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Variac Issue

B&WTube: Posts : 20
Join date : 2019-08-19

Post n°1

Variac Issue

by B&WTube on Wed Nov 13, 2019 8:12 pm

Need help with Variac issue.

First, thank you to everyone in the community who has been so helpful, as I learn about this hobby.

Now the issue: I purchased a Variac, because my AC power bounces between 119-124 V. This prematurely ruined one GL power tube. I was advised to get a Variac. I was told this one would be fine and purchased it https://www.circuitspecialists.com/variac-variable-ac-ps05kva.html

I have been swapping out lots of things and have not been totally happy with the sound. It was just missing a richness and bass punch. Tonight, I plugged the amp directly into the wall and that got me back what I lost. That is the sound I want. BUT I don’t want to prematurely blow out my amp or $40 power tubes at 200 hours.

Any advice on how to have the sound quality AND not blow things up with my fluctuating high line current?

LeGrace: Posts : 331
Join date : 2016-08-07
Location : Ontario, Canada

Post n°2

Re: Variac Issue

by LeGrace on Thu Nov 14, 2019 12:36 pm

Did you check and readjust bias when you added the variac?

B&WTube: Posts : 20
Join date : 2019-08-19

Post n°3

Re: Variac Issue

by B&WTube on Thu Nov 14, 2019 1:13 pm

LeGrace wrote:Did you check and readjust bias when you added the variac?

Yes. I am pretty religious about the bias.

deepee99: Posts : 2265
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°4

Re: Variac Issue

by deepee99 on Thu Nov 14, 2019 6:11 pm

A Variac won't protect you from overvolts or undervolts.

wildiowa: Posts : 219
Join date : 2012-03-19

Post n°5

Re: Variac Issue

by wildiowa on Fri Nov 15, 2019 3:13 am

What voltage do you have coming out of the Variac? Ae you measuring the output voltage with a reliable meter?

deepee99: Posts : 2265
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°6

Re: Variac Issue

by deepee99 on Fri Nov 15, 2019 10:43 am

wildiowa wrote:What voltage do you have coming out of the Variac? Are you measuring the output voltage with a reliable meter?

For meters, the usual Fluke and a half-dozen Harbour Freight specials, and all agree most always.
A Variac's output is governed by incoming voltage. If the line voltage goes up 2 volts, the Variac goes up by 2 volts. Down 2 on the input, down 2 volts on the output.
Another drawback to Variacs is that they and ground-fault interrupters famously don't get along together.

wildiowa: Posts : 219
Join date : 2012-03-19

Post n°7

Re: Variac Issue

by wildiowa on Fri Nov 15, 2019 11:42 am

I run mine at 119 volts...and yes that floats relative to the incoming line voltage. But it averages out I guess. 118v seemed too low and 120v too high so I split the difference. In any case it is better than the typical 125v I get here and occasionally 126v. I like Variacs. Others like the bucking transformers. Cheap line conditioners are a no no. BUT....back to the issue at hand. Would bass and overall performance of the amp be noticeably different depending on the Variac setting? I suppose it would suffer if it's set too low. But in the 118v to 120v range? Apparently, the original poster likes it "hot." And willing to perhaps sacrifice some tubes.

Dave_in_Va: Posts : 350
Join date : 2013-04-02

Post n°8

Re: Variac Issue

by Dave_in_Va on Fri Nov 15, 2019 12:07 pm

"I have been swapping out lots of things and have not been totally happy with the sound."

This could be the reason you lost your power tube, not the variac.

The exact same thing happened to me when I first got my VTA ST 70. I swapped out several sets of power tubes but didn't know at the time how important it is to re-tension the tube sockets. I lost an NOS Mullard EL 34 and the Mullard rectifier. I re-tensioned my sockets and quit swapping tubes and not a single problem since. An expensive lesson.

I use a bucking transformer that knocks my usual 123 line voltage down to about 114 - 115. No problems. My voltage doesn't fluctuate alot but it does some (usually about 2 volts).

DavidR: Posts : 29
Join date : 2017-08-10
Location : MetroWest, MA

Post n°9

Re: Variac Issue

by DavidR on Mon Nov 18, 2019 8:22 pm

Find an electrician and install a clean-up transformer and a constant voltage transformer.

deepee99: Posts : 2265
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°10

Re: Variac Issue

by deepee99 on Mon Nov 18, 2019 8:43 pm

Dave_in_Va wrote:"I have been swapping out lots of things and have not been totally happy with the sound."

This could be the reason you lost your power tube, not the variac.

The exact same thing happened to me when I first got my VTA ST 70. I swapped out several sets of power tubes but didn't know at the time how important it is to re-tension the tube sockets. I lost an NOS Mullard EL 34 and the Mullard rectifier. I re-tensioned my sockets and quit swapping tubes and not a single problem since. An expensive lesson.

I use a bucking transformer that knocks my usual 123 line voltage down to about 114 - 115. No problems. My voltage doesn't fluctuate alot but it does some (usually about 2 volts).

Dave in Va
You may well be right. I had a similar event and red-plated some very expensive KT-88s. That cured me of tube-rollin', plus I found that once you've arrived at the right tonal balance with the right driver- and output tubes, the *only* reason to pull one is when it goes pear-shaped. If I could solder my TS 6550 black plates and three RCA 5963's to the chassis, I would.

IEales: Posts : 12
Join date : 2019-03-16
Location : Florence OR

Post n°11

Re: Variac Issue

by IEales on Sun Nov 24, 2019 5:54 pm

B&WTube wrote:my AC power bounces between 119-124 V.

Does the voltage bounce or just vary with time of day, day of week, season?

There is no need to finely adjust the voltage.
Tube amps run fine @ ≈117v

B&WTube wrote:This prematurely ruined one GL power tube.

Are you sure? Why only 1 tube?
Some tubes can have short lives as can every other electronic component.

What are you running from the variac?
500VA is pretty low for HiFi.
Could be starving the amp of current.

IMO, cheap variacs are a band-aid.
If you take one apart you are likely to find a cheap switch, suspect wiring, cheap sockets and dodgy wiper connections.

A bucking transformer with excess current capability is a better solution.
It can be adjustable if necessary.
Variac Issue BuckTrans_tn

Variac Issue BuckTrans_tn

See ieLogical Bucking Transformer

cci1492: Posts : 308
Join date : 2016-05-09
Age : 59
Location : Bergen County NJ

Post n°12

Re: Variac Issue

by cci1492 on Mon Nov 25, 2019 8:58 am

I built a bucker, been working fine:

Variac Issue Img_7216

rjpjnk: Posts : 209
Join date : 2018-07-18

Post n°13

Re: Variac Issue

by rjpjnk on Mon Nov 25, 2019 11:31 am

Interesting. How does this work? Is this a special kind of transformer?

cci1492: Posts : 308
Join date : 2016-05-09
Age : 59
Location : Bergen County NJ

Post n°14

Re: Variac Issue

by cci1492 on Mon Nov 25, 2019 11:41 am

I used a 6.3 VAC step down, you can use a 5 VAC too, depending on how much you want to take off. I'm utilizing the center tap of the output to take a little off. IEales has a nice link that shows you how to hook it up:

https://sound-au.com/articles/buck-xfmr.htm

rjpjnk: Posts : 209
Join date : 2018-07-18

Post n°15

Re: Variac Issue

by rjpjnk on Mon Nov 25, 2019 11:44 am

Thanks. I just found that IEales link to the bucking transformer in the other thread.

I was wondering if we could use any high current filament transformer for this application. Sounds like that is what you did.

IEales: Posts : 12
Join date : 2019-03-16
Location : Florence OR

Post n°16

Re: Variac Issue

by IEales on Mon Nov 25, 2019 11:50 am

rjpjnk wrote:Interesting. How does this work? Is this a special kind of transformer?

It's just a regular transformer.
Size the secondary for the amount of current you want to supply and the voltage to drop
Transformers are rated in VA so 5V @ 15A is 75VA

It works by wiring the primary and secondary out of phase.
The primary opposes the voltage in the secondary.
Simple math 122-5 = 117.
Using a 5VCT, gives two choices -5 and -2.5

EzPz!

RockyAM: Posts : 27
Join date : 2008-12-14

Post n°17

Re: Variac Issue

by RockyAM on Mon Nov 25, 2019 1:26 pm

That bucking transformer than Ian shows looks nice BUT the Hammond transformer listed for his bucking device is about $40 USD + shipping. When you add in the cost of the LED's, the switch, the AC outlets, the case etc., this thing has got to cost maybe $80+ in parts and you have to take the time to put it together. Plus it also only has two switched settings -5 VAC and -2.5 VAC. I have used the 5 amp model variac from Circuit Specialists now for about 7 years. Yes it is made in China but mine has worked fine with no problems for 7 years and does give continuously variable voltage control whereas the bucker that Ian shows has only two settings. Ian said "IMO, cheap variacs are a band-aid". That may be his opinion but not all Chinese products are poorly made. Check the Amazon link below for the same 5 amp variac that I have which is also sold by Circuit Specialists. 96% of the people who bought one rated it with 4 or 5 stars. RockyAM

Amazon version of 5 amp variac

IEales: Posts : 12
Join date : 2019-03-16
Location : Florence OR

Post n°18

Re: Variac Issue

by IEales on Mon Nov 25, 2019 2:22 pm

Without putting to fine a point on it, crowd ratings are next to worthless.
Also often hilarious. e.g. "Tested it at 1500 watts, and there was no arcing between the brushes and the windings."
It's a 500VA device! Why didn't the fuse blow?!?!?!

Note that the unit nominal input is 110V. US voltage is often 120V+. Set at 110V, it outputs the input voltage.
Again it's only IMO, but the potential exists to destroy your amp with this type of device.
That is not acceptable.

The panel meter is nowhere near accurate, not calibrated and may read ±10%.
Not a problem if the user is able to set the actual output under load with an accurate meter.

I've had a 1.1KW variac for 40 years.
Even with upgraded switch, wiring and socket, it still leaves much to be desired sonically.

Dave_in_Va: Posts : 350
Join date : 2013-04-02

Post n°19

Re: Variac Issue

by Dave_in_Va on Mon Nov 25, 2019 2:33 pm

This is from the great Robinette site which has loads of good info for guitar amps.

https://robrobinette.com/5e3_Modifications.htm#Bucking_Transformer

I built a couple of these making them even simpler by dropping the power light and not using the 12% drop which I would never need. Right now I show approx. 123.5v at the wall and just about 115v at the bucker.

Variac Issue KNiVqwL

Variac Issue KNiVqwL

Bob Latino: Admin; Posts : 2843
Join date : 2008-11-26
Location : Massachusetts

Post n°20

Re: Variac Issue

by Bob Latino on Mon Nov 25, 2019 2:49 pm

IEales wrote:Without putting to fine a point on it, crowd ratings are next to worthless.
Also often hilarious. e.g. "Tested it at 1500 watts, and there was no arcing between the brushes and the windings."
It's a 500VA device! Why didn't the fuse blow?!?!?!

Note that the unit nominal input is 110V. US voltage is often 120V+. Set at 110V, it outputs the input voltage.
Again it's only IMO, but the potential exists to destroy your amp with this type of device.
That is not acceptable.

The panel meter is nowhere near accurate, not calibrated and may read ±10%.
Not a problem if the user is able to set the actual output under load with an accurate meter.

I've had a 1.1KW variac for 40 years.
Even with upgraded switch, wiring and socket, it still leaves much to be desired sonically.

Ian > I must have over 100 customers who use the 5 amp or 20 amp versions of this variac mentioned above with no issues. You appear to be an "audio zealot" with respect to your opinions. IOW > You like to trash a product even though reviews of the product are quite different than your opinion. Let me tell you > I have seen some products really slammed on Amazon reviews. This product was given very good reviews yet you pick out ONE review > "Also often hilarious. e.g. "Tested it at 1500 watts, and there was no arcing between the brushes and the windings." It's a 500VA device! Why didn't the fuse blow?!?!?!" from one uninformed user who does not make sense. Yes - The panel meter with the analog meter is not accurate and you are advised to use a multimeter to measure the exact output on variac's outlet. Note - that the newer versions of this variac do have a DIGITAL meter. The fact is that these variac's do work well for many people and they do get good reviews on Amazon.

Bob

IEales: Posts : 12
Join date : 2019-03-16
Location : Florence OR

Post n°21

Re: Variac Issue

by IEales on Mon Nov 25, 2019 5:46 pm

Bob,

If we weren't all audio zealots, we wouldn't by VTA products. Very Happy

Very Happy

I read all the reviews.
Nowhere did I trash the product.
Pointing out flaws is a public service.

The review quoted was to point out how far off the beam are some 'reviewers'

More:
"and not all of the solder joints were the best,"
"This is a very cheap build, it does not produce the power that the dials indicate, be very careful using it, or you very well could damage what your trying to protect."
"after I changed the fuse, the “stationary” electrode in the back, behind the fuse popped out. I applied some heavy-duty glue to hold it back in place and it worked fine."
"Also, the output is NOT isolated as advertised."

Additionally, there are two carbon brushes in a variac.
Carbon has 200,000 times the resistivity of copper and generates a fair amount of heat.
Note the heatsink. I'll reserve judgement on its efficacy as it sits under the unventilated top.
Variac Issue CheapVariac

Variac Issue CheapVariac

Unless the wiper is moved occasionally, performance will suffer.
Frequent, as in daily, adjustment will wear the carbon contact and performance will suffer.

An error in resetting the device could be problematic.
Inadvertent and undetected setting error could be problematic.

I err on the side of caution.

Frankly, I was very surprised when a variac was suggested for my M-125s

because variacs are test devices
because a unit made 50 years after the U.S.A changed to 120V was still rated for 117V

p.s. Love the cap.

deepee99: Posts : 2265
Join date : 2012-05-23
Location : Wallace, Idaho

Post n°22

Re: Variac Issue

by deepee99 on Mon Nov 25, 2019 7:19 pm

Robert, et al.,
I read recentl I think from Circuit Specialists, re-seller, who said their variacs may have a problem with ground-fault interrupters. I had a front-seat view of this battle; GFCI to wall, variac to follow, and sure enough, the variac died while still on warranty. Why or how often that happens, I don't know.
I still see limits to the variac's use. The new ones don't really seem to be built for longevity. Mine was a 5-amp job, more than adequate for the load assigned it. Anybody know more about that?
I don't care for a variac in continuous service because they're just passing line-voltage deficiencies along to the appliance. But when it comes to firing up an old tube radio, or bringing a new VTA amp to life the first time, a variac can't be beat. And it will give you a darling little neon digital display.

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Post n°23

Re: Variac Issue

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