Guitar Tube Amp Conversion Questions

[ToobNube]

yeah, on closer inspection, it looks like it is/was wired up correctly. ok, i'll use a 100 ohm pot instead. thank you.

[Helmholtz]

A 100R pot should be rated at 1W.
150R pot would get away with 0.5W.

[35L6]

Will that pot have 0 volts in the middle , 6.3 volts one phase on one end , and 6.3 volts the other phase on the other end ?

[Helmholtz]

The heater winding has a grounded CT. So each end of the pot should measure 3.15V wrt ground (but 180° out-of-phase). The pot's wiper will be at some voltage between zero and 3.15V.

[35L6]

Yes , of course , 3.15 V . I thought of that after I posted . With opposite phase you could null out whatever hum dominated in the rest of the amp .

[35L6]

Do R31 and R33 have to be equal ? Do you need a center tap on the heater winding ? I wanted to try this circuit but my PT doesn't have a center tap . I wonder how well this works compared to standard humdinger . This is the only schematic I've seen that has it so it seems it never caught on .

[Helmholtz]

Do you need a center tap on the heater winding ? I wanted to try this circuit but my PT doesn't have a center tap . I wonder how well this works compared to standard humdinger .

It requires ground-symmetrical heater voltage, so either grounded CT or balancing resistors.

But the method only works with a pentode.

R32 (10k) and R33 (150R) constitute a voltage divider.

[35L6]

Not just any pentode but one that brings the suppressor grid out to a separate pin . It would seem that R31 and R33 being the same is just coincidence . Does the tube amplify signal at the suppressor grid ? 150R and 10k is a pretty steep divider . By my calculations you get a max signal of about 46 mA . If I install a virtual center tap that may go a long way to eliminating hum making the suppressor grid circuit unnecessary . On the other hand it may get rid hum radiated from the transformer . This may be why you don't see it much .