Asymmetrical blue pulsing in tubes!

[Mr_bibbles]

Hey y'alll-

Ampeg v4 on the bench, very weird problem.

Sound dips when a large signal is applied to it. Power amp in, acts fine. Preamp out, acts fine. With the master dimed and the preamp about half way up, the tubes begin to pulse with a blue light, pretty intensely. I know some amount is normal, but this looks similar to another amp that had a shorted flyback diode; a pretty extreme blue glow... the strangest thing is that all 4 tubes don't glow simultaneously. One pair will pulse about a half second behind the other pair. While this is happening, you can see the speaker move back and forth on it's own, even if I'm not playing a note. Checked the output for DC, didn't get anything (although the meter did fluctuate up and down, never settling on a number.

Other notes-

You can here a slight "wub wub", usually in sets of 2/3, from time to time. Checked the voltage out of the bias section, it will raise up from -55 to -45 in times with the "wub wub". I've already replaced the electrolytics in this stage, no change.

Grid and Plate voltages fine on tubes, -55 and 515, tubes drawing about 40ma as read on bias probe.

My first instinct is Phase Inverter, but has anyone ever experienced anything like this before? Parasitic oscillation??

Thanks.

http://www.ampegv4.com/schematics.php

[Helmholtz]

"Sounds" like very low frequency motorboating, a subsonic oscillation of the power stage. Mostly caused by bad filter caps. (DC on the output is impossible with a transformer.)

[g1]

So you have the version with the distortion knob and 6AN8 tube?
I'm not clear about the FX loop results (only the 6AN8 version has the series loop with switch on return jack like that).
It sounds like you were describing 2 faults.
First the 'dip' in sound, could you elaborate?
Second, the blue flashing glow (showing the tubes are working hard).
You said, power amp in, works fine. So with large signal, no blue glow, no dip in sound?
And pre-out works fine? So you don't see the 'dip' at the pre-out? But when you use pre-out, the power amp is still connected, so you still see the blue glow and get the 'dip' at the amp output (while checking pre-out)?

[mozz]

Ive seen some 6v6's glow blue inside, actually they were 7408's. Sound was good, power output and bias was good. Just some sort of gas from the factory?

[Leo_Gnardo]

Ive seen some 6v6's glow blue inside, actually they were 7408's. Sound was good, power output and bias was good. Just some sort of gas from the factory?

Electrons get outside the plate & form a sort of cloud between the plate & glass. I notice the blue glow is brighter as more current passes between cathode & plate. And the cloud flashes brighter & dimmer when large signals are applied to the tube.

This information courtesy of the Lost Electricity Reclamation Agency.

[Chuck H]

The volume dip could be phase cancellation or parasitic oscillation sapping the available power. Either of which could also be due to filter caps. I think Helmholtz called the right course of action. I'll add that if the filters in the amp are original then they are old and need replacement anyway, but a bad ground for the filters would also ruin their function, so check that too.

EDIT: I got the right parts for the one and only V4 I've worked on at Fliptops. I think this would be the kit you need:

https://www.fliptops.net/catalog/p-1...ortion-control

[Mr_bibbles]

Thanks for the advice everyone-

First, whoops, that was the wrong schem- I believe this is the right one: https://drtube.com/schematics/ampeg/vt22-v4-70.gif



Seems like all signs point to filter caps. That was my initial thought, so I've actually already replaced those and the symptoms remain. I've double checked my work and the voltages all check out, but I will re-check the grounding schematic as I've switched from cap cans to turret mounted discreet caps. Used 47's instead of 40's in the 100/40 cans, but other than that it should be identical.


So the unanswered question so far is, why would the flashing happen asymmetrically? I've seen plenty of blue, pulsing tubes, healthy and otherwise, but I've never seen one pair flash a half second after the other pair. It always happen at a regular delayed interval of about half a second, like the second set of tubes has a slapback delay pedal hooked up to it...

To clarify further, If i play a huge chord and then mute it, I'll see the 'A' half of the tubes flash in time to the signal, and then the 'b" half flash directly afterwards.

[Chuck H]

Is the blue flash delay from side to side symmetrical? As in: A .5sec B .5sec A .5sec B ???

[g1]

Helmholtz mentioned sub-sonic oscillation. That would be the 'wub-wub' you are hearing and seeing at the speaker cone.
I think the asymmetrical flashing may be due to the PI configuration. These amps also sometimes red plate only one half of the power tubes when pushed hard due to the same PI effect.
The pre-out pwr.in jacks on these are not a series loop, they do not interrupt the signal. But it would be worthwhile to break into the circuit there and apply signal directly to R23 after disconnecting one end of R22.
I've attached a pdf of the schematic so it can be enlarged to see the component numbers better.

[Mr_bibbles]

The flashing only happens a total of once in each half so--->

Signal --> A FLASHES WITH SIGNAL --->. B. FLASHES .5 seconds later, that's it. Doesn't return to the "a" half.

I'll try disconnecting the FX loop and inserting a signal. I wish I had a scope for this one....

After I take a look at the grounds again and see if I can find any other cause of subsonic oscillation.

[Mr_bibbles]

Thanks G1-

Power Amp in works fine when fed from another amp, even when cranked up pretty loudly R22 was removed for this test. No wub wub, and only minimal asymmetrical flashing. Good call on re-testing this section.

Preamp out seems fine too.

SO: subsonic oscillation.... from the preamp section?? The flashing can be accounted for from the design of the PI- a symptom of the failure rather than the failure??

I will start by checking the Preamp B+ feed. Any other ideas from the genius bar??