Guild Thunderbass Quantum Power Transformer Replacement

[imdsaenz]

Hi everyone. I have read many useful threads here and decided to join because I need help. I have a Guild Quantum amp with the 8417 output tubes. Only the rectifier tube lights up. When I measure AC volts at the power transformer. I get 120 going in on the primary. Then the secondary I get 240v, 6.3v, 45v, and 0v going to the filaments. Also the filament windings measures 350ohms. Can anyone recommend a replacement transformer. Or what the correct voltage readings on the secondary should be? I will attach a schematic

 

[fronobulax]

Welcome. @Default ?

 

[Default]

Hi.

There is no rectifier tube on any of the Thunder Bass amplifiers. What does the tube say on it? Some do have a voltage regulator tube, but all the rectifiers are solid state diodes.

Is the hum balance pot set all the way to one side? Turn it to the middle of the rotation for now.
The power supply uses a voltage doubler. You should have around 240v on the first leg, which should go to that junction of the 250 mfd filter caps. The next B+ lead should go to the junction of the two rectifiers. The B+ from that leg should be about 530 volts. Does that sound correct?

 

[imdsaenz]

Yes that is correct. The high voltage seems to be intact and and I do get around 530v B+ The only tube that lights up is a 6gf7 and is connected to it's own 6.3v secondary. There is another 6.3v circuit that runs to all the filaments of all the other tubes. That one reads 0v. I am measuring at the end of the green wires with the amp turned on and the transformer in circuit.

 

[imdsaenz]

Here are some pictures of the underside. I bought this amp from reverb and was told it works but needs new tubes. I replaced the tubes. Still no output. There are some burn marks on the chassis. It still has death cap and 2 prong cord.

 

[imdsaenz]

Update. I swapped the 7025 tubes out for some different ones and got the 6.3v on the last leg of the output transformer. So I am guessing it was tube related. I got a guitar sound output for about 5 minutes. With minimal hum. Then blew the 50uf 50v cap that I replaced with a 50uf 150v cap. Did I mention I am a novice. I had a basic intro to electronics class about 15 years ago. Anyone know why that blew?

 

[Default]

Yes that is correct. The high voltage seems to be intact and and I do get around 530v B+ The only tube that lights up is a 6gf7 and is connected to it's own 6.3v secondary. There is another 6.3v circuit that runs to all the filaments of all the other tubes. That one reads 0v. I am measuring at the end of the green wires with the amp turned on and the transformer in circuit.

Update. I swapped the 7025 tubes out for some different ones and got the 6.3v on the last leg of the output transformer. So I am guessing it was tube related. I got a guitar sound output for about 5 minutes. With minimal hum. Then blew the 50uf 50v cap that I replaced with a 50uf 150v cap. Did I mention I am a novice. I had a basic intro to electronics class about 15 years ago. Anyone know why that blew?

< sigh > Take it to a professional. This isn't an amp to learn on. That's my honest advice. 530 volts will stop your heart, if you accidentally short the B+ through you.

Build a Champ or a tweed Deluxe clone first.

I don't know why that cap blew. Are you sure that you got the polarity right? Because, if that cap is in the bias circuit, you'll kill the amp.

Again, take it to a tech that knows what he is doing. I don't want your epitaph to read, "Default said to do it!"

 

[imdsaenz]

Roger that

 

[imdsaenz]

Also you are correct. Polarity is reversed. Even a beginner should have caught that. Thanks Default. I will take it in to get a cap job.

 

[AcornHouse]

Also you are correct. Polarity is reversed. Even a beginner should have caught that. Thanks Default. I will take it in to get a cap job.

Don’t knock yourself out on that one. When I recapped my first Thunderbass, I put that same cap in backwards. The bang and smoke scared the crap out of me, but luckily it didn’t do any lasting damage. (To the amp, that is. Still can’t get rid of this *tic*.)

 

[GAD]

< sigh > Take it to a professional. This isn't an amp to learn on. That's my honest advice. 530 volts will stop your heart, if you accidentally short the B+ through you.

Build a Champ or a tweed Deluxe clone first.

I don't know why that cap blew. Are you sure that you got the polarity right? Because, if that cap is in the bias circuit, you'll kill the amp.

Again, take it to a tech that knows what he is doing. I don't want your epitaph to read, "Default said to do it!"

This x1000.