1959 GA-20T

[Thndr]

I'm attempting to understand the 12AY7 circuit on this basket case GA20T that was given to me.


I am just not understanding how the cathode is being driven. So what I did was make a layout drawing to the best of my ability. I understand that it is grid leakage that is doing it. I just dont see a wire going to the cathode from the 2 1meg resistors..

What am I missing? Someone has already been in this attempting to fix it, then gave it to me. I want to learn all the sections, replace what's bad and play this amp.

Any help would be appreciated.


Best regards,
Thndr

[Enzo]

Your 1 meg resistors appear to be grounded to the input jacks, if that is what the circles left center represent.


Pins 3 and 8, the cathodes of a 12AX7, appear to be grounded by that common wire acrosss the top. Two things both grounded are thus connected together.

[Thndr]

Thank you Enzo again, Im concerned by the solder joints to the board that this thing may not be wired correctly, thus the exercise to lay this out. Then attempt to follow it. From measurement I know the bumblebees are toast and resistors Ive checked are out of spec. The rectifier tube took a beating it's loose in it's housing after the previous owner replaced the electrolitics. So If I knew this amp was at least wired correctly to the board it would be helpful. I am learning as you can tell, but what a journey.

Thanks
Thndr

[Enzo]

Well, keep in mind that not all connections are made with wires.

[Thndr]

I wonder if there is a layout for a GA-20T out there somewhere someone would post. I've looked everywhere.. I think Google is going to start charging me lol.

[tubeswell]

There's no layout diagrams in the 1 gibson master service manual AFAICT. Your best image of the layout is the real live amp in front of you. Have you had a look here http://forums.vintageamps.com/index.php ? (don't know if you'll find anything useful, but it might be worth a try)

[Thndr]

tubeswell,

Just registered there, I can't be the only one that has attempted a layout. Setting down this evening to make sense of the layout I made in comparison to the schematic. I took Enzo's advice and downloaded Radiotron designers handbook, a lot of info to digest.

Thndr

[Thndr]

It's been awhile...

Update, caps replaces, now I'm in troubleshooting mode.. Found one Cap reversed which caused 3 blown fuses before I found it..

Now Im fighting a no sound issue,
speaker is good, 9v bat. test
getting 403 v at the plate of the 6v6's and hearing noise when the probe touches the plate connector. So the output transformer is working
The "noise" frodm the 12ax7 was louder, but at the 12ay7 plate it was softer... Ive changed out the the 12ax7 with a new tube and the noise does not change if I put the tube into the 12ay7 position. I suspect something is wrong between the 12ay7 and 12ay7 sockets.. I'll beable to do more troubleshooting this weekend, if the rain comes .

Volume 1 pot was really stiff, I have not checked it yet but will.


I'll have to continue this later.. I dont have all the voltages with me.

Thndr

[Thndr]

The problem I have been having is no output. I've been using the Jack Darr method of troubleshooting, some strange things have been happening... the biasing resistor (200 ohm 10 watts) died, at the same time the 5y3gt mount started to smoke. I shut it off. At this point I did not know the resistor had failed. Mouser has been notified and two replacements are on order. Prior to this occurrence these are the voltage readings two days ago:

5y3gt:
Filament 4 v
Plates 337 v

6v6:
Plates 403 v
screen grid 279 v
control grids 0 v

12Ax7:
Both plates 254 v
Grid 0 v

12Ay7:
Plates 94 v
grid 0

Before the resistor blew...

6v6:
plate 460 v
Screen grid 0 v
right after the measurement is when the sucker smoked.

does it make sense that the increased voltage on the 6v6 could be due to the 200 v resistor going out? Or from these voltage measurements did something else occur?

Thanks for any help.

Thndr

[52 Bill]

... the biasing resistor (200 ohm 10 watts) died, at the same time the 5y3gt mount started to smoke. I shut it off. At this point I did not know the resistor had failed. ...does it make sense that the increased voltage on the 6v6 could be due to the 200 v resistor going out? Or from these voltage measurements did something else occur?

Thanks for any help.

Thndr

When the cathode resistor (200 ohm) died it turned off the output tubes, which then due to the lack of current draw caused the voltage to rise. Did the resistor go open? Did you test the output tubes?

The more important voltage measurements that you are not listing are the cathode voltages. The cathode voltages will tell you if the tubes are turned on and at what level the output tubes are idling. And of course the 12AY7 will not show a voltage on the cathode.

[Enzo]

It is way more likely that a tube problem caused the resistor to fail. An open cathode resistor won;t harm the tube, it will do as Bill said, it will just turn the tube off. In fact in various amps that is how they turn tubes on and off for hi/lo power. Other amps use that method to make a standby switch.

[Thndr]

When the cathode resistor (200 ohm) died it turned off the output tubes, which then due to the lack of current draw caused the voltage to rise. Did the resistor go open? Did you test the output tubes?

The more important voltage measurements that you are not listing are the cathode voltages. The cathode voltages will tell you if the tubes are turned on and at what level the output tubes are idling. And of course the 12AY7 will not show a voltage on the cathode.

When the 5y3 socket started to smoke, I turned the amp off. When the replacement resistor arrives, I'll turn it back on again with my variac and get some voltage readings for the cathode. I have checked the resistor and it is open.

I appreciate the input, I'm new to amp troubleshooting. so I need all the help I can get.

Thndr

[Thndr]

Thanks for the reply, the 5y3 is new. I suspect it is toast now, this amp was worked on before I got it. From what I was told... he had smoked the 5y3 to the point it came loose from its base, I found the electrolitic caps backwards, which may have created that issue. I have replaced the electrolitic caps properly and found a bumblebee cap on the back broken. Replaced all the bb caps, verified all the resistors, traced all the wiring. The ground from the 680k resistor from the 12ax7 cathode did not exist, so I added one.

This amp was truly a basketcase when I got it, the price was good, free. I have an amp tech that could take over and fix it.. but that wouldnt be any fun for me. Ive had my head in Darr's book as well as Morgan Jones's Valve amplifiers.

If it wasnt the 200 ohm resistor could have been a bad new tube? there was a thunderstorm going off inside that tube.

Thanks again
Thndr

[52 Bill]

...I found the electrolitic caps backwards, which may have created that issue. I have replaced the electrolitic caps properly...

Did you replace these caps or did you just rewire correctly?

The ground from the 680k resistor from the 12ax7 cathode did not exist, so I added one.

I assume that this is a typo the value should be 680 ohms.

If it wasnt the 200 ohm resistor could have been a bad new tube? there was a thunderstorm going off inside that tube.

If you mean that the tube was sparking or arcing inside the glass bulb, then yes it is a bad tube.

If the tube socket was smoking, check the socket itself for burn marks and carbon traces between pin holes.

[soundmasterg]

I wonder if there is a layout for a GA-20T out there somewhere someone would post. I've looked everywhere.. I think Google is going to start charging me lol.

My uncle has that exact same amp. I overhauled it several years ago and drew up a hand made layout for my own use. It is somewhat helpful, but once you get better with schematics, you learn to look at the schematic and find the parts in the circuit without needing a layout. The amp sounds similar to Fender 5E3's though a little more raspy in the distortion and less clean headroom.

Greg