1970 Ampeg V4 positive/negative peaks clipping as signal oscillates.

[Stan Keith]

Tried everything under the sun, but still only getting about 40 watts, then starts clipping (on scope). Sounds clean, and pretty loud up until then. But this should be putting out around 100 watts.
Tubes ok, and biased correctly. Caps seem okay. Disconnected loop, and signal seems fine up to output grids. Tried moving around lead dress. No change.
As I increase amplitude of 1k signal it starts to oscillate, and +/- peaks alternately clip then no clip.
Even though I subbed in new caps over plate supply section, I suppose I should just completely disconnect old caps. But I've already spent way to much time repairing other problems with amp. Any suggestions are appreciated.
Thanks,
Stan
Probe set on 10X https://drive.google.com/open?id=11k...IM66iXYIq8aKSD

[nevetslab]

Tried everything under the sun, but still only getting about 40 watts, then starts clipping (on scope). Sounds clean, and pretty loud up until then. But this should be putting out around 100 watts.
Tubes ok, and biased correctly. Caps seem okay. Disconnected loop, and signal seems fine up to output grids. Tried moving around lead dress. No change.
As I increase amplitude of 1k signal it starts to oscillate, and +/- peaks alternately clip then no clip.
Even though I subbed in new caps over plate supply section, I suppose I should just completely disconnect old caps. But I've already spent way to much time repairing other problems with amp. Any suggestions are appreciated.
Thanks,
Stan

Do you get the same results patching your input signal into the Preamp Output/Power Amp Input jack? What happens if you remove on pair of output tubes? Try with each pair separately, just in case it's related to tube socket position. You already indicated the signal drive thru the driver stage to the grids of the power tubes is ok. When the output stage clips early, are you seeing that at the grids of the power tubes, with the FB loop intact? All screen resistors ok?

[Stan Keith]

Do you get the same results patching your input signal into the Preamp Output/Power Amp Input jack? What happens if you remove on pair of output tubes? Try with each pair separately, just in case it's related to tube socket position. You already indicated the signal drive thru the driver stage to the grids of the power tubes is ok. When the output stage clips early, are you seeing that at the grids of the power tubes, with the FB loop intact? All screen resistors ok?

Yes to inserting signal at power amp in. I previously pulled tubes two at a time. No change. With all tubes pulled, signal is okay at grids. Same with disconnecting loop. Upon clipping, the fb loop is inserting back into circuit prior to phase inverter, so I see clipping/oscillating at grid of phase inverter. Tried to link a video of scope showing clipping, but apparently I can't link from my google drive? Anyway, I'll try to attach schematic.
Thanks so much for your suggestions.
Schematic- http://www.ampegv4.com/images/schematics/V4.jpg
Scope with probe on 10X https://drive.google.com/open?id=11k...IM66iXYIq8aKSD into 8 ohm load.

[g1]

With good output tubes you should probably be able to get a bit more than 100W.
What is your supply voltage at pins 3 and 4 of power tubes?

The google drive thing won't open for me, but it could be my old system. Anybody else able to see it?

[Enzo]

Google drive image opens for me. Scope trace looks to me offhand like a sine wave with ripple modulation. Are you triggering off the source signal or off the amp output?

[g1]

Ok, it's working for me now, or at least let me download it.
The scope shows around 2VRMS. Is that at the grids? You should be able to get around 40VRMS (56Vpk) at the power tube grids for full power.
What is the max at the grid pins when you had power tubes removed?

[Stan Keith]

Hey Enzo,
Triggering off of output. Here is another vid using my $40.00 miniscope- https://drive.google.com/file/d/11oU...ew?usp=sharing
Thanks for your help!
Stan

[Stan Keith]

Ok, it's working for me now, or at least let me download it.
The scope shows around 2VRMS. Is that at the grids? You should be able to get around 40VRMS (56Vpk) at the power tube grids for full power.
What is the max at the grid pins when you had power tubes removed?

I am getting around 40 RMS at grids.
The 2VRMS you're seeing is actually output of around 20 V RMS across 8 ohm load. Probe is set on 10X
This scope is set to 1X.- https://drive.google.com/file/d/11oU...ew?usp=sharing
Thanks for jumping in.

[Helmholtz]

As Enzo said, you don't have (parasitic) oscillation but rather PS ripple (amplitude) modulation.
You can watch the ripple modulation on the scope by squeezing the signal display horizontally (time base) until you see a bright band with the modulation on its envelope.
So I suspect the power supply, especially reservoir and screen filter caps.
What is your B+ at full output?

[Stan Keith]

As Enzo said, you don't have (parasitic) oscillation but rather PS ripple (amplitude) modulation.
You can watch the ripple modulation on the scope by squeezing the signal display horizontally (time base) until you see a bright band with the modulation on its envelope.
So I suspect the power supply, especially reservoir and screen filter caps.
What is your B+ at full output?

Yeah, I know it's modulating at a lower frequency than the parasitic area. Was in a fog after hours of troubleshooting different problems.
I subbed caps across main filters without much change, and there is no noticeable hum. I guess the push-pull output is doing it's job?
Anyway, the B+ with no signal is around 450V. Square waving output it drops to 379V.
I'm going to replace main filter caps after all. So much for an easy repair. ha
I'm still learning after doing this a few years. I few like a total newbie around you guys.
Thanks for your time, it's much appreciated.

[Helmholtz]

Anyway, the B+ with no signal is around 450V.

That's a lot lower than the 545V in the schematic.

Square waving output it drops to 379V.

And that means a lot of sag. That low B+ won't allow for more than around 60W clean output, and this only with good new tubes.
Did you check screen resistors and screen voltages?

Are all 4 B+ rectifier diodes good? What's the ACV across the PT HV winding at idle?

What's your actual mains and heater voltage?

[Stan Keith]

Screen resistors and diodes good.
ACV on PT is 400~V.
Mains voltage is 120ish. Heaters are 6.28 ~V.
Transformer appears to be OEM.
Looks like I need to replace caps and diodes at this point, even though they test okay?
Judging from PT voltage should be getting around 565 unloaded VDC, correct?
Just pulled tubes, and am getting 509 VDC.
Thanks for your suggestions. This is fun.

[g1]

Yes your B+ (509 unloaded) is low. Is there a lot of AC ripple on it?
You mentioned 'biased correctly', do you mean stock condition? They are biased fairly cold. Warming the bias will also pull down the loaded B+.

[nevetslab]

Yes your B+ (509 unloaded) is low. Is there a lot of AC ripple on it?
You mentioned 'biased correctly', do you mean stock condition? They are biased fairly cold. Warming the bias will also pull down the loaded B+.

How much plate current (per tube) are you running? You'll have to measure across the plate resistors of the four tubes, or if you first measure the DCR between OT Primary CT and each primary lead, the DCR drop and computer the current feeding each pair of tubes. Or, with a DC Current Probe, the total per pair of tubes sensed thru the primary leads feeding the plates. 30mA per tube would be typical, 40mA per tube might be on the high side.

[Stan Keith]

Yes your B+ (509 unloaded) is low. Is there a lot of AC ripple on it?
You mentioned 'biased correctly', do you mean stock condition? They are biased fairly cold. Warming the bias will also pull down the loaded B+.

There is definitely ripple. Were you able to observe videos? 60mA per side (30mA for each tube). Initially, It was biased way cold, (like 12mA per tube) with heavy Xover distortion. So Ampeg's -60ish (cold) bias voltage may explain the higher idle voltage in schematic?
Thanks