VariAc as attenuator

[godi]

Hello. I have a variAc, could this be use to make an attenuator ?
I'm not thinking to drop the +/- 120 Volts main input, I want my filaments at 6.3 Volts, but could it be use after the output transformer and before the speakers as a attenuator ?

Thank for your opinions.

[Reeltarded]

I have used variacs for almost 30 years as complete attenuation until the lights almost go out.

[d95err]

A variac is not very useful for attenuation, because by the time you get the voltages low enough to make any significant change in volume, the filaments will be way too low.

You can't use a variac after the output transformer (although Marshall uses some kind of variable transformer for their Powerbrake).

You should lookup Powerscaling (see www.londonpower.com), a technique where a simple high voltage regulator is used to reduce (or "scale") the the HT voltages of the amp to reduce output power, while leaving the filament voltages untouched.

[Kagliostro]

The only practicable way that I see to use a variac is to use it to reduce the wall voltage before the PT but you must disconnect the filament from the PT and use a separate transformer for filament winding supply as to prevent the voltage go down

reducing the B+ voltage to the pre - PI - and final tubes can give you a lower sound, but people that are lowering B+ usually want to achieve the same sound they have at higher power being at lower power

as to obtain that you must have the preamp section to standard voltage and PI with final tubes at lower B+

so is much more better to use a VVR

if your amp is a cathode biased amp you can easily build your own VVR

here some discussion and documentation about

http://www.el34world.com/Forum/index.php?topic=6899.0

Kagliostro

[VacuumVoodoo]

Who has read and understood the original question

In principle one can place an autotransformer (aka Variac) between amp output and loudspeaker.
This is how volume was adjusted in a 100V audio line distribution systems. There was a small variac-like autotransformer inside each speaker box.
But I don't recommend it at all.
Mains Variacs are wound with a wire diameter much smaller than your OTs secondary simply because you never pull such high current frm the mains as you do from your OT.
You got the right idea but wrong hardware to test it on. In one word: don't.

[Kagliostro]

Who has read and understood the original question

Sorry I misunderstand

Kagliostro

[VacuumVoodoo]

No problem. Whenever I go to Italy I'm always still not fully sober on Saturday mornings. Ciao.

[d95err]

Who has read and understood the original question

I did!

[godi]

OK, I'm goin to build a AirBrake, I don't want to blow my output transformers.

Here an other strange question, let say I build a tube amplifier without a impedance selector and I install 3 output jacks, one with 4 Ohms, second at 8 Ohms and the last one with 16 Ohms tap. Now, can I run a 4 Ohms speaker plug into the 4 Ohms jack and a second 8 Ohms speaker plug into the 8 Ohms output jack at the same time ?

[Kagliostro]

I think that the reflected impedance will change in a way your tubes don't like

Hope someone can explain this also to me

Kagliostro

[martin manning]

Here an other strange question, let say I build a tube amplifier without a impedance selector and I install 3 output jacks, one with 4 Ohms, second at 8 Ohms and the last one with 16 Ohms tap. Now, can I run a 4 Ohms speaker plug into the 4 Ohms jack and a second 8 Ohms speaker plug into the 8 Ohms output jack at the same time ?

This would be no different from connecting two 8-ohm speakers in parallel to the 8-ohm tap- the reflected impedance will be cut in half.

[ampdoc1]

I tried an unusual, but simple way to use a "variac" in an amp several years ago. I put a 20W pot in series with the output tubes heater line. It turned out for the tubes I was using that a 3 ohm pot worked perfectly. It took a few seconds (like the VVR) but lowered the output from 30W down to less than a watt, keeping full tone until the speaker excursion became very small.

[pdf64]

I seem to remember that it's a 'BAD THING' to apply B+ to cold tubes - cathode poisoning was the issue maybe? Pete.

[rdjones]

Here an other strange question, let say I build a tube amplifier without a impedance selector and I install 3 output jacks, one with 4 Ohms, second at 8 Ohms and the last one with 16 Ohms tap. Now, can I run a 4 Ohms speaker plug into the 4 Ohms jack and a second 8 Ohms speaker plug into the 8 Ohms output jack at the same time ?

This would be no different from connecting two 8-ohm speakers in parallel to the 8-ohm tap- the reflected impedance will be cut in half.

This has come up before, wasn't the outcome that a closer match (but not exact) was a 16 Ohm speaker on the 8 Ohm tap and an 8 Ohm speaker on the 4 Ohm tap ?

rd

[martin manning]

You're right rd, that would match it correctly, but you'd need a 16. I was responding wrt a 4 and an 8.