Hall VVR on Marshall JCM 800

Fatfingers

Has anyone fitted VVR to a JCM 800? I put it on my Ampmaker SE5 and it works superbly. However, I also put it on an old Selmer Treble 'n' Bass 50 which has the bass channel modified to be a Trainwreck Express clone and it didn't sound as good on that. I'm not sure whether that's because it just works better on cathode biased amps rather than fixed bias ones. I don't know. There's only one way to find out, though.

What I plan to do is replace the two Marshall rocker switches (mains and standby) with a single toggle switch with a centre off. That way I can mount the VVR control next to it and avoid having to drill the front panel at all. Does anyone know where I might get a suitable switch. Google searches have so far drawn a blank, save for Amazon USA, who won't ship to the UK (there be dragons etc).

Fatfingers

I bought a suitable switch - a 4PDT with a centre off. So I can use it as up for mains and down for standby. I was obviously planning on wiring it so that the mains was on in either the up or down position. However, having looked at the two Marshall rocker switches I've noticed a black wire on one side of the mains switch. It's connected to the neutral AC when the switch is OFF. Anyone know why? Is it something to do with draining the filter caps so that anyone poking about doesn't get a jolt? And if so, what can I do with it? Isolate it and leave it unconnected?

Here's the black wire I'm talking about (top right). When the mains switch is in the off position it's connected to the amp's mains fuse.

Here's the 4PDT toggle switch I wanted to use so that I can avoid cutting any holes in the front panel.

Misterg

Fatfingers said:

I'm not sure whether that's because it just works better on cathode biased amps rather than fixed bias ones.

Ideally you would need a VVR3 for a fixed bias amp (which tracks bias voltage in line with the HT). It looks like Ampmaker only stock the plain VVR for cathode biased amps, though.

I fitted a home-brew version to a TW clone, and (IMHO) it worked well.

Fatfingers

I have a VVR3. Just a bit puzzled about that mains switch now.

ICBM

The black wire is the supply to the neon - it comes from the 110V tap on the PT so it's independent of the voltage selection. It's not connected to the middle terminal even when the switch is up, the switch is not like that inside.

This arrangement can be a bit of a bind when you have to replace those switches, since almost all available switches have the neon wired internally to the live and neutral connections now, as well as for what you want to do.

I would use the second speaker jack hole for your VVR control. If you're trying to reduce the volume you won't be using two cabs .

Fatfingers

Blimey. Thanks ICBM. I'd never have guessed at that arrangement. How strange!

ICBM

For what it's worth I wouldn't use that 4PDT for switching high voltages either - or not unless you only use three of the poles - it's been badly manufactured. The middle terminals for the two left-hand poles in the pic are far too close together and are an arcing risk at the sort of voltages in either mains or the standby wiring in a Marshall. I'm not sure the four middle outer terminals aren't too close to the casing tabs either. Very poor...

Fatfingers

I'm actually having second thoughts about the VVR. I just tried it again with the air brake and it really sings on the setting before the bedroom one. I think I'll live with it for a bit before I do anything else.

John_A

A PPIMV works great on a JCM800, much easier mod than VVR on a high power fixed bias amp