1970s Ampeg V4 Strange Input Stage Behavior

[g1]

I remember another thread where it was found that one side of the output red-plated sooner than the other due to the PI type (not long-tail). The duty cycle is different so at hard clip it gets asymmetrical. It was deemed normal for this model.
The bias is much better now.
At just barely the onset of clipping, can you get 90V pk-pk ? Can you show a scope pic?

[Helmholtz]

The same distorted plot shown at output as in post46 https://music-electronics-forum.com/...064#post957064 (pic1). Swapped neg/pos sides and even from inside to outside with no change.

With such heavy clipping a number of ugly things happen like power tube grid conduction, PI outputs getting asymmetrical, blocking distortion, NFB dropping out and more.
Reduce signal level until one of the output signal sides doesn't clip.

I don't think a <10% drift of a PI plate resistor will make much (audible) difference.

As you are getting 50W of clean output, I don't think the amp (still) has a problem.
Maybe you cured some initial issue without noticing it.
Or the vol. knob slightly slipped so the numbers look different now.

[dubulup]

At just barely the onset of clipping, can you get 90V pk-pk ? Can you show a scope pic?

very close...but not quite 90Vpp

I don't think a <10% drift of a PI plate resistor will make much (audible) difference.

correct, I set up 3 series resistors as a test...no change; original going back in.

[Helmholtz]

very close...but not quite 90Vpp

Why don't you use the peak-to-peak and RMS measuring options of your scope?

[dubulup]

Maybe you cured some initial issue without noticing it.

you may be on to something...when removing R32/33, I noticed or knocked the ext_amp input wire loose. I went back to my intake pictures and it definitely shows this connection soldered to the wrong point (between R25/26 where the feedback ties in); it should be tied into R21/22/23 junction at the CF output. I never could get my personal amp to drive the output stage of this V4 using this connection. I'll repair this and try that again.

[g1]

very close...but not quite 90Vpp

That's perfect. Scope shot is a great example of how you can bias cold (50% in this case) and have no crossover notch at full clean power out.
Have you had a chance to try out the owners cab yet?

[dubulup]

Why don't you use the peak-to-peak and RMS measuring options of your scope?

ok so it is 90Vpp at a hair past noon...I guess you can't see the figures in this low-res photo

[dubulup]

That's perfect. Scope shot is a great example of how you can bias cold (50% in this case) and have no crossover notch at full clean power out.
Have you had a chance to try out the owners cab yet?

This week...will report back his thoughts.

[Helmholtz]

ok so it is 90Vpp at a hair past noon...I guess you can't see the figures in this low-res photo

Great little scope!
Have the Siglent SDS 1202X-E since a couple of weeks and just love it.
You can easily save a screen shot on a flash drive and post it here.

Forget the numbers on pot dials. They don't have a real meaning. Only show more - or less, but not how much.

[Enzo]

I don't think a <10% drift of a PI plate resistor will make much (audible) difference.

Unless I am confused, these are not plate resistors. Not plate load resistors anyway. They are in series and across the PI plate to plate. The junction of the two is the point taken from which the lower triode grid gets its signal.

[Helmholtz]

Unless I am confused, these are not plate resistors. Not plate load resistors anyway. They are in series and across the PI plate to plate. The junction of the two is the point taken from which the lower triode grid gets its signal.

You're right, I meant (one of) the balancing resistors between the plates of the self-balancing paraphase PI.

[g1]

The crossover notch being not mid-point in the post # 60 (with matched tubes) I believe is an artifact of overdriving this type PI. Though it is barely discernible in post # 67 it looks to be mid-point of the waveform now.
If you make the crossover notch even more pronounced with the bias, I think we will see the output is now balanced up til full power clipping.

[dubulup]

This week...will report back his thoughts.

all is back and strong on the 16R cabinet! Big thank you to everyone who read and shared thoughts, opinions and insights!

I did come across a strange pilot lamp issue. Once the new lamp showed up, I removed the original and tied it straight to 120Vac and it burned strong…no flickering. So I tapped it straight to the power terminal strip it I was still streaky. Not liking the way the new one looked, I fed this one back through the chassis and wired it in…flickering, WTF…removed again and re-wired the entire power TS. Light was steady testing and once back in the chassis it flickered. Turned out…if I pulled the lamp base away from the chassis it held steady…the moment it came within millimeters of the chassis it began flickering again. Fiddle around a little more…and gave up.

wired the new lamp in and it worked like a charm; just not as cool as the original.

[g1]

I did finally find the old thread where the imbalance of the PI was discussed. It ends up causing one side of push-pull to red-plate before the other when heavily overdriven. It is normal behavior for this model due to the type of phase inverter circuit.
https://music-electronics-forum.com/...496#post539496