77 twin reverb

[majwild1]

Ok guys I've got a 77 twin reverb 135 ultralinear, I'm going to recap it, and since i'm going to have it open I thought I would ask to see what mods work well with this amp, and where to find them. I can't seem to find alot of info on mods for the 135, I changed out the speakers and put in weber california, big difference. info would be much appreciated.

[majwild1]

Well I've dicided to do the variable nfb mod, but I want to confirm how to do this. On the 135 there are 3 wires instead of 2 for the output ( 1-black, 1 green, 1- green yellow ), I was going to use a 25k pot. would you pull both the gy wire and the white wire off the 820k and install the pot there?

[Bruce / Mission Amps]

Yours has a transfer jack so you can use an aux cabinet and the stock one and still have an impedance match.
Mount your variable NFB pot somewhere close by the jacks and break the wire to the 820 ohm feedback resistor from your "line out" jack and 4 ohm tap.
I think that is the yellow with green stripe wire going over to the eyelet board.

The hum balance pot is very frequently already blown in these amps so I'd just yank it out and use two discrete 100 ohm resistors to ground (across the filament leads somewhere) to balance the filament supply and use that hole for your variable NFB control with a small knob on it.

[majwild1]

Thanks Bruce,

While I'm at it I think I will change to a adjustable bias, If I remember right you just remove the resistor off the bias balance and add a 100k pot in its place as a variable resistor with a 47k to ground? let me know