Matchless DC30 hum

[kgreene]

Any techs out there ever encountered complaints of a DC30 having too much "hum", especially as the amp is turned up louder?
I have serviced this guys amp in the past several times. He is a full-time working pro, and did have some issues on occasion. But overall, my impression of that amp was that it was built like a "brick shit-house". The caps seem in good order, and I don't detect anything amiss anywhere else, however I do have to admit the 60/120 cycle hum is more than I would expect. Any thoughts?

[kgreene]

And tubes have all been subbed and ruled out.

[Gaz]

Heater hum perhaps. Try subbing a hum balance/dinger pot for the grounded heater winding's CT and see if that affects anything. No NFB amps with post-PI master volumes are nosier by nature.

[Diablo1]

I built a DC30 clone from a ceriatone kit. The schematic and components are identical values (caps, resistors and pots) to the DC30, and uses original spec transformers (TDS). The layout and ground scheme is different than the DC30 however. In any event, my amp is dead quiet, so I would suggest that the hum problem you're having is not caused by the circuit design.

[Gaz]

I wasn't suggesting that, just making a generalization. Amps without NFB are noisier all else being equal.

[Diablo1]

Is the hum issue the same on both channels? The preamp circuits are very different between the two channels. EF86 tubes can be more problematic than a typical 12AX7. Try to isolate the hum to determine if it's in the preamp, phase inverter or output section.

[kgreene]

I believe the hum is present in both channels. My experience with these amps is that they are very quiet typically. One bit of info: this amp sat for one or two years unplayed. I was tempted to try re-forming the filter caps, but I haven't ever had to to that on a modern amp like this.

[Gaz]

Diablo's right, pulling tubes is the quick and dirty way to isolate a hum - that or grounding grids methodically.

[Diablo1]

I believe the hum is present in both channels. My experience with these amps is that they are very quiet typically. One bit of info: this amp sat for one or two years unplayed. I was tempted to try re-forming the filter caps, but I haven't ever had to to that on a modern amp like this.

Clip in a new cap or two in parallel to the existing caps to see if the hum gets improved. That'll tell you if your caps have lost capacitance.