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Taming Ampeg Vt40

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8/7/1997 1:40 PM
Tony	Taming Ampeg Vt40 My 50W Ampeg Vt40 is killer loud. 2 1/2 is about all anyone can handle being in the same room with the thing. Unfortunately this level is no where near enough to get a "cool" tone. any ideas? Tony

8/8/1997 8:13 AM

Methods to reduce power out:

a) Master volume control mod (several bad ways, a couple of good ones)
b) on four-output tube amps, pull two tubes, one per push-pull side
c) reduce B+ by installing
- a tube rectifier, assuming there is not one there now
- a power zener diode, preferably in the power
trans CT return
d) rewire the output tubes into psuedo-triode mode by tying the screens to the plates with a resistor; this can be done with switching to go back and forth.
e) on amps with impedance selectors, mismatch the impedances - i.e., for 8 ohm cabs, set the selector to 4 ohms or 16 ohms. This cuts available power by making the output tubes work into a mismatched load on their anodes, which lowers the amount of power they can put out.
f) get less-efficient speakers; speakers vary by huge amounts in how much of the electricity they make into sound. Get ones with a softer voice...

DON'T
- use a variac, this will wear tubes out faster

What did I miss, guys?

8/8/1997 4:36 PM
j krogh	you missed the most obvious consumer available option. the hotplate/powerbrake.

8/11/1997 12:57 PM

There are two other obvious answers, and I'm not trying to be a wise guy, but:

1. Get another amp

2. Get a pedal

I like the sound of those Ampegs, and if you use a pedal you will always have that huge clean power reserve if you need to play louder. Those 70's Ampegs were designed to be an "improvment" over the Fenders of the day that started breaking up at "4" on the dial. Running the amp flat out into a hotplate/powerbrake has got to be hard on the tubes, and if you are using 6550s or the original 7027s thats gonna cost you. EL34s wear out even faster, though they are less expensive.

Just had to say that.

JM

8/11/1997 4:33 PM
R.G.	Good point. I did leave out my personal favorite technique for getting distortion in any amount: Build a small (1-5 Watt!) amplifier, drive the bejeezus out of it, and amplify THAT output cleanly. Should help with the cleaner Ampegs.

8/12/1997 12:06 AM

Hi RG

I have heard many times "Don't use a Variac" but the only reason I can find is that the cathode coating will be stripped - due to insufficient heat from the filament.

If this is the only reason then I should be able to run the heater filaments from a dedicated supply and then use the Variac to drop the B+ and bias proportionally?

I can't think of any reason why the tubes would suffer at a lower plate voltage as long as the filaments are kept nice and toasty.

Am I missing something?

See Ya

Ozzie

8/12/1997 9:00 AM

Yes, RG, I have installed coutless low level outputs on Champs, etc so players could do this. Some of the old Kalamazoo and Premier amps are good candidates for this. With the plate voltages in the high 200s or low 300s, they beak up nicely.

As for the variac thing that Austin mentions, I do this all the time when bringing up "experimental" amps, using a spare trans for the heaters. I keep a few sets of good, but expendable, tubes for this purpose. The variac is also good for bringing up an amp that has been stored for a long time. With an ammeter you can see if the current draw is going too high so you have time to back off before blowing the HV winding on the trans. Usually due to a shorted cap.

JM

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