OT: what should I look for in a variac?

[Johnny Guitar]

I was looking at Marlin P. Jones who are offering two different variacs:

One is 500VA for $39

http://www.mpja.com/directview.asp?product=15162+TR

The second is 2KVA for $89

http://www.mpja.com/directview.asp?product=15163+TR


Since I've never used one I'd like to make sure I don't get one that won't suit the most likely usage -- tube guitar amps.

Thanks
John

[Hal]

a VA is a watt.  You need one rated for the power consumption for your amp.  The smaller one _should_ be fine.

[Peter Snowberg]

The larger ones are a little easier to clean and refurbish not to mention they take overload a little more gracefully.

I would go for the 2KVA unit.

I love Variacs! What a cool invention.

[bwanasonic]

OK, I have always been under the impression that variacs are cool if you have a full time amp tech, and an endorsement deal with Marshall. But if I really can't afford to blow up my Rivera Quiana, is this something I should look into? Even if  it won't blow my amp up, will it accelerate the *aging* proccess?

Kerry m

[Paul Perry (Frostwave)]

Your amp is designed to run at the voltage printed on the back.
If you run it at any other voltage, BADNESS WILL OCCUR sooner or later.
But if you DO get a variac, get the big one, then you can use it as a non-interfering dimmer on your electric bar radiator (yeah, I've seen that done in a lab!)

[Johnny Guitar]

Hmmm, four replies, four different answers.

Ok, the reason I'm interested in getting one is not to run an amp at a lower voltage but when I look at an older amp.

I don't do this much but a few years ago I worked on a friends 30 + year old Twin Reverb (which didn't need repair but needed to be "returned to original" -- a bad modification had been poorly installed which I removed). It hadn't been turned on in about ten years.

I've heard that powering up old electrolytic caps can burn up an old amp. But if you slowly bring up the PSU voltage the caps may be able to handle this.

The reason I'm looking at this now is that I've just inherited a 50 year old reel to reel which I'd like to see if it will work but don't want to damage it.

John

[moogatroid2000]

Interesting thread. I recently bought a used one but have been hesitant in firing it up. It was one part of a four ganged set up. The main "poles" ran through all four units. This pole was cut and four variacs were then sold on their own.
I am wondering if/how I can go about testing a used unit like this?
Saving the extra dough was cool but now I must admit I am more than a little nervous about firing it up.
Anyone have any suggestions? maybe I should bring it to a real tech type and let them test it out. Sorry if my reply contribution does little to answer your own questions. Your post just got me to thinking about it again.I really want ot use it. Mostly for the same reasons. I wish to start working on my one of my amps, twin reverb of course, and would like to do the whole cap forming procedure.

[Fret Wire]

I have a nice little Staco 0-140v, 5a, .7KVA model. It's dial indicated voltages are approx. 2v low, due to household current levels rising since it's manufacture. Of course, you should always set one up using a DMM.

Handy for checking and working on amps, plus other old equipment that isn't rated for modern 120v.

[Johan]

Hmmm, four replies, four different answers.

Ok, the reason I'm interested in getting one is not to run an amp at a lower voltage but when I look at an older amp.

I don't do this much but a few years ago I worked on a friends 30 + year old Twin Reverb (which didn't need repair but needed to be "returned to original" -- a bad modification had been poorly installed which I removed). It hadn't been turned on in about ten years.

I've heard that powering up old electrolytic caps can burn up an old amp. But if you slowly bring up the PSU voltage the caps may be able to handle this.

The reason I'm looking at this now is that I've just inherited a 50 year old reel to reel which I'd like to see if it will work but don't want to damage it.

John

with amplifiers, there is another way to do this that is both cheap and effective.
pull all the tubes from the unit, so that no current will be drawn when powering up. install a BIG value resistor, like 1Mohm between the rectifier and the first cap. now you can power up your amp and watch the voltage rise on the caps. depending on the state of the caps it could take anything from a few seconds to several minutes before the voltage reach its peak.
when the voltage has reached whatever value it should be ( say 470v in a marshall or a twin..) you can remove the resistor, reinstall the tubes and fire the unit up again. if the voltage doesnt rise high enough, you have a bad cap...its a low-tech solution but it works..

Johan

[Paul Perry (Frostwave)]

Sorry, I didn't realise what you wanted it for, I thought you might be wanting to run your amp permanently at 85 or 130 mains insteasd of 110.
I don't know how common it is to have a problem powering up old unused valve gear, but I personally never have had a problem (at least with tube electronic test gear and radios stacked away for 20+ years).

[Johnny Guitar]

OK, a lot of great info and ideas. It sounds like I can use the cheaper .5kVA model which is what I was hoping for (it's been along time since I've been inside or in back of a Marshall or a Twin so I'm not sure what the power consumption is for those beasts).

Thanks as always!
John

[cd]

with amplifiers, there is another way to do this that is both cheap and effective.
pull all the tubes from the unit, so that no current will be drawn when powering up. install a BIG value resistor, like 1Mohm between the rectifier and the first cap. now you can power up your amp and watch the voltage rise on the caps. depending on the state of the caps it could take anything from a few seconds to several minutes before the voltage reach its peak.
when the voltage has reached whatever value it should be ( say 470v in a marshall or a twin..) you can remove the resistor, reinstall the tubes and fire the unit up again. if the voltage doesnt rise high enough, you have a bad cap...its a low-tech solution but it works..

Johan

Good suggestion.  You can also do this with a light bulb instead of resistor.

[toneman]

U didn't say how big yur amp is/was/willB
depends on the size of yur amp's power requirments.
VA means   VoltsAmps..."not true"  power(watts) but closeenough.
i'd go 4 the larger...more usefull down the road....
U also didn't mention if U were using for troubleshooting or
using the variac for amp pwr sag.
afn
staytuned
tone

[jplaudio]

Hmmm, four replies, four different answers.

Ok, the reason I'm interested in getting one is not to run an amp at a lower voltage but when I look at an older amp.

I don't do this much but a few years ago I worked on a friends 30 + year old Twin Reverb (which didn't need repair but needed to be "returned to original" -- a bad modification had been poorly installed which I removed). It hadn't been turned on in about ten years.

I've heard that powering up old electrolytic caps can burn up an old amp. But if you slowly bring up the PSU voltage the caps may be able to handle this.

The reason I'm looking at this now is that I've just inherited a 50 year old reel to reel which I'd like to see if it will work but don't want to damage it.

John

A Variac is not a very good tool for powering up old or suspect power supplies. It is good for making adjustments for over or under power line conditions. Most professionals use a current limiter to power up units for the first time on the bench. You can build your own current limiter for less than $20.
Heres instructions:
http://www.vintage-radio.com/projects/lamp-limiter.shtml
If you want to buy instead of build:
http://kendrick-amplifiers.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=KOS&Product_Code=KEN04-limiter&Category_Code=04