5Y3GT tube possible failure - High voltage

[Potato Cakes]

Hello, Everyone,

Just finished building a Champ Vibro and when I went to power it up for the first time it outputted a loud squealing noise, which made me very sad. Found that the rectifier was putting out 475V, which is supposed to be 355V. The AC voltage from the secondaries is about 335V from the center tap which is correct. 6.3V and 5V windings are also measuring correctly. A bad 5Y3GT is what makes the most sense to me but building tube guitars amps is fairly new to me, so this extremely high voltage from a tube rectifier is something I haven't seen. Curious to know if this has happened to anyone else. I searched the internet before posting here, but it seems that the internet is slowly becoming useless and even negative helpful.

Thanks!

Paul

 

[pucho812]

Years ago I picked up a Gibson amp on the cheap because when in 15w mode, it squeal. The amp has a 15w, 45w output so you can select between pentode or triode output modes.
Anyway, the reason the 15w mode would squeal was a factory wiring screw up turned negative feedback into positive feedback. Took a second to flip the wiring correctly and no more squeal. Check your work and solder joints.

 

[Potato Cakes]

Years ago I picked up a Gibson amp on the cheap because when in 15w mode, it squeal. The amp has a 15w, 45w output so you can select between pentode or triode output modes.
Anyway, the reason the 15w mode would squeal was a factory wiring screw up turned negative feedback into positive feedback. Took a second to flip the wiring correctly and no more squeal. Check your work and solder joints.

Would this be the polarity of the secondary winding? The transformer I have is supposed to use the color code used in the Champ schematic, which only has red for both HT leads. I've disconnected and reconnected those connections from from the rectifier tube three times now and I'm pretty certain that part is fine at this point. I've attached the schematic I am using. The only thing that looks like feedback is the 2k7 resistor going from the transformer primaries to the cathode resistor of the 2nd half of the 12AX7. Now that you mention it the sound does remind me of an op amp no feedback resistor, but the excessively high voltage still draws my attention to the rectifier tube. I'll look at it again tomorrow.

Thanks!

Paul

 

[Khron]

Would this be the polarity of the secondary winding? The transformer I have is supposed to use the color code used in the Champ schematic, which only has red for both HT leads. I've disconnected and reconnected those connections from from the rectifier tube three times now and I'm pretty certain that part is fine at this point. I've attached the schematic I am using. The only thing that looks like feedback is the 2k7 resistor going from the transformer primaries to the cathode resistor of the 2nd half of the 12AX7. Now that you mention it the sound does remind me of an op amp no feedback resistor, but the excessively high voltage still draws my attention to the rectifier tube. I'll look at it again tomorrow.

Thanks!

Paul

Yes, the secondary (winding), but the secondary of the speaker output transformer, NOT the power transformer.

 

[Potato Cakes]

You know, I looked right at that transformer color code and told myself not to wire the secondaries backwards and even took my time to make sure I didn't but still managed wire those dang secondaries on the output transformer backwards. That was the one thing I didn't check because how could I make such a silly mistake? The answer is because I'm a silly person. Pucho was 100% correct as to what the problem was. After I fixed it and got it up and running, I played through it for about an hour and it sounds really nice! However, I believe I managed to damage the tubes from the initial error as it started giving me a high pitched ringing, then the static gargles of an output tube in its death throes, then no output and just louder than normal hum. Also, the vibrato didn't work. BUT, at least I know the secondaries on the output transformer are correct!

Thanks for the help, you guys. The brain sometimes gets stuck when diagnosing what should be simple stuff.

I'll show some pics after I fix the last couple of bugs.

Thanks!

Paul