I Want To Buy A Variac

[Milnoque]

How big does a variac need to be to handle an old amplifier. Does looking at the amp's fuse tell me all I need to know? What's your usual startup procedure when you don't know anything about an amps history?

Any other advise on variac selection would be appreciated.

What are the pros and cons of using it to drop the input voltage to achieve tube saturation at lower volume?

 

[Track Rat]

VERY bad idea. When you drop the mains voltage you also drop the heater voltage to the tubes. You're better off changing B+ voltage. The variac crap is a myth.

 

[Milnoque]

That's fine about not running an amp with lowered input voltage. I'm still interested in getting one to do safe initial startups on old amps. Variacs are rated in amps for capacity. I'm really interested in knowing how many amps worth of variac will be enough to handle most old amplifiers. Can you look at the fuse and know the amp's maximun draw?

Does an amplifier's fuse ALWAYS break the main input voltage? If so what is the greatest fuse value I'm likely to encounter? Do I need to add a substantial margin for safety on the variac?

I was reading something by Mike Soldano that made me think I could use the variac to drop the output. He was saying it was safe for the amp as long as you stayed above about 85 volts, but it may or may not result in good tone.

So can you just put a pot or resistor in series at B+ to get tube saturation at lower volumes? I'm not looking for crunch. Just compression and sustain at lower volume.

 

[timthetortoise]

Wouldn't recommend putting a potentiometer anywhere near the B+, as it would probably burn out immediately and cause you some damage.

 

[JCH]

How big does a variac need to be to handle an old amplifier. Does looking at the amp's fuse tell me all I need to know? What's your usual startup procedure when you don't know anything about an amps history?

Any other advise on variac selection would be appreciated.

What are the pros and cons of using it to drop the input voltage to achieve tube saturation at lower volume?

How big- Ten amps is plenty. Mines a five, and it handles anything I use it for. I've usually only got a 1 or 2 amp fuse in it for added safety.

Fuse- That'll only tell you if it's blowing fuses.

Startup- For intial startup I use a dim bulb tester. They're cheap, and very easy to build. What it does is put a light bulb in series with the primary voltage. If there is a short the bulb will burn brightly. If the bulb glows dimly the amp is probably safe to fire up. It's also a great tool for testing power transformers, and other things. I built mine with a switch that will by-pass the bulb, and added an inline fuse.

pros/cons Many vintage amps were designed to run at lower voltages. I've got an old silvertone power amp from an organ that's rated at 105v. Many are 115-117. The current I get out of my wall is usually 122.5 or so. Anyway I've got a couple small amps that sound noticably better at 115 than they do at 122.5. Drastically dropping The primary voltage can lead to a host of problems. It upsets the whole food chain. Ohms law is our friend. IMO Five volts less than what an amp is rated for isn't going to hurt anything, but there might be a sweet spot there. If you want saturation get an attenuator or better yet, build one.

 

[Track Rat]

That test rig is pretty cool.

 

[punkin]

Hotplate...

 

[Milnoque]

So I bought a 7.5 amp variac for startups. I knew about bulb testers but I like toys. Thanks for the advice.

Now tell me about attenuators, please. This is my thread so I guess I can highjack it.

 

[JCH]

http://www.webervst.com/lpad.htm

This should get you started.
Check out 6db attenuation @ 8ohm. Its a very simple one. You use a 4r in series, and an 8r in parallell. The final output is 25% of original. G/L

Variacs are handy. I'd be pretty lost without mine.

 

[GONZO-X]

out of all the attenuators i've tried so far (powerbrake, hotplate, power soak, gibson plate) i like the weber mass lite the best.