Fender Vibro Champ bias question

Ivan Funk · From: Hamburg Pennsylvania, USA

I have a 1976 Fender Vibro Champ that I restored to original specs and recapped, etc...
Then I started looking at the bias. (6v6GT tube)
I gather from research that the way they came from the factory they run hot.

I tend to obsess over voltages but some say to not worry about it and leave it as is.
I'm more a fan of clean headroom than bluesy grit.
I experimented with different bias and these are some results:

originally:
488 ohms cathode resistor
23.6v voltage drop
.048a current
348v plate voltage
dissipation 16.7w (120%)?

changed to:
817 ohms
28.7v
.035a
367v
12.8w (91%)?

My math might not be correct.

My questions are:
A. Is it safe to leave it as it was or should I set the bias a bit lower?
B. If set it lower with a greater voltage drop, do I need beef up the 25uf/25v bypass cap to a 50v cap?

Thanks,

Bill A. Moore · From: Silver City, New Mexico, USA

Bias is a suggestion, it can be hotter or colder, most 5F1's, and 5E3's I try to set close to 100% at idle. If you like the sound, and the trem operates like you want it to, 91% is fine.

Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks Bill.

What about the original stock readings at what looks like 120%? Is that too high? The 6v6 get's really hot but it sounds fine.

If I go with the 91% specs is the 25uf/25v bypass cap ok or should it be 50v?

Bill A. Moore · From: Silver City, New Mexico, USA

I'm sure they're fine, when building a new circuit, I buy 50V caps, but wouldn't change the 25V you have.
Look at this article, he really breaks things down!
https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Jeff Highland · From: New South Wales, Australia

I've got a Vibrochamp clone that I built, never checked the bias though.
Great little amp, love the vibrato (really tremolo)
A really good mod is to add a pot to the Negative feedback loop which moves it close to tweed teritory for when you want more crunch.
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Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks Jeff.

I might try that just for fun but ultimately I want this particular amp to be silverface clean and not tweed crunch.

I guess I'll split the difference and go with a 750 ohm cathode resistor.

George Biner · From: Los Angeles, CA

I personally wouldn't run a tube over it's power rating, it won't kill it but it will shorten the life -- and I don't you'll notice much different sound going to 91 from 120%
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Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

If it sounds great at 91%, I see no reason not to run the tube cooler. I keep my power tubes as cool as I can while still sounding good. This is a Class A power section - stuff like crossover distortion is not an issue. Of course, running too cool will cause sonic issues also. I agree that bias is to taste, as long as the tube isn't getting too hot or red-plating.

If I was replacing the resistor, I see absolutely no reason not to also upgrade the bypass capacitor with a fresh 25/50. Only is I was obsessively trying to keep everything totally original would I avoid doing that. That is a 48 year old amp - I would not be inclined to push the voltage drop over the voltage spec. Yeah, I know that in the old days, there was some built-in extra tolerance in components. But on a nearly 50 year old amp? I dunno, I just think it's prudent to over-spec the voltage rating a bit when you're pushing it.

Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks guys.
All great helpful info.
I'll go for coolest level wile retaining the best sound.
What about the idea of also changing the cap value to achieve the same cut off frequency? (when changing resistor value)? For example, if changing to 750 ohm, also changing the cap to around 15uF?
I guess I'll try it and see how it sounds.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

I'd play around with values and see what sounds best.

I have this tweed Champy thing in a Pro Junior box with its original 10" speaker that was made for my old shop years ago that I picked up last year. The preamp bypass circuits were changed and with a 12AX7, the sound is really raunchy, even at low volumes. I put in a 12AY7 to drop the gain some, and that helped. But I need to experiment with those values. I also would prefer to get it a little more blackface/silverface clean, but without as much mid-scoop. I have a 71 Vibro Champ, totally stock. Sounds great - even pedal steel at very low volumes. But mostly it's a guitar amp for me.

Ivan Funk · From: Hamburg Pennsylvania, USA

Nice. Perfect.
Yes I mainly got this Vibro Champ because I wanted the silverface sound for playing pedal steel quietly at home at night.

I settled on 750 ohm cathode resistor and a 15uF/50v bypass cap (for the 6v6). 12.6 watts dissipation - about 90%?

The preamp values are all stock.
A fun project. Really good sounding amp. Inspiring.
Also a side note - the previous owner installed a Jupiter 8SC speaker in it. Really good sounding speaker.

I don't really use tremolo (although it is the best tremolo I've heard) so I'm tempted to try to find a regular non-Vibro silverface Champ if I ever see a good deal...

Thanks everyone for the helpful input.

J Fletcher · From: London,Ont,Canada

So when calculating the tube dissipation, subtract the cathode voltage from the plate voltage , than multiply by cathode current , to get power dissipated by the tube . Personally I would go with the stock values , unless you are having problems with the amp . Jerry

Ivan Funk · From: Hamburg Pennsylvania, USA

Thanks Jerry.

I'm liking the sound right around 90% dissipation.
Most enjoyable playing through this amp.

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