby from what I have been told.

There were at least 4 mis-connections found.. what a mess.

Your help as always is appreciated
T

[52 Bill]

The Wife is a bit PO'd from my extended time consumption

First rule of married life "Happy wife=Happy life"

From what I am reading from the schematic -450 dc is way high, should I proceed with 6v6's?

Without the tubes installed all of the voltages will be higher than expected.

Also I am receiving -440 dc volts a the ground side of the second 10K resistor.

I don't understand the 10K is not connected to ground is it?

A cord was plugged into channel 1 during these readings.. apparently if a plug is not in it's in standby from what I have been told.

I don't see that anywhere in the schematic.

I assume that when you write -450 dc that you are really measuring a positive voltage. Your black meter lead connected to the chassis ground and the red lead connected to the circuit.

If you feel that the wiring is correct to the schematic, then add the tubes and see what the voltages are in the circuit. Remember that once you add the output tubes you need to have a load or a speaker connected to the output transformer.

[Thndr]

Thanks for the quick replay!

ugh your right I have the red to ground..
putting the 6v6's in I'll report back soon.
T

[Thndr]

here we go...
Channel 1 I am able to get noise when I touch the connector. plugged in my strat it played but not very loud at full volume.
Light is brighter but not like it was a week ago. I can see the light bulb filament but like in a cloud.. if that makes sense...

Voltages:

5y3:

Pin 2 filament = 194 dc
pin 4 plate = 203 ac
pin 6 plate = 200 ac
pin 8 filament = 195 dc

6v6:

Pin 3 plate = 191 dc
pin 4 screen grid = 146 dc
Pin 5 control grid = 20 mv ac/2.1mvdc peak
Pin 8 cathode = 7 dc

12ax7:

Pin 1 Plate =112 dc
Pin 2 grid = 0
Pin 3 cathode = 0
Pin 6 plate = 0
Pin 7 grid = 4.4 mvac
Pin 8 cathode = 0

[Thndr]

Update:

Channel 1 (Mic) is working good!! The 53 year old Jenson kicks!
Channel 2 is dead for the moment, it would appear I missed something. I'll go back through sections again.
50% there!!!

Thanks everyone for all your help!!!

Thndr

[Jazz P Bass]

Is it not time to ditch the lamp limiter?
Your heaters cannot be very happy.

[Thndr]

You have a point, I thought I would keep the lamp limiter on while I troubleshoot channel 2. I didnt think about the heaters.. Thanks for pointing that out Jazz.

I played on ch 1 for 10 minutes, tone is good, needs a slight bit of reverb.

T

[52 Bill]

Great to hear you got it working. Remove the limiter and check the voltages compared to the schematic. Now it's time to start testing all of those coupling caps.

[Thndr]

Both channels are now functional... the wiring I did was correct, the pot must of had some crud in it.. it would fade in and out the more I worked the pot the better it functioned. I'll replace all the pots in a couple of weeks.
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?

The amp was on for 45 minutes tonight without the current limiter, the first 10k resistor is hot but not like before.. I can put my finger on it for 2 maybe 3 seconds. before it would immediately scorch my finger so this is an improvement.

I think a new set of pots would improve this amp a lot.. I noticed this weekend one of the 500k pots had been changed to a 250k that cant be good.

Again thanks for all the help.

T