Residual Hum on Matchless DC30 Combo Amps

[nevetslab]

One of my steady clients sent over two of their Matchless DC30 Combo Amps. I think that's all they have in their inventory. I checked our inventory, and see we have four....one that I've never serviced. What I have noticed with this beautifully built amp...is they all have significant HUM when you turn either channel all the way up. There's more gain in Ch 2, which has the EF86 Pentode preamp tube. One of the two had squeal on the Pentode channel, which I was able to cure by swapping out that tube with one that has a shield within a 1" tall tight circular rubber compression ring on the body of the tube. I also had on one of the two amps that came in, substantial hum on V2 tube that follows the volume control for Ch 1, as well as the Driver tube. Both are now as tame as I could get them.

I've noticed all of these DC30 amps appear to have the same high degree of hum. Not that one needs the volume pots turned all the way up to encounter that, but....it still bothers me. I've never made a project out of this, nor has any client requested their DC30 to be cured of it.





This morning, after swapping V1 & V2 preamp tubes with each other, which did yield better residual noise/hum than before, I connected the output of the amp to my HP 3580A Spectrum Analyzer and collected these two plots of the residual hum/noise...one with the Volume controls at full CCW, the other with that trace plus Ch 1's Volume at full CW setting for comparison.



I've thought about connecting my HP 712C Power Supply up to this amp, to see what the residual noise looks like without it's healthy-looking Power Xfmr turned on and powering the circuits.

Anyone out there find similar behavior with their Matchless DC30 amps?

[greggoad]

I just worked on a DC-30 and had the same impression. There was a lot of hum for a circuit like this. When I look at how all those 6V AC heater wires are run i'm not surprised. There are heater wires right next to the pots and input jacks. Not something I'd do. I also noticed all of the shoulder washers for ground isolation on the input and speaker jacks were breaking. It looks like they used short jacks and overtightened and that deformed the isolation washers. I think some ground loops had been created by unintentional contact with the chassis. This helped the noise floor but did not get rid of the hum. I know it's not filtering because the hum is totally gone if you pull the PI tube. I often eliminate hum in amps for customers, but with an amp of this value with this much wire running everywhere it's not a can of worms I wanted to open.

Did you figure anything out?

I've heard reports of these amps being dead quiet, but that was not the case with this one. But looking at layout and grounding decisions I'm not surprised.

Greg