Has anyone used a variac with their valve amp? Does it work well, any damage to your amp, how low in volume can you go, does the early saturation sound good, and anthing else that's interesting.

I found a couple of variable transformers in the lab I'm doing my final year project in (Physics is what I do), I'm half tempted to knock up a simple variac from them, but I want to know what experiences people have had with them.

I have an ashdown fallen angel, which isn't all-valve (ie it has pre-amp and power amp valves, but things like the fx loop are controlled by IC's) so there's going to be a threshold voltage for how low I can go before it starts making strange noises, and the dc heaters of course, but it would be nice to get early saturation, and variac's seem so much more geeky than attenuators, which is what draws me to them...
variacs don't directly hinder your amps volume. they simply supply your amp with less (or more) volatge from your wall outlet. Many vintage amps ran at less volts than today so some people use a variac to reduce their incoming voltage to preserve the amp. You can also use a variac to slowly increase the voltage to your amp if you've just done some work on it. Many people who reform electrolytic capacitors use a variac to slowly bring up voltage. Some people also reduce incoming voltage so that they can increase the bias current in the amp. eddie van halen used to do this. Other then that, attentuators and variacs are totally seperate tools
I understand that they are separate, and I am aware of what most people use variac's for (drop down to a steady 117-120V for old amps, down to about 90V to make it run more efficiently), but there is the use where you drop the voltage quite dramatically (ie down to about 55V) and you can decrease the volume a fair bit and still have full power valve saturation. Has anyone done this?
I would really like to know about this too. I want a nice 50W all-tube amp for going loud, but to scale back the power for recording and personal practice. I dont want attenuators to mess with the tone. I also looked at London Power's power scaling kit, but I dont know much about that. A little hesitent to start slapping pieces onto a high value amp

edit - someone said to be really careful using these things. What's the risk to your amp using them?
Last edited by Jeep_Guy at Mar 29, 2007,
no, lowering the voltage wont give you lower volume distortion, it will lower the current going through the tubes and lower the volume, but the tubes will still break up at the same volume. You might be able to do it by lowering the voltage then biasing your amp a lot hotter. But then if you use the amp without readjusting the bias you might blow your tubes.
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So what are the options for getting a higher-wattage amp quiet enough without losing tone? Attenuators, variac, power scaling?
Valves running at lower voltages will saturate sooner. The THD univalve has a hi voltage and low voltage switch, when on low voltage it saturates sooner. However, the voltage difference isn't low enough to really make it much quieter, and the univalve has an attenuater. What I meant when I first said variac was running the voltages down to very low voltages, less than half.

If you want to make a high wattage amp quieter the best idea is to use an attenuator. Nothing else will get it down to bedroom levels. Yellow Jackets, removing valves and variac's will reduce volume somewhat, but not enough to provide bedroom level valve saturation