Harmony 400.... loud hum

[Doc Stitches]

Harmony 400 amp problem.

Nothing but loud humming noise comming the speaker.

Looks like R1/02/20% ( wired to 6 & 7 on 12AU6 socket ) overheated and shows no reading....what is a good replacement resistor? The schematic on amp says 1 MEG ? ...if I am reading it right.

I will get an isolation transformer and do the safety upgrade on this one. I don't look good with straight hair !

Thanks again for the Red Knob help.
Jim

[bnwitt]

Can you share your schematic. I only have one for the Harmony 400A and h400. Neither show a resistor between those pins.

[Bruce / Mission Amps]

I am not familiar enough with this amp to comment on it specifically.

It is not really unusual to have a resistor to ground on a pentode's screen lug
like this... it serves as a B+ voltage divider.
That is to keep the B+ voltage lower then the plate (anode).
I'd try the 1Meg 1watt resistor and see what happens.
The junction of that point is usually connected to a high voltage filter cap so I'd also put a .1uF-600v cap across the 1Meg resistor, right to ground, allowing a very low impedance to ground.

[Doc Stitches]

Hi Bruce. Thanks for the input. I'll follow your advice.
Jim

[Doc Stitches]

Can you read this schematic? If not give me your mailing address and I will mail a good copy.
Jim

[bnwitt]

Doc,
that's the H400 I had attached to my first post. Funny thing is I don't see a resistor connected between pins 6 an 7 in that schematic.

I do see a 0.02mf 20 volt capacitor going from pin 6 to chassis ground which could be also done by connecting it from pin 6 to pin 7 which is itself grounded. Are you sure that's a resistor?

[Doc Stitches]

Thanks. You are right it is a cap. Th R1 stamped on it threw me off. The amp does work now and sounds good, ...but the buzz is still there.....maybe just a little quieter. It has all 3 original ( Harmony stamped ) tubes.

[bnwitt]

Well if capacitors C4A, C4B and C4C are original, I'd say replace them all to eliminate hum.