Beating between two notes

[BloozeGit]

Hi folks,

I've built a Derailment , my first gain-ish amp compared to my other 5E3 and Champ-like projects and while it's working pretty well, there's just something that bothers me.

Whenever I play double stops on the high strings at high-gain settings, there seems to be an oddly discordant low frequency that I can only guess is some sort of beating phenomena. It happens through both 8" and 12" speakers, so I don't think it's a speaker issue (barring coincidence).

Can anyone explain what's going on here and how to fix it? Or am I just hearing things?

[pdf64]

It could be 2 things-
1. inadequate reservoir cap (the 47uF after the rectifier). Try measuring the Vac across this cap, at no signal and then cranked, when the beating is occuring. Or just double up on that cap, ie to 100uF, to see if that helps.
2. too much sub harmonics getting through, need more high pass filtering in the signal chain. Try reducing the 470k after the master vol control to 220k, or reduce the 0.02uF cap feeding the master vol control to 0.01 or 0.0047uF.

[mooreamps]

I don't know if this helps, but something to look at. When I do a build, I use the single point ground. OK, fair enough. I had a small problem; like this ; in one amp I built . It turned out, I had a ground wire pinched and shorted to the chassis. This in effect, created a ground loop under my high impedance pre-amp. So, initially, I did not see it when doing resistance checks on my circuit card with a mult-meter. But the problem would always go away when I powered on the amp, and then lifted the circuit card off the chassis... If this is not the case, perhaps if you could post a schematic for us to look at.

good luck,
-g