V4 addition of adjustable bias pot

[danb435]

I am wanting to add a variable bias pot to my V4 and have some questions. The existing fixed bias is (ideally) -28.5V from a voltage divider consisting of 39K and 22K resistors. Wanting to use a Fender-style 10K bias pot, I am planning on switching the resistors to 33K and 15K respectively, with the 10K bias pot in series between them. I am assuming this will get the adjustment range about where it needs to be while staying pretty close to the current draw of the original fixed circuit. Is my thinking correct? Next question, I've noticed in my Mesa power amp that the fixed bias resistor is a blue metal oxide explosion-proof resistor, the only one in the amp. What is the reason for this? Last question, the only tidy spot I can see to mount the bias pot in the V4 chassis is fairly close to the hum balance pot on the back panel. Since both pots have a grounded metal can, can I assume this will shield the bias pot from any AC noise from the hum pot? I also intend to twist the wiring from the PC board to the bias pot to reject any induced noise in these wires. Am I on the right track here? Thanks all.

[Liquids]

I am wanting to add a variable bias pot to my V4 and have some questions. The existing fixed bias is (ideally) -28.5V from a voltage divider consisting of 39K and 22K resistors. Wanting to use a Fender-style 10K bias pot, I am planning on switching the resistors to 33K and 15K respectively, with the 10K bias pot in series between them. I am assuming this will get the adjustment range about where it needs to be while staying pretty close to the current draw of the original fixed circuit. Is my thinking correct? Next question, I've noticed in my Mesa power amp that the fixed bias resistor is a blue metal oxide explosion-proof resistor, the only one in the amp. What is the reason for this? Last question, the only tidy spot I can see to mount the bias pot in the V4 chassis is fairly close to the hum balance pot on the back panel. Since both pots have a grounded metal can, can I assume this will shield the bias pot from any AC noise from the hum pot? I also intend to twist the wiring from the PC board to the bias pot to reject any induced noise in these wires. Am I on the right track here? Thanks all.

Honestly, the best approach is to use a pot (+ set resistor) to dial in the sound you like, then measure what you dialed in, and then use the closest actual measured resistor value to that resistor.

The stock bias resistor is the 75k. It is simply bias to ground. A bias pot need not be wired like a volume pot - you're simply taking a negative DC voltage (txf tap, recified, filtered), and controlling bias by by getting it closer to ground via more or less resistance between the voltage and ground. So you don't need series resistance.

Once you have it where you like it, odds are, the next set of tubes you install will sound good at that bias spot too. But if you must know you dialed it in by ear, you can rewire the pot, dial in, measure resistance, and install equivalent resistor again. You'd have to dial it in anyway if that was your ideal...

The V series amps don't have a place to mount a chassis pot, like, say, a fender amp does. So you have to shim in some kind of trimpot joined to a stock resistor and wire it up. If you are bold enough to drill a holl into that rather sturdy chassis (fender chassis are a lot easier to drill I suspect), or you are able to use an existing hole cleanly for a true bias pot, you're better off. Vibration will be particularly unkind to a floating bias resistance pot+resistor, be it in moving the wiper or shorting it out, etc. But, some people haven't had issues with a shimmed bias pot causing damage...

Even with those 'designed as bias pots" like in older fenders have--and their part replacements which are available, the pot is not super likely to rattle around, but vibration has been known to affect the resistance drifting and hence the bias point even with those, I'm told by amp techs. Pots are more likely to short than resistors, and a pot's resistance can be more affected by heat if not mil spec (and head is inherent when there is noteworthy voltage across a resistor/pot, let alone near very hot tubes in a closed chassis, hung upside down, etc).

If you want the tone you dial in, the most reliable way to keep it there is a stock resistor...but, if you want to keep a variable resistor in there, indeed, good luck. I myself once tried using a trim pot (two different variations on trimpot designs), and I dialed it in myself. I measured, and the tone I liked most was so close to the stock 75k resistor, I nearly peed myself.

Do you want a crunchier sound? Do you want a thicker, more pronounced midrange with more subtle highs (at the expense of less distinct lows)? Do you want maximum low end power? What are you hopes for biasing by ear? IF you know that, and how higher vs lower vs medium biasing will affect stuff, you'll know what to listen for, and or possibly avoid needing a pot at all--could just swap to the next higher or lower resistor value, or leave it(!), depending on what you want and what you feel you are getting currently.

I also still think the EQ wafer replacement with discrete components--let alone value changes--offers far more significant tonal change than bias. But one thing only bias can do is get your amp to idle higher, causing it to "break up earlier," and/or eat tubes faster. Basically only biasing can do that...

[danb435]

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.

[Hangman]

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.

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.

[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.