Re: Hum Measurements

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.

For a full wave CT rectifier one of the transformer HT leads usually has
more resistance than the other. So a line frequency component (60 HZ in NA)
arrives at the filter. The filter is less able to attenuate the 60 HZ than
the 120 HZ in the ripple, so could arrive at your filter output fairly
large. Have a look at the HT at the filter output. One easy fix if that is
the case would be to add a small resistance in the HT lead having the least
resistance so that the rectifier will see equal sources.

If you are using the chassis for the heater returns as some do you should
change the heater leads to twisted pair. The common of the amp & it's PS
should be connected to the chassis only at the signal input connector. Be
careful with the CT lead from the HT winding to the first cap. A serious
ground loop could develop altho in that case the interference would be at
120 HZ. Cheers, JLS

> 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.
>
> 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

.

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