Crate VC50 bias

[rockon1]

My VC50 runs the tubes too hot. Its fixed bias with -16 vdc available. trying to figure out the simplest way to dump less to ground and get more - bias voltage to the tubes. Thoughts? Thanks Bob


Schematic-


http://blog.fillmorepedal.com/files/...ge_club_50.pdf


....

[Jazz P Bass]

Are we talking temperature or bias current?
Did you measure the idle current of the tubes?
-16Vdc is not that bad.
Peavey runs there Classic 50 (4 EL34) at -14Vdc.
Which is hotter than -16Vdc.
Test point 17 indicates -15.6Vdc.
Although TP26 & 27 indicate -15.7Vdc which feeds the 100/15K voltage divider.
That then becomes the bias voltage at TP30.
Looks to me that it is what it is as far as colder.

[rockon1]

Bias current. Most tubes run from 85-100+ % idle dissapation. I havent measured at pin #2 but Im sure that 15.7 isnt getting to the tubes. Bob

[rockon1]

OK I measured the - bias voltage at pin 2. I cant believe Ive got -14.8 at the tubes. The plate voltage is around 400vdc. At that voltage I need tubes that idle at around 21 ma just to get down to 70%. I guess I'll try ordering the low current draw tubes for it. Of course as the current draw lowers the PV goes up....Bob

[Jazz P Bass]

It would be nice to know how much current the amp is pulling from the mains at idle.
I recall a 6BQ5 quad I had that ran really hot.
Peavey Classic 50, with fixed bias. (-14Vdc)
The amp was pulling 100 watts at idle.
I installed a "normal" quad & it dropped to 78 watts.
I guess this is where GT gets there 1-10 ratings from.

[Enzo]

I see bias voltages flying left and right, but the only mention of tube current was hypothetical.

What current if flowing in these tubes? Bias voltage is irrelevant, what matters is tube dissipation, and we won;t know that until we measure tube current.

[rockon1]

I see bias voltages flying left and right, but the only mention of tube current was hypothetical.

What current if flowing in these tubes? Bias voltage is irrelevant, what matters is tube dissipation, and we won;t know that until we measure tube current.

Well most tubes I have(Ive got a bunch) pull from 25 to 30+ ma at around 395 DVC hence the mention of 85-100+% current draw. I just spent the last couple hours going thru about 30 Sovtek 6BQ5's and found 2 sets that will idle around 21ma thats roughly 70% idle. It would be nice to be able to run any tubes in it(adjustable) and not just the coolest ones I can find. Oh well at least low current draw with get down to 70%. I'd prefer 60%. thanks,Bob

[Pedro Vecino]

Yes. This amp has a very strong bias adjustment. Only fit well Sovtek EL84 with lower grade and is not easy to find. Itīs better to make an adjustable circuit.
Regards

[rockon1]

Yes. This amp has a very strong bias adjustment. Only fit well Sovtek EL84 with lower grade and is not easy to find. Itīs better to make an adjustable circuit.
Regards

Yep thats what Ive found. Making the bias adjustable(at least more negative) is easier said than done though it seems. Most of the available - bias voltage is already making it to the tubes. Bob

[zoranrataj2000]

Bias mods

Yep thats what Ive found. Making the bias adjustable(at least more negative) is easier said than done though it seems. Most of the available - bias voltage is already making it to the tubes. Bob

I do it like this.1 OHM resistor is for eazy measuring bias.

http://shrani.si/f/L/Gk/3z9PBkw0/cra...0-bias-mod.jpg