Ground A/Off/B switch on a Mark IV?

[ketiov]

I've asked in other places, but have no idea what it does. Did some searching here as well but got completely irrelevant topics...

It's just a toggle switch above the input power transformer with A/Off/B options.

This looks like an earlier Mark IV. No reverb switch and has an EVM speaker.

Thanks in advance for any help.

 

[Stonge]

I just talked to Mike B. about this. The middle (OFF) is the quietest the amp is going to run. So keep it in the off position whenever possible. The A and B..is to find a ground, if you getting shocked by the amp. (like if the club has horrible grounding). But always return it to the off position whenever possible. That is how the amp is designed to run at the quietest (noise).

 

[ketiov]

I just talked to Mike B. about this. The middle (OFF) is the quietest the amp is going to run. So keep it in the off position whenever possible. The A and B..is to find a ground, if you getting shocked by the amp. (like if the club has horrible grounding). But always return it to the off position whenever possible. That is how the amp is designed to run at the quietest (noise).

Alright thanks. That's where it's been since I got it.