Tubemeister 36 Hum on Crunch and Lead channels

Hi Folks, need some help please. I have a Tubemeister 36 which is silent on the clean channel but there is a low freq hum on the crunch and lead channels.
This hum is there with no guitar plugged in. The hum increases/decreases with the adjustment of only the master controls. All other controls have no effect on the hum.
I understand that tube amps have some hum but the hum on my amp starts around the 9oclock position. After 12 oclock, its pretty loud.
I have replaced all the tubes with new ones and tried the amp at different locations.
Have anyone of you experienced this and is there a fix. Thanks.

Hi Edster, sorry to hear about your problem. The TM36 is the most difficult model for me to diagnose as I only have knowledge of the TM18 and GM36 schematics, the TM36 has never escaped into the wild as far as I know. The TM36 has infinitely less complex circuitry than the GM36 with its four channels and digital pots but significantly different circuitry to the TM18 which is only a two 12AX7 setup. I have had a TM36 myself before my GM36 came along but I never had any problems like the ones you describe. All in all, anything I can suggest is only speculation I am afraid, and this one is a tricky type of problem.

All I can think of at the moment is do you have anything in the Fx Loop? If not then try putting a patch cable or a guitar cable into the Send and Return. That way you take the signal out and pump it back in unchanged but you take the switching on the sockets out of the signal path. These switches are a common source of problems on a huge number of amps. If that helps then try cleaning up the sockets with some contact cleaner spray. That's NOT WD40 or similar types which are really penetrating and lubricating sprays. You would need a specific contact cleaner or something like isopropyl alcohol on a swab. And bear in mind it is the switch contacts not the signal contacts I am talking about here.

Does the amp play as it should on all three channels apart from the hum?

Thanks for your suggestions Bordonbert. I have tried the patch cable in the fx loop with no luck. I did clean all the tube sockets with CRC contact cleaner too.
The amp works on all 3 channels with no hum on the clean channel. I have new Tungsol preamp and Electro-Harmonix power-amp tubes.
The only control that varies the hum is the master volume on the crunch and lead channels. This is with no guitar plugged in.

I'm not doubting you as your description is as a "low frequency hum" Edster which is specific. Is that really the case, would you describe it as a "mains" type of thing at that pitch and with a soft rounded character even though it may be loud? Many people describe a buzz at any frequency as a hum when it isn't really. Hum is mainly at 50/60Hz or 100/120Hz (UK/US) caused mainly by the mains breaking through into the signal line and is fairly soft in nature and quite a low pitch. Anything which is rougher sounding with maybe a buzzing quality and of a higher pitch is not usually caused by the same process.