Joseph Meditz wrote:

I've recently rebuilt the circuit board of my Champ clone that I built
a number of months for a number of reasons the biggest being an
objectionable amount of hum. 60 Hz was plainly audible plus along with
a much louder 120 Hz buzz.

Now I am very happy with the results of the modifications both in terms
of greatly reduced hum and improved tone. However, there is still a
tiny bit of audible hum. I know it is not coming from the pre amp
since its level doesn't change with the volume setting and because it
is still there when I remove the 12AX7.

When you remove the 12AX7 you are disabling the NFB which plays an
important part in keeping the hum in the circuit at a reasonable level. As
well, pulling the 12AX7 allows whatever PS Hum voltage is still at the 3rd
filter cap to be applied to the 6V6 G1 & be amplified. While the 12AX7 is
plugged in it does quite a good job at attenuating that same hum voltage
since it's plate forms a more effective voltage divider with the 100K plate
load then the following 6V6 G1 220K resistor alone. All not obvious without
a careful look at the circuit.

Cheers, John Stewart

On the scope I see a small signal at the speaker terminals. Removing
the speaker and connecting a 10 Ohm resistor I observed a signal of
about 2 mV p-p. It is mostly a 60 Hz with a smaller 120 Hz component
both of which are audible.

Further measurement revealed some faint higher frequency noise of about
7 mV p-p. From these measurements I conclude that in the worst case the
power delivered to the 10 Ohm load is less than 1.5 uW!

The resulting hum is very low in my opinion as I don't recall ever
hearing a quieter guitar tube amp. Nevertheless, on my next project I
will endeavor to reduce it further. I was just surprised that such a
small signal would be audible at all. Is that in line with other
builders experience?

Joe