315V Plate voltage on 6G6B Bassman

[Underwood]

I measured 315V plate voltage on both tube sockets. I think it is supposed to be over 400.  What's the deal?

[bigsbybender]

Check the voltage off the rectifier. That will at least tell you if there is trouble involved with the choke.  Without further details it could be a lot of things... 315 is rather low, especially since most vintage amps are "over-volted" with modern electrical feeds. Leo built that amp for 117v mains, while I get 126v at my wall...then put that through a step-up transformer.

While on that topic, check your AC mains voltage at the plug.  It has popped up here where people had been in old buildings that had very large voltage drops in their electrical service.

If memory serves the "B" 6G6 has a solid state rectifier.  If your voltage is at the rectifier, then switch your meter to AC and measure the leads from the Power Transformer that go to the rectifier, take your reading and multiply it by 1.4 and see if it is near what the rectifier reads, If not, you may have a problem with your rectifier.   

Also have you inspected your filter caps? 




j.

[stratele52]

Yes check filter capacitors and a bias too hot drop plate voltage

[Geezer]

Yes check filter capacitors and a bias too hot drop plate voltage

Dittos......especially if the caps are the original 50 year old (almost) units.

[Underwood]

The caps are pretty new. I will check the rectifier.   THANKS

[6G6]

If my memory is correct, I think you should be seeing something like 430-440V.

[Underwood]

  I may have found something that may be the reason for the low plate voltage.  The output tubes are wired a bit differently from the schematic. They both have a 7.3K resistor from pin 1 to 5. They also have a 4.7K resistor instead of a 470 ohm. The pin 8 is wired to ground but not connected to pin 1.  THere is another wire that is connected to a different pin also, but I can't remember off hand which one.

[PRR]

> output tubes...a 7.3K resistor from pin 1 to 5.

That can NOT be right.

If really true, the tubes, OT, and PT will melt in minutes.

[sluckey]

Fender wired most all the AB763 output sockets with a 1.5K between pins 1 and 5. 7.3K is a very odd value. I don't believe CCs were ever made in that value. 4.7K seems a bit large for a screen resistor too. Sounds like someone has been modernizing that old amp.

[Underwood]

Your are right, but the funny thing is that the amp sounds great and I only discovered this while checking the plate voltage for new tubes I bought. The red wire from the circuit board is also supposed to go to pin5 on the second tube and it goes to pin3.

That can NOT be right.

If really true, the tubes, OT, and PT will melt in minutes.
[/quote]

[sluckey]

The red wire from the circuit board is also supposed to go to pin5 on the second tube and it goes to pin3.

Are you sure? The only thing that should be on pin 3 (plate) of either tube is a single wire from the OT. One is usually blue and the other is usually brown.

[HotBluePlates]

That must really be pin 6, as shown in Sluckey's snippet of the layout. Pin 6 isn't used, but is a mounting tag for pin 4's (the screen) series resistor. A wire should run from pin 6 of one socket to pin 6 of the other socket.