1959 GA-20T

[Jazz P Bass]

You really should be measuring across pin 2 & 8.
And without disconecting the wire to the cap, 10K resistor & the output transformer, that may be goofing with the reading.
As it is, the smoldering socket makes no sense.
You may have a bad power transformer.

[Thndr]

You really should be measuring across pin 2 & 8.
And without disconecting the wire to the cap, 10K resistor & the output transformer, that may be goofing with the reading.
As it is, the smoldering socket makes no sense.
You may have a bad power transformer.

Sorry it's been a long day...

After your last reply, I disconnected the "8" pin wire and measured between 2 and the wire. it reads 4.35 vac.

I suspect the transformer is ok, from these readings.

I have two schematics for the GA20T... one feeds the 200 ohm from a transformer tap between the heater taps, the other does not. I gotta figure out which is correct for this amp.

I pulled the 5y3 socket, and inspected it under 5X magnification, no damage or cracks noted.

I suspect the wiring is fubar'd and is causing the problem.

[Jazz P Bass]

I have two schematics for the GA20T... one feeds the 200 ohm from a transformer tap between the heater taps, the other does not. I gotta figure out which is correct for this amp.

IF the heater winding has a center tap, then it goes to the output tube cathodes.(adds Vdc to the heaters)
If there is Not a center tap, then there is no connection.
If you want to prove out the transformer, you could rig up a temporary solid state rectifier.
That would isolate the high voltage Vac winding from the 5 Vac heater winding. (which I have a suspicion is the cause of the smoke).

[Thndr]

Jazz,

I found a wax substance from the pin 8 to pin 6, cleaned with Goof off. No melting so far.

5y3:

Pin 4 to ground = 300 vac
Pin 6 to ground = 300 vac

Pin 2 to ground = 414 dc
Pin 8 to ground = 414 dc

the DC voltage is way high... at 110 ac it shot up to 456dc

T

[Thndr]

5 volts measuring at 2 & 8 @ variac set to 110vac with no tube in

334 vac on both measurement & pin 4 to gnd and pin6 to gnd. variac set to 110vac

What am I missing?

[Thndr]

Found the 10K open (destroyed by heat) replaced and received 320 v at pin 8..

Then the 10K started to melt the solder joint at the grid, and the resistor is looking bad again... I checked it 9.2 ohms.

Something is very wrong here...

[Jazz P Bass]

Did you install the tubes.
You need to load it down.

[Thndr]

yes all tubes were installed.

Today I did some more reading.. armed with a better understanding... I hooked the amp to a 60 watt current limiting light bulb then to my variac and pulled all the tubes. Slowly I turned the variac up in increments of 25 vac checking the bulb for brightness up to and including 115vac. The bulb was dim at 115vac which from what I read and your kind input tells me the transformer is ok. I then proceeded to perform voltage checks with this setup, they are as follows:

5y3 filament to filament = 5.5 vac
6v6 filament to filament = 7 vac
5y3 pin 4 to chassis = 349 vac
5y3 pin 6 to chassis = 349 vac

No DC voltage of course from filaments

Please correct me if I am in error but this tells me the transformer is ok.

All the tubes are new, barring any defective tubes ... it would be my assumption something is wired incorrectly or there is a bad component.

If all this sounds correct, can I put the 5y3 in alone and applying the same amp/light bulb current limiter/variac process without having cap guts blow up on me? If I can then I measure both ac and dc voltages if the light limiter doesnt go bright on me.

If my mind is right on this I could do this for each section until the short/bad component/miss wiring is located.

Your thoughts would be appreciated.

Thndr

[Jazz P Bass]

I thought that you had the 5Y3 installed.
Put it in.

[Thndr]

I put it in brought it up to 100 vac, the light turned bright and the 10K resistor heated up. I'll get some readings at 90vac with in the hour.

with only the 5y3 in @ 90 vac light almost bright (10K very hot):

Filament to Filament = 3.13 vac
Chassis to plates = both 4 & 6 read 210 ac
Chassis to filament = both 2 and 8 read 210

Thanks for hanging in there.

What's interesting to me is the other 10K doesnt heat up.

Thndr

[Thndr]

I'll trace the circuit out tomorrow evening.. I did find something interesting the 10 mfd after the second 10k resistor is showing 8.8ohms and never drops .. either it is bad or there is a short?

[52 Bill]

I'll trace the circuit out tomorrow evening.. I did find something interesting the 10 mfd after the second 10k resistor is showing 8.8ohms and never drops .. either it is bad or there is a short?

Test the cap out of circuit, does it still show a low resistance? If one of the 10K resistors is heating up, there is something in the circuit that is causing it to do so.

One problem that I am having, is that I don't know where in the circuit the 10K resistor you are referring to is. There should be one 10K resistor that connects directly to the rectifier tube and to the first of two 20uF filter caps. This is the first node of the power supply and connects to the output transformer center tap. Let's call this 10K resistor number one.

The other end of that resistor should connect to the two output tubes screen grids and to the second 20uF cap as well as to the second 10K resistor. This is the second node of the power supply. It also feeds the tremolo oscillator tube.

The second side of the second 10K resistor, which we will call number two, is connected to a 10uF cap and supplies the preamp and driver sections. Which of these two 10K resistors is overheating?

If any of the three filter caps were damaged from being installed backwards they can short and cause the resistor to overheat. Take resistance readings across each of the filter caps and if you find a constant low reading like you mentioned earlier, remove one end of the cap and retest the resistance with the cap out of circuit. If the reading remains low replace the cap with a new one. If the cap reading starts low and rises to some high value, it may be ok. I say may be ok because it might test ok with the low voltage of you meter but short under high voltage.

If the cap tests ok, then retest the circuit connection to see if the low reading remains there. If it does, then you need to trace out other components and connections that are causing the low reading. Maybe a wire or a part lead touching the chassis, or something similar.

[Thndr]

52 Bill,

It is the first 10K coming off the rectifier that is overheating. The 10uf, I cut the negative side lead, and it builds resistance. Did the same with both 20uf's both build resistance. So the filter caps MAY be alright.

Last night when I measured 8.8ohms it was from the 10uf side of the 2nd 10k resistor to the 10uf negative or ground. I just measured without the 10uf in circuit from the 10uf side of 10k (2nd one) to gnd (6v6 pin 1) and still have 8.8 ohms showing. I hope that made sense..

Another thing, I only have the 5y3 installed.

I'll trace the circuit out this evening, and look for the items you mentioned.

Thanks again everyone for all your help!

Thndr

[52 Bill]

It is the first 10K coming off the rectifier that is overheating.

That node of the power supply feeds the screen grids of the power output tubes and the tremolo tube via the depth control. If you read the resistance from the second side of the first 10K to ground what do you get?

Last night when I measured 8.8ohms it was from the 10uf side of the 2nd 10k resistor to the 10uf negative or ground. I just measured without the 10uf in circuit from the 10uf side of 10k (2nd one) to gnd (6v6 pin 1) and still have 8.8 ohms showing.

This node of the power supply feed both preamp tubes as well as the driver/phase inverter. Follow the wires and see if you notice any shorts in the wiring. Check the two 220K resistors on the plates of the 12AX7 driver tube, the 100K plate resistor for the 12AY7 mike preamp tube and the 510K & 1M resistors at the 5879 tube. It may be easiest if you start removing different elements from this node until you find the cause of the low ohm reading.

Another thing, I only have the 5y3 installed.

Leave the other tubes out of the amp until you get the power supply problems sorted out.

[Thndr]

I'll check them after work tonight.

also going to take a mirror and see if anything on the backside is touching the chassis.

T