‘69 era 50 Watt Plexi build squealing problem

[Webby]

Hi.
Sorry about the length of this query, but need to explain my problem fully to hopefully get a diagnosis from all you experts out there!
Back in 2013, I built a ‘69 era Plexi from the Metroamps PDF Instructions, and fitted a LAR MAR PPIMV to it. It sounded pretty awesome from day one, but it’s always had a kind of distant ‘background’ level high pitched squeal when gain is at or near max level. Just recently, I bit the bullet and purchased a Tone King Ironman II 100 Watt Power Attenuator, which allows me to play my amp maxed out at bedroom volumes. It does sound awesome while you’re playing the guitar, but as soon as you stop playing, the high pitched squeal is very noticeable. It’s not pickup squeal because I have tried my Les Paul at high output levels with other amps, and there is no problem. If I touch the metal Attenuator casing however, the squealing gets really loud! This doesn't happen with other amps, so I guess the fault must be with my amp. Some kind of screening issue?
Driving me nuts, so any suggestions would be greatly appreciated! TIA

[danman]

Since the squeal only occurs with the volume near max, it is likely an oscillation in the circuit caused by the amp's lead dress. Some careful adjustment of the socket and tonestack wiring may help to lessen the issue. If you could post some clear pics we may be able to spot something that may help.

[neikeel]

I presume that you have 5k6 or 4k7 grid stoppers on pin 5 of your output tubes??
Also that you carefully routed your preamp tube wires as per the instructions?

Plate wires go down to the chassis floor and along to the tube socket and grid wires are straight from the board then down to the socket. The oscillation is usually manifest from interaction with the grid wires picking up and magnifying stray signals like antennae.

Also check the routing of the presence wire away from other wires (this is usually dependent on pot position).

[Webby]

Hi.
Thanks for your replies.
All of the wire routing follows Georges original PDF instructions and photos religiously. 5k6 Grid Stoppers on Pin 5 of both output tubes, plate wires to chassis floor, preamp tube wires twisted and run along where they should, Grid Wires straight from the board, etc. Will check the Presence Wire routing, but will have followed the photos/diagrams to the letter, so I don’t know why that would be any different from the rest of the instructions?
The Presence Pot is exactly where a ‘69 era 50 Watt Marshall’s Pot would be, at the left side of the chassis (looking from the front of the amp).
Will post some pics as soon as I can. Wish I could post a sound clip! The squeal is immense when you touch the attenuator housing! A low frequency regular pulsing sound as well, that gets slower the longer you have your hand on the metalwork! I’ll check back soon!

[danman]

Disconnect the purple NFB wire from the output jack and try the amp again. Just want to see if disabling the NFB circuit will stop the squeal. The amp will be a touch brighter and break up a little sooner but it's safe to run it with the wire disconnected.

[Webby]

Hi.
Actually, because of your comment, I have noticed that the negative feedback wire is actually connected to the
8 ohm tag on the impedance selector and not to the positive on the output jack!! The common on the selector is connected to the output jack, but not the feedback wire!! Doh!! That’ll probably be it! I’ll get back and let you know that hopefully the squealing has gone! 🤭

[Webby]

Hi all.
Connected NFB wire to positive on jack .... Nope! Still squealing. Disconnected NFB wire ... Nope! Still squealing!
I have temporarily removed LarMar MV, and put back to standard just in case the fault was in the MV mod, but nope! Still squealing.
Have posted lots of pics to see whether anybody can spot anything obvious. Note: I used Classic Tone Transformers/Choke, and not the Martran Transformers suggested in the kit, so wire colors may vary to the original instructions.

[Webby]

Since the squeal only occurs with the volume near max, it is likely an oscillation in the circuit caused by the amp's lead dress. Some careful adjustment of the socket and tonestack wiring may help to lessen the issue. If you could post some clear pics we may be able to spot something that may help.

[danman]

Connected to the 8 ohm tap is fine and will ensure that you always get the same level of nfb no matter where you set the impedance. Attaching to the output jack will allow the nfb level to vary with different impedance settings. I still think it's likely a wire that is picking up some interference from another wire and causing the oscillation. Without a scope to track it down, you will likely need to try adjusting the lead dress in different parts of the circuit and see if it helps.

[Webby]

Thanks Danman.
I have only just joined this forum, so I haven't figured out how to attach photos!
I will try again tomorrow on my Desktop PC, Smartphones are rubbish with this sort of thing!
I knew I must have connected the NFB Wire to the 8 ohm tap for some reason!

[danman]

You will need to use a photo hosting sight to post pics on the forum.

[Webby]

Hi Danman
I’ve put Google Photos on my smartphone, I’ve just got to work out how to use it to post photos on here now! I’ll be back ...

[Webby]

OK?!! Here’s a link to the photos in Google photos ..... Hope it works!

https://photos.app.goo.gl/dVbpYtivLVWYoWUH7

[Webby]

I think its working! Please spot something obviously wrong with my build and put me out of my misery!

[neikeel]

I have had a look at your build - looks nice and tidy

There are no glaring errors that I can see.

When I asked about wire dress I talked about keeping preamp wires apart and the plate wires and grid wires apart. I see a couple of places where that is not the case.

I would also shorten the grid wires on the output valves (that twisted loop around top corner of the board)

Does your amp cab have an aluminium shield inside?