V4B - Dual Bias Control - External Set Up

[Roman Ships]

I have a 1971 V4B and I'm adding adjustable bias control with external test ports and pots.

I could use some input advice.

Cathode Test Ports:
I was planning on putting a test port with a 1 ohm resistor from cathode to ground for each tube. I have seen it done with linking the left two cathodes and the right two cathodes. Thus you could divide the current by 2 and get an average. But then I wouldn't know which tube is pulling more current. Is 4 cathode ports totally unnecessary?

Control Grid Test Ports (Bias control):
Since I want to bias the tubes in pairs i would swap out R35 & R36 (100K) for 75K and install a 50K Trim Pot in series with R35 and R36 respectively.

So I would need two test ports here, choosing one control grid pin per pair of tubes. Or do I need none since I really only care about current through the cathode?

Plate Voltage Test Ports:
One Test port and assume they are all the same given all the plate resistors are functioning correctly.



Does this sound like a good plan? I see people usually put a pot in with R49 but I assume that is just because it is the most convenient location if you are biasing all four tubes together.

Any advice / feedback would be excellent.

[Hangman]

It really depends on the kind of precision you want. I tend to think that 4 bias monitor ports is a bit much, but if your tubes are super mismatched it may be worth your trouble.

if you want to have independent bias control of each half you could just have two bias pots where R75 is, and have the output of each go to a different side.

or you could follow the silver face fender style of balancing the power amp instead of biasing.

alternately you could combine the two. what do you want to do?

Here is my opinion: I don't think most of this is particularly necessary. Ampeg biases these amps pretty cold (around 60% max plate dissipation), and they actually sound good that way. I have tried biasing them hotter, but they get pretty muddy and lose definition.

personally, Ive stopped installing bias pots in V4s because they seem happy without them, and installing them and boosting the bias doesn't make them sound better. (as it would on a mesa boogie or silver face fender)

I have a 1971 V4B and I'm adding adjustable bias control with external test ports and pots.

I could use some input advice.

Cathode Test Ports:
I was planning on putting a test port with a 1 ohm resistor from cathode to ground for each tube. I have seen it done with linking the left two cathodes and the right two cathodes. Thus you could divide the current by 2 and get an average. But then I wouldn't know which tube is pulling more current. Is 4 cathode ports totally unnecessary?

Control Grid Test Ports (Bias control):
Since I want to bias the tubes in pairs i would swap out R35 & R36 (100K) for 75K and install a 50K Trim Pot in series with R35 and R36 respectively.

So I would need two test ports here, choosing one control grid pin per pair of tubes. Or do I need none since I really only care about current through the cathode?

Plate Voltage Test Ports:
One Test port and assume they are all the same given all the plate resistors are functioning correctly.



Does this sound like a good plan? I see people usually put a pot in with R49 but I assume that is just because it is the most convenient location if you are biasing all four tubes together.

Any advice / feedback would be excellent.

[Roman Ships]

It really depends on the kind of precision you want. I tend to think that 4 bias monitor ports is a bit much, but if your tubes are super mismatched it may be worth your trouble.

if you want to have independent bias control of each half you could just have two bias pots where R75 is, and have the output of each go to a different side.

or you could follow the silver face fender style of balancing the power amp instead of biasing.

alternately you could combine the two. what do you want to do?

Here is my opinion: I don't think most of this is particularly necessary. Ampeg biases these amps pretty cold (around 60% max plate dissipation), and they actually sound good that way. I have tried biasing them hotter, but they get pretty muddy and lose definition.

personally, Ive stopped installing bias pots in V4s because they seem happy without them, and installing them and boosting the bias doesn't make them sound better. (as it would on a mesa boogie or silver face fender)

My interest really isn't increasing bias, I just want to know that it is in a good range and I want to be able to do so without opening the amp... plus its a fun project.

It has been too mushy for awhile in my opinion. Perhaps I should wait until I have the rest of the issue figured out and then assess whether I should install a bias control.

Clarification: You said R75, I don't believe there is one. Did you mean the 75k resistor I wanted to put in?


Also, I took out the hum pot and was going to tie 100 ohm resistors from each heater to ground. I read this was a good alternative.
That sound ok?


thanks,

[Hangman]

I'd say install the 1 ohm resistors on the cathodes of each tube, and use the multimeter to see how the tubes look. that should give you a pretty clear idea of whats going on, you can always add the bias monitor ports after that if you think that it needs it.

yes, R49 which is a 75K resistor. Sorry about that, looked at the schematic, read the wrong value.

2 100ohm resistors are fine for replacing the hum balance pot.

My interest really isn't increasing bias, I just want to know that it is in a good range and I want to be able to do so without opening the amp... plus its a fun project.

It has been too mushy for awhile in my opinion. Perhaps I should wait until I have the rest of the issue figured out and then assess whether I should install a bias control.

Clarification: You said R75, I don't believe there is one. Did you mean the 75k resistor I wanted to put in?


Also, I took out the hum pot and was going to tie 100 ohm resistors from each heater to ground. I read this was a good alternative.
That sound ok?


thanks,

[Roman Ships]

When you said I could put two bias pots in place of R49 for independent controls,
I'm not sure how I see that would work. After R49 it still passes the bias caps and
That resistor across the caps. So anything in R49 position would affect bias on all tubes.

Maybe I'm missing something ?

[Liquids]

Here is my opinion: I don't think most of this is particularly necessary. Ampeg biases these amps pretty cold (around 60% max plate dissipation), and they actually sound good that way. I have tried biasing them hotter, but they get pretty muddy and lose definition.

I second this, just for the record. Also, below 60% doesn't sound bad at all either! I think I prefer it lower, personally, but that is just me. Plenty of mids can be dialed in (or out) on this amp, unlike most other tube amps. It's fenders or marshalls or voxes, where the bias becomes a tonal thing that's useful. Here, hot bias just isn't needed, especially if you run the amp relatively clean (and maybe beyond).

There's nothing quite like the big fat lows this amp provides, though, and those shine best and most defined at 'cool' bias, which in this amp, is relative, as the plate and screen voltages are far from 'cold/cool' given the tubes specified.