Post Phase Invertor Master Volume Mods for a Model T

[Rick Himel]

I just had my early 70's Model T serviced and in an attempt to "tame the beast", I had the original maser volume modded to a PPIMV. This amp definitely did not like this modification.

Is anyone familiar with Master mods for Model Ts and what works for these amps?

[Infernal Doom Fanatic]

Why would one tamper with perfection?
One Word: BLASPHEMY!

[loudthud]

Try replacing the Master Volume pot with a 1 Meg AUDIO ( sometimes called LOG ) taper. It will expand the area from 0 to 1 to be between 0 and 5. You might also want to do the same with the channel volume controls. The second version Model T ( the one nobody can seem to find a schematic for ) with the mid shift switch has a second section on the Master Volume pot that is wired back to the junction of the input of the second stage. It seems to me this would reduce the amount of overdrive you could get with the Master Volume turned down but I can't try it because the second version amp that I own is missing a power transformer.

Another thing to try is to remove two of the 6550's and set the impedance switch to half of your speaker cabinet impedance. Remove the two closest to the back of the amp. This will reduce your power output to 75 watts or so.

[Ryan_77uk]

Does it damage the amp at all running it with two power amp valves out?

[loudthud]

This type mod is suggested in several books by people like Aspen Pittman and Gerald Weber so I wouldn't expect any problems. Just remember to change the impedance switch to half of your cabinet's impedance rating. It's probably less damaging than running with the impedance switch set too high or running with a resistive power attenuator. The only disadvantage is that your matched quad of 6550's won't be as closely matched anymore.