Variac

[AL1]

Can anyone give me step by step instructions on how to use a variac to power up an amp after having new filter caps put in? Do you set the voltage say to 70 and if so how long to you keep it there and then how many volts do you raise it from there, so on and so on. Also do you turn the amp on the same way, power then standby?

[PaulC]

The main thing is to NOT have any tubes in the amp when doing this. That includes a tube rect if it uses one. Plug in a SS rect into the socket. The reason is the B+ will come up while your tubes are running really cold on the heaters.

Put the variac on zero and turn the amp on and stby off. Bring up AC mains while monitoring the B+ voltage and mains current. Alot of people will tell you to do this over several hours, but that's IMHO a waste of time with new caps. It wont hurt anything if you want to do that, but I've never had any problems with new caps. One thing to remember is without a load on the pwr supply you might actually overshoot the voltage ratings of the filter caps. Don't go over about 80% of their rating. Check all the voltages (including bias supply). If all's cool then move on to what ever's next... Later - PaulC

[Jon C]

I was trained by a well known amp tech to use the SS rectifier, start at 20v, leave it for a few hours/overnight, then bring it up 10v per hour til your at 120v ac.

Other equally well known and competent techs have pointed out that the Fender factory did not follow this technique on *any* of the thousands of now classic amps they built, many of which have run for 20-30 yrs. They just built 'em and then turned them on.

[JacksonAmpworks]

This is only necessary when you are dealing with old caps or caps that have not been used in a long while. New caps that you get from a supplier will be fine to start using immediately. On new amps, I believe (just personal opinion) this is just a voodoo type thing that some people have said that makes people think they will get more out of a cap by starting it up over several days as opposed to just letting it rip.

I have built many amps over the years, and I have never had an issue with this. HOWEVER!! I have always used new caps. If I found an old JTM45 that had been in the closet for 30 years, I would definitely bring it up slowly so as not to hurt the caps.

Again all of this is personal opinion and the result of things that I have read and heard. I am definitely open to correction.