Hum in OD Channel of an ODS HRM

[JammyDodger]

To All,

I've noticed that I have more than acceptable level of hum in the ODS/HRM channel. If I switch to the clean channel, it goes away. All volume controls seem to effect the hum as does the entrance trimmer. I've tried moving wires around with little effect? Any suggestions?

Cheers, Mike

[Structo]

Well you are getting hum in the high gain area of the amp.

Is it 60Hz or 120Hz hum?

Does it hum with no guitar plugged in and the volume and master turned up or down?

Make sure your heater wires are twisted tight and routed away from signal carrying wires.

Either have them over the top of the tubes or on the chassis floor behind the tubes.

Also, make sure your standby wiring is away from the ac wires near the transformer. I had a problem in my amp and this was the culprit in mine.

So mainly make sure wires carrying ac are away from dc wiring in the preamp stages.

[heisthl]

Lots of possibilities, other than what was already mentioned it could be relay supply grounding (remove the relay hot lead and use a 9 volt battery to test), could be something in the OD circuit is grounded to somewhere other than the front ground bus (visual inspection). Could be you didn't use the minimum 5 pieces of shielded cable (both pots and entrance trim). If non-HRM could be the snubber caps are too close to the heaters. I just finished a build and the only hum in the OD is when drive and level are both above 3:00 o'clock which I think is pretty typical. I doesn't matter where you tie the shielding on the shielded cable runs but I have seen cases where if you use the pot lugs and are not careful the bare sheild can also make contact with the back of the pot, ruining the front bus grounding scheme. Also not to insult your intellegence, but never ground both ends of the shielding.

[JammyDodger]

Hey All,

Thanks for the replies. I did look closely at the relay. I had butchered the board pretty bad and thought that there may be the problem. As it turns out, I think I had a bad preamp tube. I moving things around and tapping inside, I noticed that V1 seem kinda microphonic so I swap some other tubes in. Hum went away! It's a bit noisier now since the V1 was a nice 7025 but I can live with that.

Cheers, Mike