02-26-2016, 12:29 PM
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)
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)
(02-26-2016, 08:42 AM)Roman Ships Wrote: 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.