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

[Stan Keith]

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.

i measured resistance between CT and primaries, then voltage. Ohms law gave me 60mA per pair of tubes.

[Stan Keith]

I'm going to go ahead and replace the original filter cap cans. I should have done that right off the bat. Most of the time I can just parallel caps to quickly find the culprit. But I ran into at least a couple repairs in the past where paralleling caps didn't work, and had to remove caps from circuit to confirm they were bad. Not so easy with these Ampegs though.
I'll post the results in about a week or so. Taking a few days off to ride motorcycle in Lassen County, Ca. Fun fun.
Thanks for your help, guys.
Stan

[Helmholtz]

I still think the most plausible explanation for low loaded B+, excessive sag and strong ripple would be an open or disconnected rectifier diode. This would leave the amp with half wave rectification causing increased B+ sag and higher ripple. In this case the fundamental ripple frequency would be 60Hz. So analyzing the ripple could tell.

[Stan Keith]

I still think the most plausible explanation for low loaded B+, excessive sag and strong ripple would be an open or disconnected rectifier diode. This would leave the amp with half wave rectification causing increased B+ sag and higher ripple. In this case the fundamental ripple frequency would be 60Hz. So analyzing the ripple could tell.

I did check diodes several times (in circuit). Perhaps they are behaving differently with voltage applied, and under load. I'll go ahead and replace those as well. I'll report back soon.
Any tips on isolating ripple with scope? I tried your suggestion of adjusting time base to "flatten out," but wasn't sure I understood what I was looking at.
Thank you so much for your help troubleshooting this
Stan

[Dave H]

Any tips on isolating ripple with scope? I tried your suggestion of adjusting time base to "flatten out," but wasn't sure I understood what I was looking at.

Does your scope have a 'line sync' option? If it does, slow down the horizontal sweep to 10mS/div, turn it up to clipping and you should be able to see the synced ripple modulating the 1kHz signal envelope. Measure the time between the ripple peaks to determine its frequency.

[Stan Keith]

Does your scope have a 'line sync' option? If it does, slow down the horizontal sweep to 10mS/div, turn it up to clipping and you should be able to see the synced ripple modulating the 1kHz signal envelope. Measure the time between the ripple peaks to determine its frequency.

Awesome troubleshooting too!! I really need to learn this scope, I have a Tektronix 2430A. No line sync option on front panel that I can tell. Does anyone know if this may be a feature I would access in the many hidden menus?
I'm thinking I need to order a new "modern" scope?
Thanks so much Dave H!

[Helmholtz]

I really need to learn this scope, I have a Tektronix 2430A. No line sync option on front panel that I can tell. Does anyone know if this may be a feature I would access in the many hidden menus?
I'm thinking I need to order a new "modern" scope?
Thanks so much Dave H!

I really don't think you need a new scope. This is a very good scope, I have the same. It offers excellent trigger capabililties. Just press "Source" in the trigger area on the right and select "Line". It may be necessary to adjust the trigger level to get a steady display. Always a good idea to read the scope's manual.

Do you have a high voltage 10:1 or 100:1 probe?

[Stan Keith]

I really don't think you need a new scope. This is a very good scope, I have the same. It offers excellent trigger capabilities. Just press "Source" in the trigger area on the right and select "Line". It may be necessary to adjust the trigger level to get a steady display. Always a good idea to read the scope's manual.

Do you have a high voltage 10:1 or 100:1 probe?

Awesome! Thanks for the info. Just checked on scope, and line sync is indeed there. Unfortunately, I just started getting the 9000 trigger fail on self-diagnostic at startup.
It happened before, and I was able to bring it back trying different auto diagnostic, and self test. But I forgot what I did. Funny thing is, the triggering seems to be working. I'm able to adjust to where it's not jumping around, and I can easily check output wattage. Any tips on this 9000 fail problem?
Just purchased a digital power supply to calibrate with the removal of internal jumper.
I put another amp on the bench to repair before I take off for a few days.
Yes, I do have 10X probes.
Thanks again (everyone)
I feel like I am learning from the best.
Stan

[Stan Keith]

Thought I should pop in with an update, and put a finish on this thread.
Replaced all filter caps. Measured 105 clean watts!
Something I should have done right off the bat, and not take up everyone's time. But again, I measured caps with cap meter, and they all tested good with low ESR.
Probably missed the bad one thinking I'd already measured it. Or meter is inaccurate.
Anyway, thanks for everyone's input.
Cheers!
Stan

[The Dude]

If you just checked ESR with an ESR meter, that's not a great indication of a good cap. An ESR meter is like a tube tester, in that if the cap shows high ESR it's bad. If it doesn't, it could still be bad. I doubt there's anything wrong with the tester. It's just a limited test. For instance, if a 100uF capacitor is down to 20uF, it might still show good on an ESR tester because it's still a 20uF cap. However, it's not going to provide the filtering the circuit was designed to have.