questions about floating paraphase inverters

[antieatingactivist]

I have a few theoretical questions about floating paraphase inverters. I am currently trying to troubleshoot a severe imbalance in an early 70's ampeg v2. Here goes...

Is there a reason that there are very few guitar amps that use this style of inverter?

Ampegs are meant to be played clean. I play it turned all the way up so that the power section distorts. From what I've read, the designer didn't intend on it being used this way. Would turning the preamp all the way up overload the phase inverter, causing an imbalance?

is this style of inverter very picky about using the correct tube that it was designed around. The amp had a "mystery tube" when I got it. It has much lower gain than a 12ax7. would it be worth putting a known 12au7 in?

if there were something going wrong in the NFB circuit, could this cause problems? Could the OPT have something to do with this?

changing to a long tailed pair...good or bad idea?

if something else in the power section was off balance, could it cause an impedence change that would throw off the inverter balance. would pulling the power tubes remedy this?

[teemuk]

is this style of inverter very picky about using the correct tube that it was designed around. The amp had a "mystery tube" when I got it. It has much lower gain than a 12ax7. would it be worth putting a known 12au7 in?

Well, at least according to schematic the 12AU7 is the tube used in the phase inverter.

[antieatingactivist]

Well, at least according to schematic the 12AU7 is the tube used in the phase inverter.

Yes, I was just wondering how much of a difference having a 12**7 in there that the circuit didn't call for. maybe I should order a 12au7 just to be sure.

[Rhodesplyr]

Is there a reason that there are very few guitar amps that use this style of inverter?

All Leslie cabinet amps in the 147 family (unbalanced input) use 6SN7 or 12AU7 paraphase inverters. For some reason, they seem to have fallen out of fashion in the early 1960s in favor of split load and long-tailed pair inverters.

[Diablo]

I have a few theoretical questions about floating paraphase inverters. I am currently trying to troubleshoot a severe imbalance in an early 70's ampeg v2. Here goes...

When you say it's imbalanced, what do you mean? What are the symptoms of this imbalance? According to the notes from the "Tube Wizard", the floating paraphase inverters are an excellent choice for a PI. My early 1970s Ampeg V4 uses the same PI as the V2, and it works great with the 12AU7 tube. And these amps were routinely played at high volume.

[antieatingactivist]

When you say it's imbalanced, what do you mean? What are the symptoms of this imbalance? According to the notes from the "Tube Wizard", the floating paraphase inverters are an excellent choice for a PI. My early 1970s Ampeg V4 uses the same PI as the V2, and it works great with the 12AU7 tube. And these amps were routinely played at high volume.

I am getting extremely different AC voltages coming out of each side of the inverter. This is causing one side of the power section to red plate even at low idle current bias setting. Every cap has been replaced and I checked all the resistor values.

I have found a few threads on various forums with the same problem on V series ampegs. No solutions though.

[Rhodesplyr]

I am getting extremely different AC voltages coming out of each side of the inverter. This is causing one side of the power section to red plate even at low idle current bias setting. Every cap has been replaced and I checked all the resistor values.

I have found a few threads on various forums with the same problem on V series ampegs. No solutions though.

Do you have the test equipment to check for an oscillation outside the normal audio range--either very low or very high frequency? These can be hard to find without an oscilloscope.

[antieatingactivist]

Do you have the test equipment to check for an oscillation outside the normal audio range--either very low or very high frequency? These can be hard to find without an oscilloscope.

No i don't. I have been shopping for an oscilloscope, but haven't gotten anything yet.

It just seems strange to me that only one side of the PI would be oscillating. I just checked the DC voltages. They all seem fine. I also disconnected the nfb and switched the outputs. Sure enough, the overworked power tube changed sides.

This might be a dumb question, but can a resistor test fine on a multimeter, but actually not be doing its job correctly? At this point, it seems like the only thing that could be wrong other than one of the brand new signal caps being bad, or every PI tube that I've tried being bad.

could an oscillation in the preamp make the phase inverter go haywire only on one side?