Anyways, there's several ways to deal with it that I'm sure most everyone here is familiar with, elevating the heaters, DC supply, and even a regulated supply. I was feeling lazy that afternoon and didn't feel like trying to find a space for any of that. First thing I tried was simply putting some 1uF caps across the 100R heater balance resistors already in the circuit. That's worked fairly well for me in the past but in this case it was insufficient. It was one of those, put one cap in it makes one side better and the other worse, put both in and its back to how it was before. I just happened to have a spare 1mH common mode choke left over from a power supply project I completed recently, heater current is 1.8A, choke was rated for 2A so why not give it a try. It definitely helped but more capacitance was needed to really get the cutoff frequency low enough to fully suppress the noise. I ended up settling on 220uF per leg of the transformer to ground. This completely cleaned up the heater waveform on the scope and knocked out and absolutely massive amount of noise. The amp is now in that scary quiet category where it has a bit of hiss makes you think 'oh thats probably a bit loud' and then you turn up your guitar volume, hit a chord and the whole room vibrates.
There are some practical limitations to this approach however. 220uF has an impedance at 60Hz of ~12 ohms. That results in about half an amp of ripple current in the caps and will shorten their life if they aren't rated to handle it. Really a choke with 10 or even 100x more inductance is needed, so we can use correspondingly smaller caps to get the same level of attenuation. Compared to alternative solutions this can get expensive (relatively speaking) depending on the amount of current needed. 100mH is pretty much out of the question unless you want to roll your own (not hard but kinda a pain especially on a toroid). 10mH rated around a few amps is pretty common though. I'm probably going to order one along with some 22uF caps to give a try.
You might think why not just put it on the primary side where the currents are lower and its potentially an option. In my case the AC mains current at full tilt for this amp is about 800mA so less than half. The big problem is that any caps from AC mains to ground need to be Y2 rated safety capacitors and these are typically quite bulky and only come in fairly low values.
The best approach would probably be to just do the preamp. Maybe even skip the PI depending on the number of tubes involved, and since its the secondary side, we can use basically any appropriately rated cap we want. As to its effectiveness I can't say how it compares to something like heater elevation as I haven't tried that in this specific circuit. The upshot is we get a nice well defined source impedance and can consistently and easily guarantee a certain amount of attenuation of the incoming noise. FWIW this is now the quietest guitar amp I own by far.
Anyway, I don't know how useful that will be to anyone here, but maybe it was at least interesting.
) but it might be. If you have a single biggish transformer supplying AC mains voltage to several units, somebody on the same feed line could do this.