100W Plexi variable power mod.

[danieldeveau]

Any suggestions as to how to efficiently mod a 1967 100W Plexi so that I can put a half power switch?

[mhuss]

A half power switch will make the amp *barely* quieter. I don't recommend you bother.

A better solution would be to use some sort of Power Scaling as originated by Kevin O'Connor and documented in one of the TUT books (4?)

--mark

[CaseyJones]

Put a restrictor plate on it. That'll slow 'er down!

Anyone stay up and watch Fontana last night?

If it's a real Plexi you shouldn't mess with it because it's a valuable amp. If it's a reissue you should still leave it alone, tryin' to make a 100 watt amp play at bedroom levels makes about as much sense as pullin' a handful of spark plug wires off yer Cadillac in hopes of gettin' better gas mileage. Of course Mark is right, a 50 watt amp with 100 watt transformers will still sound more or less like a 100 watt amp. You'll lose around 3db spl which is negligible.

It takes about as many parts to build a replica plexi preamp with a smaller output stage as it does to add a proper power scaling circuit.

[BluzMike]

Any suggestions as to how to efficiently mod a 1967 100W Plexi so that I can put a half power switch?

Rather try a good attenuator such as the THD 16 ohm Hotplate or the Ultimate, which is actually a re-amp. I have compared most of the attenuators on the market with some friends. The Ultimate sounds the best, probaly also due to its impedance mismatching (constant 32 ohm load) in combination with its coil value; I may recommend it with EL34 amps into a 16ohm cab only. Unfortunately nothing for JTM45 or JTM45-100s, but you can still use the also great Hotplate. As long as you only dial the amps down around 30 - 40%, their tone will not change dramatically.

Do not forget about speaker efficiency, as replacing G12H-30Ws or blue alnicos by less efficient speaker will lower the sound level also a step.

I have not tried out powerscaling so far, but from my understanding that O'Connors approach is to coordinately lower the high and bias voltages whilst keeping the heater voltage constant, I would be largely surprised if a scaled down amp would sound nearly the same.

Unfortunately there seems no way to keep the tone when lowering the loudness level.

[ampgeek]

Here is a nice little discussion/test of powerscaling that I refer to often.

http://bruceclement.com/music/psk.htm

Good luck,
Dave O.

[Allynmey]

I'm going to use Dana's boards and double up on the mosfets for power handling.