Help with some OT oscillations,please

[catalin gramada]

Hello All,
Friend of mine asked to mod a Marshall 900 combo into a 1987 circuit. I reconfigurate the layout and changed the OT tranny to standard one. The new layout was optimised as beter as was able for, but at the end I received some high pitch oscilation when volume, treble and presence was dimmed to 10. I investigate the source and responsable for this oscillation are the long wires from OT primary to the power tube sockets (I can even modulate moving this wires a bit). The big problem for me is the original chasiss design which put the OT at one end and the power tube sockets and power caps at the other. I have to run 15 (fifteen) inch of wires to connect one to another. At the moment the primary wires runs parallel closely tied through a shrink tube. I want to try to twist them or run to a shield tube but I'm not sure if working.
So what should be the simpliest way to get rid of this oscillations in Your opinion ,how should I run this long primary wires , please ? Thank You
Best Regards
Catalin

[uneumann]

Based on your description, I suggest shielding and bundling the 3 wires to the OT primary. Best option is to shield each separately, but a common shield may be ok. Wire insulation rating is a concern. Ideally, you can run hi volt wire in a braid big enough for 3 wires. Bundling will reduce magnet fields.

Other options are to run the wires on top of the chassis using the chassis as a shield. Or, since you have space around the wires, you could tape then to the chassis using foil tape. stewart mac sells copper foil with sticky backing. That will shield and hold them.

[catalin gramada]

Thanks for Your answer
I have to add I THINK it is not a self oscillation phenomena I belive but dependent by the level of "hights" have to amplify,so it is evident present just when all controls are dimmed. I thought before it is from PI an changed the fizz cap over the PI plates from 47p to 56p and also helps a little bit. So oscillation are evident present just in extreme hights amplification, but could be wrong cause I didn't put a scope over to see what is happened at low levels, maybe there are still there... Thank You

[Enzo]

Get your grid wires as far as possible from those wires. Your OT wires appear to be a good two inches out from the rear panel, why not move them back against the rear wall.

Sure you could find some high voltage shielded wire for the OT leads. Or cut a length of right angle stock, and run it down the length of that space between the preamp tubes and the OT wires as a shield.

[uneumann]

Right - I think we're both just guessing a bit here. I can't tell what the root problem is... but I have had similar problems. Any long wire with high gain in front of it (in this case the whole preamp) is suspect IMO. In your case, the OT lines are also carrying relatively high currents so inductive coupling is also a concern. It's common to shield grid lines, but it's the big signal lines that tend to be overlooked. If you shield them, you do much to remove the source of the problem. Having your plate lines run right along the preamp tubes and circuit board can't be a good idea. You're stuck with the layout, so you have to deal with it.

You asked for suggestions - these are my thoughts based on the case you presented.

[catalin gramada]

K, everything good till now. I used a aluminium tube to pass the OT wires through. 90% of my problem gone/ main oscillation goes away, nothing is suspected to be modulated whatever tried to move something arround, it is stable, and shielding helps a lot. Thank You guys. Tried to twist the leads or keep it separate inside the tube did not do any difference.
But still I received a very high pitch swirl at the very end of controls. Seems it's time to look in other place to cure it.
Thank You very much
Catalin

[catalin gramada]

The root of problem was sensitive area of the grids which develop couplings as Enzo said. Little more wiring job and everything working allright now. Thanks for Yours support.
Cheers
Catalin

[GainFreak]

Marshall is usually using a metal shield around the OT to avoid coupling I guess. Also I'm always running a shielded wire from OT to PI. Maybe these will help as well.

[catalin gramada]

Allready done.Thanks
Catalin