I finally got the whistle out

[Wayne Alexander]

My A1a had been plagued since the first day with a very high-pitched oscillation that would really get loud with the presence and the (post-phase inverter) master volume all the way up, though it was less with a Hotplate in use. I had rebuilt various sections of the amp, shielding the negative feedback wire, the wire from v1b grid to the volume control, the wires from the volume control to the bright switch, I put little (100pf)caps across the plates of all the preamp tubes, none of that got rid of it completely. I finally rewired the bright switch (I'm using a Carling DPST on-on to switch between a 100pf and a 250pf cap now). With no cap on one side of the switch and a 500pf cap on the other side, switching the cap in made a large jump in volume and gain, with too many overtones. The stock 100pf and 500pf values tended some reason to cause that oscillation that had been plaguing me, when I finally switched to 100pf and 250pf caps that did it. Possibly it was the cap brands- I was using Cornell Dublier Silver Micas and now I'm using some other unknown silver micas that I got from Antique Electronics. Anyway, if you're getting a high-pitched oscillation, swap components on your bright switch; I'm unclear at present whether shielding the wire between that and the Volume pot, or going to a 250 instead of a 500pf on one side made a difference. I also put a piece of sheet metal the size of the chassis under it in the head cabinet. I don't think it related to the oscillation, but it seems better to close the bottom of the chassis with metal than leaving it open, bolted into the wood chassis. That shields it better and makes it a little less noisy, though I get a fair amount of background hiss at idle, like many gainy amps do. It doesn't interfere with the sound when the thing's in use.

[Elcabong]

Good to hear that you fixed the problem. If I understand correctly, you had to replace both capacitors to eliminate the problem. This is weird.
You may have to put the cap back in to be sure they were the problem

[Wayne Alexander]

I'm not sure if it was the fact of a 500pf being in the system, or the particluar caps themselves, but it was driving everyone crazy, so I'm glad it's gone. I tossed the old caps, so there's no way to know for sure. I suppose Cornell-Dublier caps silver mica caps could be microphonic, though this is a first for me. Of course, the Express is somewhat of a volatile design, all sorts of things that would work in a normal amp cause oscillations, etc. in the Express.