Bassman 6G6A 510v on plates

[shaun]

Heyo. Hope all are well. I researched this, but it seems to be a grey area. Or a gray area, as the case may be.

I entirely recapped a Bassman 6G6A, including bias caps. Apart from the freakishly treble-sounding Bass channel (which I assume is due to the fixed 27k resistors in the tone stack), it has a very high B+. It was designed for 470v on the 6L6 plates, which is already high, but I have 510v.

How long can a pair of Sylvania 6L6GCs stand that, I wonder? The rectifier diodes read okay on my meter, so the amp seems to be doing what it was designed to...when we had 110vac in the supply. All resistors read very good, especially for older carbon comps. A couple of voltage readings are out of whack (circled).

Some people love the Bassman, but the 6G6a...perhaps no so much.

I guess I could change out the 56k bias resistor to bring the bias down. Suggestions greatly appreciated.

[sluckey]

Recheck those circled voltages. You started with 421V then went thru a dropping resistor that BOOSTED the voltage up to 483V. Then that 483V went thru another dropping resistor and got BOOSTED up to 510V. That's opposite of the way most power supplies work. 

[Latole]

 
 IMO, power tubes's bias is very very cold. Tubes don't drop power supply voltage .

You wrote  "Apart from the freakishly treble-sounding Bass channel." 

It's looking cold bias

[ac427v]

Were all measurements done with the standby switch in the "play" position?

I'm just double checking for apples and oranges to try to understand what you are seeing.

[Latole]

Were all measurements done with the standby switch in the "play" position?

I'm just double checking for apples and oranges to try to understand what you are seeing.

He can't read those ( and any )  voltages if stanby was not in "Play " position

[pdf64]

What VAC do you measure at your wall outlet and across the amp’s heaters, one probe on pin 2 terminal, the other on pin 7.

By the mid 50s, North American mains voltage was 117VAC, see the 5F6A schematic.

Anode voltage at low anode current (eg idle) is immaterial. What’s the HT VDC when the amp is putting out its max power?

[shaun]

I use a Variac set to 120vac. Stand-by switch turned on - so, full power.

I am unable to account for the strange voltage readings after the dropping resistors, but I re-checked them several times. I will do so again today. I wondered whether there was some sort of short - perhaps in the Normal channel - that could be responsible for those weird voltages.

Thanks for the responses. I will re-check and post my findings.

[sluckey]

I am unable to account for the strange voltage readings after the dropping resistors

Those voltages would make perfect sense if you swapped the numbers for nodes A and B.

[shaun]

Those voltages would make perfect sense if you swapped the numbers for nodes A and B.
[/quote]

You are absolutely correct, Steve (if I may be so bold) - I got them backwards - not the first time I got something backwards :).

The question still remains whether 510v on the 6L6GC plates is okay. A bit more research, and I found it is not uncommon. But there's no simple fix either. So the amp is what it is, I guess.

[sluckey]

I'm Okay with 510 on the 6L6s. The current flowing through the tube needs to be considered too.

You may be interested in building a little Buckaroo! Only cost about $20. Look at page 2 of this pdf...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[shaun]

Thanks! Very cool page. I have wondered about bucking trans for a while now, so there are some questions answered. Lots of other good stuff on there too.