Need advice on wiring variac switch

[bmf5150]

Well I decide I wanted the variac switch in my amp that drops everything including the heaters just like the suhr sl68 and George's amp does.i have heyboer power tyranny with the extra 150v primary to do so..here is the wiring layout below that I came up with.just want to make sure it's correct,get your guys opinion..

Here is my power tranny wire color code
Primary
white-0,black-120, black/white-150
Secondary HV
red-185, red/blue-152, red/yel-0, red/blue-152, red-185 @ 280mA.
bias
blue-0 blue-100v wound w/ same wire as HV, 280mA
fil
grn-3.15, grn/yel-0 , grn-3.15 @7.5 Amp

[VelvetGeorge]

Wire it like this:





george

[bmf5150]

Thanks George,one last question,how do I incorporate the light to the switch.my indicator light is 120v off the switch

[VelvetGeorge]

The top white wire terminal and the black wire from the PT (120V) terminal.

george

[bmf5150]

For bias,do you have to bias the 120v switch position first then the 90v variac mode second?

[VelvetGeorge]

Yes, bias in normal operation first. Then switch variac mode and bias for that.

george

[bmf5150]

Thanks George

[bmf5150]

One last question George,the bias pot on the board controls the standard mode correct,but when switching to variac mode the bias pot on the switch interacts with the one on board??wouldnt the 150v tap primary be on the same side as the bias pot on the switch???

[vh junkie]

The two pots are supposed to interact. The switch mounted pot is shorted out (bypassed) in HIv mode. That is when you adjust the bias on the board for proper HIv bias. Then for LOv mode you leave the board bias pot alone and adjust the switch bias pot for proper LOv bias. At this point, the two pots are in series.

[bmf5150]

Thanks Robert

[VelvetGeorge]

Exactly.

Think of it as the switch adds more resistance in series for variac mode. This lowers bias voltage, increasing bias current in the tubes.

George

[bmf5150]

Thanks for the explanation George

[RagnarMarGunn]

George and all, what is the required rating in amperage for the switch ... and if someone could explain why, the fuse before power switch in 100W MV is rate 4 amp. I need to do this mod on my Metro 100W MV an would like have a switch that is as small as possible, so basically i'll need DPDT before standard power switch which is the Marshall one, just to select between 220vac & 240vac on one side and bias on other. That selection would be done with Marshall power switch off.

[RagnarMarGunn]

No worries, have it figured out, Variac wired on my Metro 2203 Friedman BE-100 Specs with some of my own twists, sounds great.

[jloewenheim]

George, any chance you can repost your diagram of how you wired the variac switch? For some reason, that link does not seem to work.

Cheers
Jake