Variac Recommendations

[Matt J]

Hey everyone!

As I've gotten more and more into amplifier building, I've added more tech tools to my work bench - i.e. - a current limiter, dummy load, etc. Some I've purchased, others I have made myself. I've decided it would be a pretty good idea to add a variac to my list of tools. This is so A) I can slowly power up an amp on first start ups and b) I can run my amps at a correct 120 VAC and achieve the proper PT secondary voltages since the wall voltages in my home can vary throughout the day.

I'm looking for feedback and recommendations on a decent variac. I've seen some of the very nice ones, but their price is a bit over my budget. I'm looking for something less than $200.

Any recommendations?
Thanks!
- Matt J.

[pompeiisneaks]

I did two things wrong with my purchase trying to save money:

1. I got a 3A one, and some amps go upwards of 6A, so get a 10A one minimum
2. I didn't get one with isolation as well, get an isolation transformer as well.

https://www.amazon.com/gp/product/B006N ... 4903190978

(thats my amazon affiliate link for one I'd like to get, but you don't have to buy from amazon OR use my affiliate link I'm just testing it out in theory, as I'm adding the links to my videos for people to 'support the channel' )

There are likely much better quality ones than that one too, I'm no expert on the best brands. Just make sure you get the two items sorted I mentioned that I messed up.

~Phil

[Matt J]

I did two things wrong with my purchase trying to save money:

1. I got a 3A one, and some amps go upwards of 6A, so get a 10A one minimum
2. I didn't get one with isolation as well, get an isolation transformer as well.

Thanks for the recommendation, Phil!

Could you go into greater detail about having an isolation transformer? What effect exactly does having one do when you're using a variac?

- Matt J.

[sluckey]

IMO an isolation transformer is not an important consideration when choosing a variac for use with guitar amps.

[pompeiisneaks]

Interesting sluckey. Could you elaborate? I was told by tons of different amp techs that they're pretty much mandatory when working on unknown amps, it protects things a lot better. I didn't fully understand why, but I'd love to, guess I shouldn't suggest it unless I fully understand I guess eh? What's the difference in cost of isolation vs non isolated usually?

~Phil

[sluckey]

Well, the line cord is not isolated. So why would you want the variac to be isolated? What advantage do you see? What things will be protected a lot better?

I'm not saying isolation transformers have no value. There was a time when there was a big isolation transformer in nearly every radio/tv shop. They were needed because a lot of equipment, including test equipment had one side of the line connected to chassis.

I have one of those expensive 10A variacs. Doesn't have a built in iso transformer. I've never needed one. Kinda like windshield wipers on a goats ass!

[sluckey]

Phil, just wanted to say I'm not poo-pooing isolated variacs. If price is comparable and I was shopping, I'd probably buy an isolated model. It can replace two pieces of test equipment. Although I see no advantage for an amp that is already isolated due to power transformer, there is still a safety advantage when working on old tech portable equipment that has no power transformer and may have one side of the line connected to chassis ground. So, if you will be working on an old Kay amp with no power transformer, or an All American 5-tube superhet, or a '60s B&W TV with no PT, you need an isolation tranny for safety. May as well combine the isolation tranny and variac.

Here's a pic showing an isolated variac and a more common auto-transformer variac...

[pdf64]

I've got a big variac (non-isolated). I got it free when my workplace was having a clear out. I'm glad I've got it but the damn thing is like a boat anchor and doesn't really earn its keep. If I didn't have it I might think that a 'proper' tech's test bench should have one and I'd be in the same position as you. But in reality, for most anything you might think of, something else, generally a light bulb limiter, can do it better and with less faffing around.
As an amp user, trying to stabilise mains voltage seems a fools task, just creates a barrier to playing the damn thing.
I concede that on the test bench, it's handy to tweak the mains to maintain it at the nominal voltage when running some tests and so achieve constancy.
And if I need a bit more power out of a big soldering iron, eg for chassis soldering, it's useful to run it via the variac turned up to max

[pompeiisneaks]

Sluckey,

thanks for clarifying, that makes sense. so my real mistake was the 3A vs something higher I've popped the variac fuse once due to an amp drawing nearly 3.2A and it had like a 4 or 5a fuse..

pdf64, I've used mine a few times on amps I get that are used and I don't know the status and slowly bring them up on the variac to ensure I don't blow something up. New amps I just use the light bulb limiter too.

~Phil

[trobbins]

Mine is just a vintage 300VA open-frame unit I fitted in a spare computor enclosure. I use mine on the bench for initial powering and setting up of restored amps, and not as a full-time line voltage adjustment for equipment. As such 300VA is enough to cover any large amp I'll ever have on the bench, and final benchmark testing would be done straight from mains.

If you are competent with mains AC then going to 2nd hand options (ebay for me) is fine as there are so many about going for almost zilch - but I can see that a new unit is appropriate for many.

The variac effectively replaces the need for a bulb limiter for me (which I don't have) - but I rarely power up unknown equipment, and typically only power up valve equipment.