Amp cuts out when driven, HELP!

[WRC34]

I just built a 40-ish watt amp using an old Stancor PT, a Classictone OT and an old Merit choke.

The amp has 6SL7 cascaded one side into the other, into a 6SL7 LTP phase inverter, into a pair of 5881 (the real ones - old Tung Sols) with a 5AR4 providing DC to the unit. It has only one knob, a volume control, placed between the two sides of the first 6SL7.

Initially I built it with the first 6SL7 parallel, but the amp sounded muddy and barely broke up when cranked. After switching the first tube to run in series, the amp has been "cutting out" at higher volumes, like completely shutting off and it takes a few seconds for the sound to come back. From what I have been researching, it seems like the output tubes are being driven into cutoff.

I have done many things to try to fix this - plate resistors on V1 have been switched from 100K on both sides to 55K on both sides and now it has a 180K on the input triode and 100K on the second half.

Plate voltages are currently:

6SL7 (V1)
pin 2 - 150VDC
pin 5 - 180VDC

6SL7 (V2)
pin 2 - 250VDC
pin 5 - 263VDC

5881s
pin 3 - 418 VDC
pin 4 - 398 VDC

I have changed the bias feed resistors from 220K to 200K to 100K to attempt to control the grid drive, no effect.

I upped the grid stoppers on the output tubes from 8.2K to a whopping 55K - nothing.

All coupling caps on preamp stages are .01uf, and from phase inverter to output tubes are .022uf. These are the fairly conservative values people tend to suggest when trying to diagnose this problem, but the issue is still prevalent.

The PT is a Stancor PC-8422.

Could it be a bad preamp tube?

Any advice would rule since I am about to jump out the window.

[WRC34]

more info -

cathode resistors on V1 are 3K bypassed w .68uf and 1K bypassed by 2uf

issue persists with many changes in power tubes - 6L6, EL34 & KT66 all cut out like the 5881s

with a 6SN7 in V1 it is much more stable, but will eventually cut out in a similar fashion. This tells me that I might have some sort of blocking distortion going on with V1. As noted above, I have tried a wide range of plate resistors with no improvement.

[strelok]

It might not be an issue with V1. The 6sn7 has lower gain so its going to take being further up on the volume control to push the amp into the same region of operation that's problematic. What's the bias on the power tubes?

[WRC34]

Bias is around 60% depending on tube type - for the 5881 with plate voltage at 418VDC I have bias set to approx 28mA (one tube draws a few mA more, so the other one is probably at 25mA), with KT66s the plate voltage jump up to around 440VDC, and I set the bias for about 34mA, EL34s around the same.

BUT as I adjust bias (which I am reading in mV on my meter, which I believe translates directly to mA) the plate voltage varies too, the more bias mV I dial in the lower the plate voltage goes, and vice versa. It's not extreme, but it does fluctuate.

[M Fowler]

I'm not picking up on any clues yet.

I've built about the same circuits but with Volume and Tone controls.

Used everything from 12AX7 values on the 6SL7 100k plate/1k5 to 270k plate/2k7 which is more stock values such as Ampeg 15N.

PI LTP worked great 6SL7.

Will keep following incase someone has some ideas here

Mark

[Littlewyan]

I'm guessing when you stop playing the sound comes back again?

[WRC34]

Yep, when I stop playing the sound comes back, but it takes a while. Seems like an extreme case of 'grid clamping' or blocking distortion or what have you.

The chord or two I'm able to eek out before the cut off sound glorious, but obviously there's an issue.

When I do start playing the bias jumps as I play, which I think is normal, correct? Plate voltage also goes down in accordance with bias current going up. This seems normal to me. The meter will read 28mA or so at idle and jump up into the 60s, then the cutoff happens.

I have read about 'dumping gain' via resistors in the preamp sections. Though would I achieve this, what values and where I should place them I don't know.

I have built a few other amps using 6SL7 as phase inverter as well, and had no problems until now.

[Littlewyan]

Bias moving on the output valves is normal but it should always settle back down. Have you got any negative feedback in this amp?

[WRC34]

No negative feedback

[WRC34]

other things I've tried, none have helped -

Originally built with 1M audio as volume. Switched out for 500K

Upped tail resistor in LTP to 47K (from 22K)

Added grid stoppers in series with all coupling caps in preamp (220K value), after coupling caps, directly onto tube sockets

I do have a slightly different filter cap B+ arrangement going in the amp. It goes like this:

5AR4 pin 8 -> STBY switch -> 47uf 500V cap, OT primary & choke -> 47uf 500V cap (choke returns here) -> 3K 5W resistor -> 30uf 500V cap & output tube screens (which each have their own 1.5K 5W resistor tied to pin 6) -> 15K 5W resistor -> 22uf 500V cap (which feeds plates of LTP) ->3K 5W resistor -> 16uf 475V cap (which feeds plates of V1)

[Littlewyan]

Hmm a tough one this is!

Have you measured voltages as you've recreated the issue? Could be something is getting starved.

[JazzGuitarGimp]

Do you have grid leak resistors on all your preamp stages?

[stretch2011]

Hmm i don't know a whole lot. But my friend built a tweed deluxe, 1 input with a parallel first stage. It was driving so hard it would literally delay notes. His power transformer was right on the edge if not over the edge of being too small.

What's the chances your power transformer doesn't have the current capacity to keep up with the output tubes being driven into cutoff?

[Firestorm]

My SE amp will do exactly that if you bridge the effects loop with a wire instead of an effect. I think it's an extreme case of blocking distortion. You charge a cap to the point it won't conduct anymore. Signal won't pass until it bleeds off. I was surprised how long it took, but you could do the math and confirm. A single 12AX7 triode can do up to gain of 100, but we don't wire them that way. More like 50 bypassed and 30 in bypassed. Put another gain stage in series without enough attenuation between and you've got 50x50 or 30x30. Scary numbers.

[WRC34]

Thanks for all your suggestions guys. I really tried to hold off asking here as long as possible!

It was driving so hard it would literally delay notes. His power transformer was right on the edge if not over the edge of being too small.

Yes, I was wondering that myself. I knew going into it that the PT would be working pretty hard beyond it's specs. The HV secondary is 325-0-325 and rated at 150mA. I built a 50 watt EL34 amp using a Stancor PT that was rated for 175mA and it works beautifully, so I didn't think it would be an issue. This afternoon I tried running the amp with a 5V4 and a pair of 6V6s to test this out as I don't think the 6V6s would work the transfrmer as hard - same problem.

I think it's an extreme case of blocking distortion. You charge a cap to the point it won't conduct anymore. Signal won't pass until it bleeds off. I was surprised how long it took, but you could do the math and confirm. A single 12AX7 triode can do up to gain of 100, but we don't wire them that way. More like 50 bypassed and 30 in bypassed. Put another gain stage in series without enough attenuation between and you've got 50x50 or 30x30. Scary numbers.

This. Sounds exactly like what I have going on. I suspect the solution may be...

Do you have grid leak resistors on all your preamp stages?

I don't think so - these would be resistors tied to the grid and ground? Please forgive my ignorance!