PT: mechanical hum - heating up - help

[bakerlite]

Ok - this jcm 2000 is breaking my heart - one of those tsl 100 .

The history of the amp is it was given to me and i was told ("she kept blowin fuses".... yeah i know, like for a reason...)
sure enough both HT and mains fse were toast - PT was melted on the primary side and tubes were worse for wear.

Basically  despite initially passing the light bulb limiter test with a new PT it has a savage mechanical hum and is getting real warm. I do notice on the limiter that in standby we have "nightlite/25w brightness" when you flick the standby you get the normal inrush birightness  and return to nightlite brightness but then she sort of pulses up towards what a 50watt bulb would
be so there must  be some sort of a short somewhere.


All voltage check out out on the PT both connected and disconnected.

I do notice that the hum is worse when connecting the HT leads (bias and heaters connected it sounds like a normal PT - hook up the HT and she growls a bit when you take her out of standby)

Have tried with completely different tubes, no different. The main board is new so all filter and bias caps are fine.

have triple checked those pesky Marshall ground connectors and soldered them for good measure.

I get about 20 minutes before I am afraid to leave her on any more -  but perhaps I am being too careful - maybe these marshall PT's run hot with a bit of hum? i do reckon this is too hot though.

she is biased at about 75ma over each pair.

If the last PT melted on the primary side is that relevant? maybe she was drawing too much juice.


It has to be said despite the amp picking up a small bit of the mechanical hum into the speakers she does sound very very good on all 3 channels.

still i reckon if I played her for 30 minutes she'd cook the PT.

stumped