V4 addition of adjustable bias pot

[danb435]

its the same general idea with the distortion version V4. you would just adjust R215 instead.
I would be careful with rebasing the V4. They don't really like to be biased as hot as a fender or a marshall. the hard fast rule that 70% max plate dissipation at idle is where tube amps need to be set is all nonsense. let the amp decide where it likes to be set. I have found that the V4 tends to sound its best biased at about 60% max plate dissipation at idle. many other V4 users from the ampegv4.com forums and here have experimented and the general consensus is that 60% is the spot to be. which is roughly where the amps are set from the factory. I think the distortion models are set a little lower... but I can't recall exactly.
The only time I recommend pushing it higher, is when the tubes are replaced with KT-88s which isn't really possible in the early V4/VT-22 or VT-40 (due to space limitations) but is possible in the V2 and Distortion V4/VT-22.
6550s can be used in the Distortion Model also (due to the lower screen voltages). I have set the bias on V2s as high as 70% MPD at idle when there are KT-88s in there, and it sounds sick!

as for metal film resistors, you can use carbon film too if thats what you have, Ampeg did.
The advantage of using metal film is that the temperature coefficient is smaller by a factor of 10, so over a given temperature the resistors value doesn't drift as much with the temperature.
here is a nice calculator that Vishay has on their website if you're interested:
http://www.vishay.com/resistors/change-r...alculator/

typical TCR value for carbon film is 500ppm/C and for metal film its 50ppm/C

metal film resistors also have less thermal noise, but i don't think that wouldn't be noticeable in this case.

Once again, much thanks for the information. DanB

Thanks Liquids, very good information. I was a bit confused about the 75K bias resistor, then looked at the early 70's schematic and sure enough there it was. My V4 however is the '76 Magnavox era distortion version, which appears to be a different animal. That said, I still get the gist of the point you were making and it does make a lot of sense, taking into account all of the heat, possible vibration and banging around, that fixed resistors are a wiser and more robust choice. Regarding metal film resistors for bias, are these better because they are more stable under higher temps, quieter than carbon film, or what? I'm sure the blue color has some mythical properties that create a supernal cosmic effect on Halloween, but I'm not sure that's enough to special order them just because...
Thanks again for your sharing your wisdom.