Presence Squeal

[reswot]

I've built an amp loosely based on a Trainwreck Express/Komet/Weber Java. Basically a Trainwreck circuit, but with tube rectification, and a Lar-Mar PPIMV.

The amp sounds good, but I'm getting a horrible squeeal from the Presence control (it's interactive with the PPIMV, as well). I've poked around with chopsticks, moved the wires in the Presence/feedback circuit around, re-routed the PPIMV wires, and then basically rewired the latter two controls. For a while, I had the wires for the Presence and the PPIMV totally out of the amp to get them away from everything else. I had the PPIMV wires twisted tightly, but even switched them out for shielded wires.

I also put flashing in the head cab underneath the chassis -- and, yes, it does make good contact with the chassis.

I'm still getting a squeal, though, and nothing has made any difference, whatsoever. I actually liked the way I'd wired the amp initially, but I now have the Presence and PPIMV wires flying across the top of the board, rather than underneath...

The squeal goes away when the Presence is turned all the way up. Is this indicative of the need to swap the blue/brown wires from the OT?

[PCollen]

This does not sound like an OT wiring problem, and the oscillations I have heard when the blue and brown OT wires are not connected to the proper tube is more of a lower frequency oscillation..like a woodpecker going at it, or a jack hammer.

One thing confusing to me is that you say it goes away when the presence is turned UP. My experience has been that it goes away when the presence is turned DOWN (below 3 or 4), and gets worse when presence is turned higher on the dial. You might want to triple-check your NFB connections, makeing sure you are connected well at both the presence pot and PI tail resistor . You can disconnect the NFB wire, insulate/isolate the free end so you don't short to something, and then see if the squeal is still going on as before. If not, you may very well have your OT wires to the tube plates reversed.

[Colossal]

One thing confusing to me is that you say it goes away when the presence is turned UP. My experience has been that it goes away when the presence is turned DOWN (below 3 or 4), and gets worse when presence is turned higher on the dial.

Yes, this is exactly right. I have seen this condition in numerous Marshall amps where they start oscillating once both channels (Plexi) are dimed and presence is increased to about 5. The oscillation sounds like a high pitched whistle.

Reswot, PCollen is right; check that your presence pot is wired correctly as it would be expected that the noise increase, not decrease on turning up the presence. I would consider rewiring the presence circuit like the 1987 circuit (see enclosed schematic) as a means to keep DC off the pot. It may not however cure the oscillation issue. That is likely related to lead dress unfortunately or the proximity of the NFB lead to other sensitive parts of the circuit. The Express circuit can be a cruel mistress when everything is seemingly perfect.

[reswot]

I'm pretty sure that everything is wired correctly (I've gone through it twice), but will go back through it again.

I'll check the 1987 wiring and see if that does the trick, too...

[reswot]

Thanks. I removed the NFB wire from the impedence switch and the squeal went away. Then I swapped the blue/brown wires and tested teh amp. Still no squeal, so I reconnected the NFB, and.....



NO SQUEAL!

I'm good, now.

[Cliff Schecht]

It's funny how different the flipped negative feedback squeal is from amp to amp. Sometimes it appears as a nasty motorboating that will easily blow an underrated speaker (done it!). Other times, it's a simple sinusoidal (relatively) high frequency signal that is barely audible, if at all, but is still of course unwanted as well. Sometimes you end up in-between these two extremes but I think the most common case is the HF squeal.

[azatplayer]

That is odd. Whenever ive got the leads reversed, (always the reason youre in a wince ready for it on startup) its been unusable. Massive droning. Instant turnoff and swapping.
BUT, i did just finish my first express, and deliberately left the presence circuit out and went with a cut as i was using a larmar.
Made the amp very different, so in the sakes of science, i tried the presence in it. Could never get it stable. Squealed like a pig, and was worse with the presence going down, as you experienced. This is very interesting. I put the cut back in, tried a few different values and had the same thing. Its rock solid with a 250k pot and .0047 cap. I used the Aikin cut, 1meg pot and 500pf cap, again squealed.
I assumed the leads were ok out of the box as it did not do the motorboat/ drone on startup and runs great with that cut. Could it possibly be reversed? Seems thats what you found. Very odd.

[tubeswell]

that is GFB induced oscillation for sure. 1 'cure' I have tried with success (that I found out about from the guys at the ampage forum) on a marshal style amp that I built that had this last year, was a chunky 5W 220R - 470R resistor across the OT secondary (that's right - between the speaker jack terminals) - it doubles at OT protection and seems to kill oscillation. Worth a try

[azatplayer]

Yes i was reading about this when determining whether to go with the diodes on the EL34's of my express build. My layout ended up a bit close on the jack area so i went diodes. BUT will definitely give it a go in future builds.