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March 2000  Vintage Guitar Article

Amp Tips

by Dan Torres

Torres Engineering

“Vacuum tube Power Play”

This month I am taking a break from the on-going Ampeg project to outline a few misunderstandings about tube amps.

As we get farther and farther from the golden age of vacuum tubes, basic knowledge becomes more difficult to get, and more remotely in the past.

There is considerable misunderstanding about channel switching amplifiers and output tubes.

#1. Amps with two power output tubes. 6L6, 6V6, EL34, EL84, 6BQ5, 6550, 7027, etc.

With just a few exotic exceptions, all tube amps have all their “features” channels, reverbs, effects etc. come together at the “driver tube” and everything runs through the power amp section in mono.

Many believe that their two output tube amp, for example with two EL84 tubes, has one tube for the clean channel and the other for the distortion channel of a channel switching amp.

That is not how it works. Both tubes are used at the same time in a “push pull” circuit. The channel switching happens before the signal gets to the power tubes.

Additional confusion with this type of amp is that power can be cut in half by removing one of the two tubes - again, this is not how it works. The two tubes are a “team” and taking one out would make half of the output signal and the output transformer search for something to amplify it, creating a horrid tone called “crossover distortion.”

Half power on a two tube amp is done (usually) by changing the way the tubes operate - both tubes remain in the circuit.

#2. Amps with four power tubes.

A bit harder to understand, and quite a source of confusion. (Usually amps in the hundred watt range.)

In general, again with some exotic amp exceptions (the Torres Engineering Double Dee Luxe design included) just having four output tubes does not mean the amp had two separate output/power channels, each with a separate pair of tubes.

If it did, there would have to be two output transformers, PLUS, the power transformer, and two sets of speaker output jacks powering completely separate speakers.

The most common “internet information” is that you can take two of the tubes out and cut the power in half - right, but which tubes?

Again, the amp is a push pull amp, with the two tubes on the right being one side of the push pull circuit, and the two tubes on the left as the other side of the push pull circuit.

Taking out the wrong tubes will, again, send the amp and the output transformer into crossover distortion - an ugly tone that no one can use.

So which tubes do you take out? (Or disconnect.) Take a look at the drawing for clarity. Either the two tubes in the middle, or the two tubes on the outside. Not both right hand or both left hand tubes.

Cut the power in half? Again, misunderstandings. This does not drop the volume to half, or anywhere close.

All other things being equal, it takes 10 times as much power to make a sound appear to be twice as loud to our ears. So, the opposite is true, it takes 1/10th as much power to sound half as loud.

Cutting from 100 to 50 watts is not 1/10th, it is only half. 10 watts would be half as loud (with everything else equal.)

But cutting the power will allow the amp to distort or break up earlier, at a more reasonable volume.

There are other concerns with removing or disconnecting tubes. The output transformer was made for the four tubes - there is what is called a “primary impedance” - a load that the amp and tubes expect at the input of the output transformer.

For example, a 4 x 6L6 transformer primary may expect an impedance of 1,900 ohms from the tubes. Taking two of them out doubles that impedance to 3,800. You did not change the transformer, so now there is a considerable mis match and the transformer has no way to “fix” it other than putting out a different secondary impedance (to the speakers.) Depending on the “winds ratio” of the transformer, now the speaker impedance is wrong, doubled, or close to doubled.

The amp will still work, but not as well.

Why all this? Simple, a 100 watt amp with two tubes removed doesn’t sound as good as the same model amp really made as a 50 watt amp with the correct transformer.

The “10 times” audio factor is also the reason audio taper pots are used in guitars and amps to control powered audio signals. The taper is adjusted so that 1/10th as much power appears across the pot at 5 as at 10, otherwise, we don’t hear very much of a taper at all, most of an “on-off” condition.

This article is for entertainment only. Any additional help will require the amplifier to be in our shop for service.

Copyright © 2000 by Dan Torres

All rights reserved

may not be reproduced in any media existing now or in the future without permission.

Ok, here's the same warning rap. But read it every time. Warnings and disclaimer Tube Amplifiers, all components and related products are electrical products with extremely high voltages that if mishandled or if used carelessly or for improper purposes, can cause life hazards and serious personal injuries. Such equipment is dangerous even when turned off or unplugged. You shouldn't be working inside the amp unless you know what you are doing. Don't work on an amp plugged in. Always discharge the filter capacitors before starting work. If you don't know how, contact us for a reprint on how to do it ("What the Hell are filter caps?") Solder neatly, and use the smallest amount of solder you can. Be careful, but have fun.

Dan Torres is one of the most sought after tube amp experts in the world. Dan is the author of the definitive book on Tube Amp Technology, “Inside Tube Amps”. The latest new stuff from the Torres Shop are a whole series of new easy to build complete amp kits. Check out the catalog or web site for the latest. New technology every day!

This article was created, researched and written by the author. Caution is advised when dealing with electronic and electric materials or valuable musical instruments.

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Messing around with your tube amp power tubes!
By Dan Torres
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