PT Questions

[mumford]

I am in the process of converting a bogen challenger CH618 into a guitar amp. It shows its age in that it is designed to take AC or DC power.
My plan up to this point is to get it working before deciding if I want to make further changes. That said, I am getting rid of all of the DC input power sections.

I've attached both the original schematic and one showing the changes I've made so far. (Please disregard my notes on which caps I've replaced to date. )

I find the PT puzzling. It seems to have dual primaries, but one of them (the lower one) appears to just supply the heaters on its own without using a secondary. A friend I trust, who is far more knowledgeable than me, suggested I eliminate the line in pink and just ground the wire coming from the bias. what I think it happening there, though, is that that line is being used to power the heaters when using AC power. Am I right?

Any thoughts on the circuit in general and my changes are welcome.

I would just apply AC to the transformer with it removed from the circuit to see what's going on, but I need to get a multimeter that is rated above 400v first. I have done basic continuity testing.

[Bob-I]

I disagree. What that connection is doing is raising the heaters above ground by using the cathode voltage. It's a commonly used practice for quieting amps.

[mumford]

So what I have done in the power section is acceptable, and I should leave the pink wire?

[martin manning]

Keep the pink wire and lose the 0.1uF cap across the filament winding (just tape the yellow wire off). The 0.1 cap is likely there to keep the vibrator contact points from arcing. I don't see any need for the 0.02uF and 2k2 in series on the secondary either.

[Structo]

Be sure to include a properly rated fuse on the hot leg.

[Dr-Joned]

It's not really a dual secondary, just the way that it was drawn. The winding on the bottom left is for the 6v heaters. That is sorta funky way of taking a centertap (pink) to feed through the 2k7 to the Cathodes. I would leave it.