Weird sound through cabinet with tube amp (video inside)

[ScoopMyMidsDaddy]

Alright so about a week ago I made a post regarding a vintage marshall kit build I got that was pretty wonky. Well I got it working and it ran through my 8ohm 12" speaker I was using for a load with no issue. Sounds great actually.

Here's is my current issue. When ran through either of my 4x12 cabinets I get this sound once the tubes warm up. The oscillating sound you hear is actually a second or two after I switch the amp to off. Amp doesn't say what ohm its output is but I tried both a 8ohm cabinet and 16 ohm cabinet. Both have this same sound.

https://streamable.com/ege94l

[Helmholtz]

When ran through either of my 4x12 cabinets I get this sound once the tubes warm up. The oscillating sound you hear is actually a second or two after I switch the amp to off.

Not clear what you mean here: Strange sound with amp on or after switching off?

Can you post a schematic?

Do you have a scope?

[ScoopMyMidsDaddy]

Not clear what you mean here: Strange sound with amp on or after switching off?

Can you post a schematic?

Do you have a scope?

No schematic yet I'm in the process of making one because this was a random home built, no scope on hand but my buddy has one if we need to go down that route.

The video posted starts right as I turn the amp on. Once the tubes warm up it starts making the loud noise through the cab, once I flip the amp off it goes to that oscillating sound as the amp powers down. This doesn't happen through a single 12" 8 ohm speaker I used. I tried two different cabinets with two different speaker cables.

[g1]

So it does this sound with 8 ohm 4x12 cab, but not with 8 ohm speaker? Using same speaker cable?

[Chuck H]

Microphonic tube maybe?

Plug the amp into the 1x12 cab that it seems to work with and then tap the tubes (including power tubes) to see if any are grossly microphonic.

EDIT: Oh, and does the amp do this when no guitar is plugged in?

[xtian]

I replied to OP's thread on TGP. I'm guessing positive feedback. Try disconnecting NFB and see if issue persists.

[Chuck H]

I replied to OP's thread on TGP. I'm guessing positive feedback. Try disconnecting NFB and see if issue persists.

Good call. But why is SMMD asking here when he clearly had competent advice at TGP?

Anyway... There will be differences in the feedback signal @ frequency between the 1x12 and the 4x12 so this would explain the cabinet sensitivity and oscillation at shut off. Instability was my first thought but I didn't consider why it might be speaker sensitive. I'm remembering a build I did long ago where the OT secondary ends were colored backwards. The amp worked (sort of) ok in most situations but the tone had a little reedy timbre and the amp was unstable at some extreme settings. It took me a bit to figure that one out since I didn't have instant screeching one usually experiences with reversed OT leads. Point being, it's possible to have such an error in the wiring without the classic and obvious synptoms.

[ScoopMyMidsDaddy]

Good call. But why is SMMD asking here when he clearly had competent advice at TGP?

Anyway... There will be differences in the feedback signal @ frequency between the 1x12 and the 4x12 so this would explain the cabinet sensitivity and oscillation at shut off. Instability was my first thought but I didn't consider why it might be speaker sensitive. I'm remembering a build I did long ago where the OT secondary ends were colored backwards. The amp worked (sort of) ok in most situations but the tone had a little reedy timbre and the amp was unstable at some extreme settings. It took me a bit to figure that one out since I didn't have instant screeching one usually experiences with reversed OT leads. Point being, it's possible to have such an error in the wiring without the classic and obvious synptoms.

Posted a few places because more eyes on the situation can't hurt and any advice is greatly appreciated. Will try swapping the leads, I was just unsure with it working on the single speaker.

Microphonic tube maybe?

Plug the amp into the 1x12 cab that it seems to work with and then tap the tubes (including power tubes) to see if any are grossly microphonic.

EDIT: Oh, and does the amp do this when no guitar is plugged in?

Amp does the same thing with no guitar plugged in. Will try the tapping method on it.

So it does this sound with 8 ohm 4x12 cab, but not with 8 ohm speaker? Using same speaker cable?

Same speaker cable. Tried with two different 4x12s and also different speaker cables.

[Chuck H]

Don't just swap the OT leads! Extra work and more invasive. Can cause extra problem with human errors. xtian suggested disconnecting the feedback loop. That is what I would do first. Just disconnect one end of the resistor on the board with a lead running to the impedance sector on one end and the presence control on the other. But only one end of that resistor. Other connections must be maintained. This will eliminate the feedback loop. Please try this and report.