Using variac on tube rectified amps

[diamondsouled]

I've heard a variety of things about using a variac on amps with tube rectification; such as subbing the tube rectifier with a plug in diode sub.

Is there a safe way to do this? Safe for the rectifier tube as well as the amp?
Cheers

Lar

 

[audiodon]

The only problem I've noticed is that solid state tube rectified units will start producing audio around 50-60 volts on the variac. Tube rectified units don't charge the tube rectifier enough to power things until close to 100 volts.

 

[gadget73]

SS diodes will ramp up the B+ starting almost immediately. A vacuum rectifier probably needs a good 90v or more before it will begin to conduct, so you won't be able to bring up the B supply as gradually as with a solid state rectifier. Honestly unless you're using old caps, this shouldn't matter. If you want to re-form them, SS rectifier subs would probably be the way to go.

 

[highfigh]

I've heard a variety of things about using a variac on amps with tube rectification; such as subbing the tube rectifier with a plug in diode sub.

Is there a safe way to do this? Safe for the rectifier tube as well as the amp?
Cheers

Lar

You can do that but unless the solid state rectifier is done correctly, it won't have any significant voltage drop, like the original tube did and the design called for. This can take out electrolytic caps if the amp is running on higher line voltage and the working voltage rating isn't high enough. In a HiFi amp, the stiffer B+ is fine but in a guitar amp, it really doesn't sound as good, IMO, unless it's part of the original design. Low line voltage isn't the worst thing for a tube rectifier, but you don't really want to run it like normal at that time. If you use a Variac to lower the line voltage to 110 because the amp was designed for that, even 117 is within the original tolerance. The B+ can be significantly higher but electrical service has a +/- 10% range in a lot of places.