A closer inspection of Mati’s problem revealed, that it is not a case of a simple humble HUM.
Actually, what it turned out to be, is that the device was HOWLING like a hoard of wolves. Screaming, like if it were fighting for it’s right to live on this planet. This was definitely not hum. These were oscillations. That came to be more or less like this, as soon as the anode voltages settled and the heaters heated up ….
So the reason is definitely not “hum”. These are rude oscillations. Like in an Oscillator. Speaking of oscillators, … what is the difference between an oscillator and an amplifier? In simplest words, using a mental shortcut, … the difference is in 180 degrees.
180 degrees of phase shift. When you are building an amplifier, it is very easy to mix up the cabling of the output transformer. When you do that, you might turn up feeding the negative feedback loop with a signal, that is not NEGATIVE in phase, but positive.
Positive Feedback, which destabilizes the whole device.
Not an anti-phase signal, but an in-phase signal. When you do that, you have essentially converted your amplifier-to-be into an oscillator-that-is. Not a very nice thing to your ears, and potentially very dangerous to your speakers, as it will howl and screem to the full force and extent of it’s power capability … I am not sure that your speakers, especially tweeters, can withstand such a brutal load for more than several seconds. If you hear it howling … switch it off immediately, and take care of the screwed up phase.
But how do I “change” the phase within the feedback loop ?
Easy. Let us suppose that this is the output stage of the PP2012 circuit, slightly modified:
You can change the phase, as received by that lower wire going back to the input stage, in one of two ways,
a). the not so elegant, but quick an safe swap of the wires of the secondary of the output transformer:
b). the more elegant, and conceptually “correct” way to do it, but rather not so safe, as it involves fiddling with four wires on the primary side of the output transformer, like this:
Whichever way you do it, you shall obtain the result that you have converted your havoc shedding screaming siren into a well behaved amplifier.