Variac tested

[Colas LeGrippa]

Hola amigos, lately a friend of mine gave me a VARIAC  that he used in his class of physics  in a local University. Made in USA, it loooks much sturdy. Immediately, I had a thought for the great Eddie Van Halen and of course, I wanted to plug one of my  builds in. I choose my Lil' Gripper amplifier ( SE, 2x 12ax7, 1 x EL34, )
and plugged it in the VARIAC.  Then I said to myself:  "Colas, if you lower the ac supply to the amp, you re lowering the filament supply as well which is probably not a good idea ( this, I am not quite sure cause if the filaments are like light bulbs, they can be lowered without any harm). Anyway, I thought that it could be interesting to hook up a separate 6.3 V supply transformer, plugged direct in the 115v while the rest of the circuit of the amp would be plugged into the Variac, so that the tubes electrons emission wouldn't be affected by a lower than.normal filament voltage .

[acheld]

Why not plug the amp into the variac, and then lower the voltage while measuring the heater supply?  IF you can't your B+ down enough for taste, then a separate 6.3VAC transformer.   

But, it all sounds too complicated for me . . . 

What do you expect to achieve by lowering your voltage? 

[Colas LeGrippa]

......suite)


So I was right. First, it seems that a separate heaters circuit  removes a tiny little bit of hum while the tone of the amp is crisper. Up to now, the sweet spot is 70V. I use this amp as a preamp and is plugged  via a jensen transformerr to a clean 50W  amplifier ( my own circuit : 1x preamp, 1x PI, 2 x EL34 ).  The tone is exactly what I ever wanted:  Kim Mitchell, Van Halen and other hard rock heroes. At low volume  ( a volume just low enough to hear me sing with no mic ), I get great definition, mellow highs, not muddy bass, very rich overtones and infinite feddbacks doubled with tube saturation.


My.only one question:  will a tube  heater filament be damaged by , let's say 3VAC supply or simply won't yield a good petformance ?


Gracias !


Colas
 

[sluckey]

My.only one question:  will a tube  heater filament be damaged by , let's say 3VAC supply or simply won't yield a good petformance ?

The filament will not be damaged. In fact it will probably last a gazillion years. However, doing so may not heat the cathode enough to give good performance from the tube. Let your ears be the judge.

What is the rating on your variac?

[Colas LeGrippa]

10 A,  110V.in,  from 0V to 140V

[kagliostro]

Ciao Colas

I didn't understand well what you did with the filament supply

At the end the good result was feeding the whole amp with the Variac or feeding only the power section disconnecting the PT 6.3v windings and feeding the filament with a separate filament dedicated transformer to maintain the 6.3v on filaments ?

The more, the 70V you indicate as the perfect spot are referred to the output of the Variac or to the HT level (B+ Voltage) ?

Which ever is the good spot B+ why don't arrange a PS as to have it as B+ and avoid the use of the heavy Variac ?

If 70V is your B+ spot with a 50VAC (or an easier to be find 48VAC) transformer you can solve your problem

Franco

[Colas LeGrippa]

Ciao Franco,


The sweet spot of 70V I refer to is the Variac output, which corresponds to aprox 230VDC B+.


YES, I have hooked up a separate filament transformer plugged in the  wall outlet in order to maintain the  regular voltage of 6.3V.


The rest of the amp is plugged in the  variac.


Eventually I could swap the pt for getting rhe desired B+ voltage in order to get rid of the variac. For now I find it interesting modifying my B+

[sluckey]

Eddie would be so proud!

[Colas LeGrippa]

It's a POWERSTAT make

[acheld]

That's a classy Variac, much more so than my newish China Red.

I find that Sluckey's "zero" mod PZEM-061 panel meter has been very helpful to me over the past year or so. 

My line voltage varies from 119 to 124 throughout the day, and with the Variac and the meter I can keep a constant line voltage to my builds while testing and trying different ideas. And having the ammeter visible and "always on" has saved my butt from a mistake I made recently.

Check out http://sluckeyamps.com/misc/Amp_Scrapbook.pdf and scroll down.  The meter itself is not expensive.

And, it sounds like I should try out the EVH experience!

[sluckey]

It's a POWERSTAT make

What model number? We may have the same variac.

[Colas LeGrippa]

I l check it out tomorrow, I have left  my studio already tonight

[Colas LeGrippa]

3PN 116 B


That,s the variac type no.


Colas

[sluckey]

My 3PN216C is a later model. They're the same except for cosmetics. The panel meter that acheld mentioned is a very useful mod I did several years ago.

[Colas LeGrippa]

Indeed  very useful mod !


Where.did you get the multi meter from ?

[sluckey]

Amazon has them. If the zero mod sounds like something you want then get the PZEM-061.

     https://www.amazon.com/HiLetgo-Multimeter-PZEM-061-Voltmeter-Transformer/dp/B079JNY5VD/

[Colas LeGrippa]

Ok thanks amigo