Squealing oscillation - help!

[EFK]

I've got a situation where I am forced to use very long PI leads. I'm also getting a squealing oscillation when I barely begin to turn up the volume - I know the problem is with these long wires but I'm not sure which and I could use some help trying to figure out what to do. What is more likely to cause the squealing - long parallel plate leads (1 and 6)? Or long parallel grid leads (2 and 7)? ***Would twisting the plate leads help?*** (would love some thoughts on this) How about shielded wire for the 2 grid leads - could this add capacitance that may cause other issues? Or is this less of a concern in the PI? Moving all of these wires around does effect a change, but it still squeals so I need to do something more proactive than just moving them. The more advice the better! Thanks!

[daz]

Did you just build this or put in a new OT? I imagine you know this, but in case you don't, reversing the primary OT wires to the power tube plates may be what you need to do. The amp will do exactly what you're experiencing if they are backwards, and the way you tell is just install the OT and swap the leads if it squeals.

[JHow]

I think in general, it's the grid leads that more sensitive to oscillation. They can pick up signal from the plate leads and other sources and introducing it back into amp as feedback. Many times people will use shielded grid leads, grounded at one end to help this. Shortening the grid wire can also help, but isn't a guaranteed fix. Also playing with the lead dress can help - which is trial and error, "chopsticking". Using solid core wire can help you here, since you can find the dress that kills the squeal and get the wire to stay in that position.

[Merlinb]

***Would twisting the plate leads help?***

Yes. Or at least, it won't do any harm and is worth a try.

[pdf64]

Also try fitting tube socket mounted grid stoppers on the phase splitter (10k-33k) and power tubes (1k5 to 10k, if not already).

[tubeswell]

You can shield PI leads without too much worry about capacitance (as long as they ain't miles too long). try it and see if it helps