Voltages idle and under load

[surfsup]

Reeltarded can you elaborate?

swapped primaries makes a nasty noise and messing with PPIMV is the only time I have done that

I took out V1 (stage one and two both fed off the same filter cap). I jumpered teh input to the grid of V2 (the cold 10k stage) which is just after the TS. If I put a maxon overdrive pedal in front of the amp to boost the signal in, the oscillation happens still. I am assuming I can eliminate the components for stage 1, stage 2 and the TS?

It sounds like the note has a tone on top like a big thin sheet of metal getting whacked and then flubbing down to nothing. Only when the amp is overdriven though. (but i might not hear it set to clean).

[Reeltarded]

if you cross the grid feeds it's the same as reversing OT primaries. That is what I meant. If you search here for reversed OT primaries you will find similar outcomes to what you have described, at least audibly.

In my amps any volume causes the scream though.

Installing PPIMV in previously built amps did this to me 20% of the time, at first. I have been watching this thread wishing I knew more to help and it's all I could come up with. You guys are way more techinical that I.

[surfsup]

Are you referring to neg fb turning into positive fb? I don't have feedback installed.

[katopan]

It can still cause a problem even in an amp which doesn't use feedback. Worth swapping the OT primaries and seeing.

[Gaz]

It can still cause a problem even in an amp which doesn't use feedback. Worth swapping the OT primaries and seeing.

Really? Can you elaborate? I've never heard about that before, but I hope it works for surfsup.

[martin manning]

The only thing I can think of is that if there is an unknown lead dress issue, swapping the OT primary leads could eliminate it by flipping the phase of everything downstream.

[surfsup]

I will try this tonight! Thanks!!!!

[surfsup]

Okay I busted outta work at 8hrs on the dot. I reversed the leads from the PI cc's to the MV. seems slightly better but not a fix.

New info I noticed today. I left in V1 (first two stages) and took out V2 (3rd stage cold clipper) and turned it on. I get guitar coming out of the speaker.

How is this possible?

I would think nothing should come out of the speaker. I have two other amps (unfortunately both SE not PP) and pulling the second PreA tube results in pure silence. Since the PI is after this empty socket, I would think no signal should go through since effectively the PS is the only thing connecting the PI to V1. Could this be a bad filter cap(s)?

[Reeltarded]

Slightly better?! Alright I rock!! Yay! (kddng)

You have finally nailed down your issue. You have a signal loop. That accounts for the the trashy phase and confusing noises.

Look at how the first tube MUST be coupled somehow to the PI entrance, right? Even if it's just a little parasite, something is touching something in there. Your problem. There it is.

[Gaz]

I agree, that's gotta be the source of the nasty noise to some degree. In my experience, I've had a similar problem where I still got guitar sound when shorting the grid of the PI. It was very faint, and did not cause a problem like yours, but was annoying because sound would still get though with the master (pre-PI style) on zero. At first I thought it was the pot until I investigated some more.

After some searching online, I thought it may be a grounding or PS decoupling issue. This was in spite of using Merlin's/Kevin O'Connors grounding, and decoupling every stage with it's own filter cap.

After trying many things, I was never able to fix it, and just decided to live with it as it didn't effect the sound of the amp (at least that I could hear). In the end, I attributed it to some sort of capacitive coupling on the board, like the TS circuitry being too close to PI entrance cap... I guess I'll never know.

Looks like there's some more desoldering ahead of you!

[surfsup]

Looks like there's some more desoldering ahead of you!

Yay! Can you feel the excitement in the room?can ya? huh??? kidding aside, I *am* happy to have made a discovery, even though it is painful.

something is touching something in there

Nothing is touching anything and all turrets are spaced .375 on center like every other build on the planet. What would cause this? A bad tube socket (i'm using ceramic sockets)? Bad pot? I could use a little suggestive guidance here please. Thanks so much so far everyone! and all that's left to do is everything...

[katopan]

Pull your valves out and start checking continuity against the schematic. You're on your way now because that definitely shouldn't be happening.

About the swapping the phase in an amp without feedback.... what Martin said.

[surfsup]

Ok so with v1 out but all other tubes in, i don't have any guitar from the speaker. So it is definitely something with v1 inserted, or surrounding v1. Jumping the input to v2 seems to sound fine but i don't get enough gain to push it.

But i don't get it. Even if v1 was doing something weird, how does the signal get to the PI with v2 out?

[Reeltarded]

That is the question. I would drain it and set the meter for tone then listen very closely for the slightest detection between PI grids and every part of the signal path before it without preamp tubes installed. Agree with katopan 100%.

You are really close, no matter how far it seems.

[Structo]

Is this an 18 watter?

I didn't really see where it was identified.

Anyway, I've had bad pots do all kinds of weird stuff.

Learned my lesson last build.

I always measure the total resistance and sweep of the pot before solder.