B+ dropped from 427V to 339V !

[slidincharlie (Carlo P)]

This IS my unlucky day (see my thread about flames in my Vibrolux)... and all is happening two days before an important gig at a blues festival!

My '77 Deluxe Reverb used to have the B+ at 427 V.
Yesterday this amp sounded bad, very (and I mean VERY) distorted with the volume at 4. A nasty, dirty distortion.

I pulled the chassis out, and the B+ is 339 V (bias: 20 mA)!
The power tubes are two 6V6G Westinghouse (coke bottle) with maybe 20-30hrs life.

What may have caused such a drop in the B+ ?
Is there maybe a failing resistor somewhere in the power section?

TIA.

[Amp Kat]

Is that with the tubes in or out ?. Pull the tubes and check again. Could easily be a tube failure no matter how old they are or how much use. What is the neagative grid voltage on pin 5 ? and what is the screen voltage ?

[slidincharlie (Carlo P)]

Do you mean that either one or two bad tubes may give such symptom?

B+ is 408 V (that is, almost how it's supposed to be) without power tubes.
Negative grid voltage on pin 5 is -27 V with or without tubes.
Screen voltage on pin 4 is 405 V without tubes, 338 V with tubes.

[Alex/Tubewonder]

Check diodes in the HV rectifier bridge. you could be getting half wave rectification only which would give the values you're measuring.

schematic is here

[slidincharlie (Carlo P)]

Alex,
this amp is a Deluxe Reverb, not a Blues Deluxe, and it has got a 5U4GB rectifier tube.

BTW, shouldn't pins 4 and 6 of the rectifier tube read the same?
Well, pin 4 reads 315 VAC, while pin 6 reads around 5 VAC.
This happens with two different rectifier tubes.

[Enzo]

REmove power from teh amp and discharge everything. Now pull the rectifier tube. Measure resistance from pins 4 and 6 of the recto socket to ground. We are checking the power tranny winding for an open section. For that matter, you could just measure from pin 4 to pin 6 and forget the center tap. if that does not have continuity, the tranny is open.

And yes, that would be what Alex described

[Alex/Tubewonder]

What Enzo said.
Looked at wrong schematic, delux is delux is a delux

What's the probability of having two power transformers gone south in one day? I think you should buy a lottery ticket

[Ptron]

What's the probability of having two power transformers gone south in one day? I think you should buy a lottery ticket

Or don't leave the house.

[slidincharlie (Carlo P)]

REmove power from teh amp and discharge everything. Now pull the rectifier tube. Measure resistance from pins 4 and 6 of the recto socket to ground. We are checking the power tranny winding for an open section.

Pin 4 to ground reads 109 ohm, while pin 6 to ground reads open...There is no continuity between pins 4 and 6.
Is the power tranny dead?

There is a guy here who can redo windings. Do you think I can ask him to rewind only the bad section?
This is a Weber replacement. I don't know if it's still under warranty.

[Alex/Tubewonder]

Well, it's dead for this amp.You have one section of HV winding intact aand it can be used with a SS bridge rectifier but will have a little less current capability. Keep it for when you build an 18W amp or something similar.

It's replacement time for this PT. A word of warning: we had a number of amp builders here in Sweden reporting blown Weber PTs. I don't think rewinding will work, there's simply not enough room on the core for HV winding with properly calculated diameter of copper wire. Get one from Shinrock or Welter in Germany. Also Arie-Jan at Marble Amps in Holland will custom wind a good PT for you at reasonable price in a week. He's a good guy. Tell him I sent you.

[slidincharlie (Carlo P)]

Good news:
I asked Ted Weber if the tranny was covered by a warranty (I bought it from him last December), and he said that he is going to replace the tranny.

[Alex/Tubewonder]

Good news:
I asked Ted Weber if the tranny was covered by a warranty (I bought it from him last December), and he said that he is going to replace the tranny.

Very good, I wouldn't expect anything less than this from Ted.