Silverface MV Twin Hums On Reverb Channel

[Ted]

I've got a 70s Silverface Master Volume Twin Reverb here.
All electrolytics have been relaced and its got a fresh set of valves - including matched Svetlana 6L6s.

It hums on the reverb channel. Looks like 100Hz on the scope.
All voltages are within 5% of the values on the CBS 010182 schematic.

Turning the Tremolo intensity up to 10 with the trem off seems to reduce it. Turning on the trem reduces it.

Removing V4 makes it go away altogether. Shorting the grid of V4b to ground with a cap stops it too.

Playing through the reverb it sounds horrible - like its full of broken glass.

With V6 removed its possible to set the hum balance so there's no audible hum at all.

I know that the effects channel of twins tends to be noisier than the clean channel (which incidentally sounds fine with V4 removed) but there's something not right with this one...

[MWJB]

You seem to have already isolated the problem to V4b or earlier (V2, V3, V4a or b). Pulling V6 will kill the hum, if the problem is earlier in the circuit.

I assume that you have tried substituting V2, V3, V4 for known good valves (try swapping V1 for V2 and V4)? Do you have another reverb tank to try, what did the reverb sound like before you changed V2/V3/V4?

I know you say that voltages read good, but it might be handy for you to list dc voltages at V2-V4.

[pdf64]

The reverb on 70s MV twins tends to sound bad when the MV is turned low. The reverb driver V3 is underbiassed with only 470ohms, so tends to overdrive, which sounds bad.
Does the reverb tone improve with the MV high, channel vol low?
If the valve sub doesn't help but above does, could try taking the 470 cathode resistor on V3 up to 2k2.
Re the hum, is this with a footswitch plugged in?
Have you got a different (but equivilent) reverb tank you can plug into the amp?

[Ted]

Well. A couple of things were confusing the issue....

The tremolo LDR was exposed and was picking up 50 Hz from my working light and the heater balance pot had a broken track.

The 100 Hz hum is there regardless of whether there's a pedal plugged in.

Channel vol low and MV high makes the reverb sound a lot sweeter.

I'm going down the pub. More tomorrow.