low power switch buzz

[dscottguitars]

Hello again...

I'm finishing up this amp and have a loud buzz problem when I switch it into low power mode.  It is really quiet with only slight hiss in high power, but when I switch to low power I get a nasty buzz.

The voltages on the plates are:

High-490v
Low-340v

Adjusting the bias does not help.

I've done this before but not with such a spread of 150v.  The other amps I did this with were 420v down to 320v.  I didn't get any more noise from them either.

Any suggestions as to why with this one?

I'm really looking for that low voltage grind like Eddie Van Halen's brown sound using a variac on the wall plug.

Thanks...

Daniel

The picture is not current.  I finished hooking things up but it shows the layout.  I have the bypass caps grounded because the footswitch is not finished either.

[eleventeen]

I doubt this has anything to do w/your noise, but why do you have a switch that turns off the heater on the 5Y3?

Never mind, I see it's to be able to select between tube & SS recto.

[dscottguitars]

I think I have found where the problem is but I don't know why.

I noticed on the first drawing that the switch is not in the correct place.  I put in before the main reservoir caps as shown in this drawing.  My other amps are like the first drawing too.  For some reason I thought it would be better with the switch in front of the caps which is the only reason I changed it.

Can someone please explain why this would make a difference?  I will hook it up like my other ones tomorrow and see what that does.

[dscottguitars]

That took care of the problem.  Now I would like to know why...