Marshall Plexi Build problem

[ElPedro]

Hi everyone.  I haven't run across this before....So I'm hoping someone has and can point me in the right direction.


I am working on building a 50w Plexi clone built off of the ceriatone '67 Plexi Lead layout. The only change I knowingly made was to eliminate the 1/2 power switch, and then connect pin 6 on both output tubes together, and then one of the pin 6's to the the first 8R2 resistor (S1 on the schematic).  Layout

The problem I have is when you flip the amp off of standby, it sounds like a foghorn sometimes, and sometimes it's a squealing sound.  While the tubes are warming up you can strum the guitar and hear it thru the speaker cabinet.  I noticed that with no output tubes in, the plates are in the 400v + range (can get specifics if needed), but with tubes in they are around 180v.  I have tried several different sets of tubes. The foghorn or squeal changes a little bit when you fool with the knobs on front, except the volumes - they don't have an effect.

The transformers are ClassicTone 40-18023 (PT) and 40-18025 (OT).  There's a Classictone choke in there as well..I think a 40-18032.

At this point, not being sure about what the problem is, I'm not turning it on any more...until I have an idea of whats going on.

Thanks in advance!

[PRR]

"Foghorn" can be reversed NFB.

Disconnect "X" from speaker jack to board (kills NFB); or reverse Blue and Brown OT leads (proper fix, but not a quick check).

[sluckey]

In addition to what PRR said, check the voltage on pin 5 of both output tubes. The amp should be on standby or the output tubes should be pulled for this check. Pin 5 for each tube should be about -40 to -50vdc. Use the bias pot to set to the maximum voltage. Don't put the tubes back in until you have this negative voltage on both pins 5.

[ElPedro]

That was it...The phase was inverted on the OT.  Duh 

I HAVE done that before, but it didn't sound like this.  I should have suspected, especially when the secondary lead colors didn't match the original Marshall...Now that I think of it, I believe there was actually a note on the schematic for the OT that mentions this. 

This situation was a good reinforcement that its best to take a step back from the project when you cannot see the forest thru the trees...

Thanks very much guys!

[sluckey]

Good deal. I approach every new build that uses NFB from the OT as if it will be the wrong phase. There's actually a 50/50 chance that it will be wrong. Most times the symptom is an obvious loud squeal or howl. Sometimes it's more subtle. Either way, that's one of the first things I verify when first powering up a new build.

[Tone Junkie]

Yes I just automatically leave them long until I make sure. I remember my first plexi build had all kinds of problems. Came here you guys helped me fix it. Its been a few years, was the start of my addiction. LOL.
Bill

[MartinB]

I did exactly the same thing on my recent Plexi build.  Because I used a PPIMV it didn't start oscillating until tge MV was turned up (and when it did it was deafeningly loud) and it would give a brief squeal when switching to standby.  It's odd that so many transformer suppliers don't clearly identify which way round they go!

[alerich]

Sometimes you won't know it until you turn it up a bit. I built a Marshall Studio 15 clone and had the OT leads swapped. Played it like that at apartment volume for several months with no ill effects. It sounded great. Took it to my girlfriend's house where I could air it out and {squeal}.

Wouldn't the number of gain stages (phase inversions) impact which way the OT leads would be hooked up in a given amp?


 

[PRR]

> don't clearly identify which way round they go!
Wouldn't the number of gain stages (phase inversions) impact

And, for push-pull, which way you connect the two PI outputs to the finals.

And where NFB is injected.

In theory, if the OT polarity is known, you can work it out ahead of time.

In my experience, the "worked out" polarity is usually wrong.