Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: chocopower on June 09, 2019, 12:34:15 pm

Title: Cheap "variac". To good to be true?
Post by: chocopower on June 09, 2019, 12:34:15 pm

Hi there?
I found this in ebay. Is safe to use as variac for tube amp work?


https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.es%2Fulk%2Fitm%2F323583637603 (https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.es%2Fulk%2Fitm%2F323583637603)

Title: Re: Cheap "variac". To good to be true?
Post by: shooter on June 09, 2019, 04:35:30 pm

probably, the specs and my Spanish are sketchy  :laugh:
I'd make sure your "test" amp has a good fuse  :icon_biggrin:

Title: Re: Cheap "variac". To good to be true?
Post by: kagliostro on June 09, 2019, 04:57:23 pm

I think that isn't usable to feed amplifiers

seems to be a voltage regulator not a real Variac

Franco

Title: Re: Cheap "variac". To good to be true?
Post by: DummyLoad on June 09, 2019, 06:25:27 pm

agree with franco: seems like a solid state phase control rather than a variable transformer.

i wouldn't use it.


--pete



BTW: the word "variac" started out as a general radio trademark for their line of autotransformers (variable transformer) first filed in 1933. general radio is defunct and a holding co., Instrument Service & Equipment, Inc. owns the trademark now. 

Title: Re: Cheap "variac". To good to be true?
Post by: chocopower on June 10, 2019, 03:19:34 am

Thanks.  Just what i thought.


I found this analysis (in English)
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.es%2Fulk%2Fitm%2F323583637603 (https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.es%2Fulk%2Fitm%2F323583637603)

Title: Re: Cheap "variac". To good to be true?
Post by: sluckey on June 10, 2019, 08:38:41 am

Quote

I found this analysis (in English)

Not my English.  :icon_biggrin:

That is the same link as in your original post.

Title: Re: Cheap "variac". To good to be true?
Post by: vampwizzard on June 10, 2019, 09:11:01 pm

Vary voltage up to 4000W. P = VI.. at 120V youre good up to 30watts of total current draw. (90% efficiency, 33.3 watts at 100% efficiency)

For small amplifiers this may be ok, but I would personally use a dedicated variac.

Title: Re: Cheap "variac". To good to be true?
Post by: DummyLoad on June 10, 2019, 10:14:58 pm

Quote from: vampwizzard on June 10, 2019, 09:11:01 pm

Vary voltage up to 4000W. P = VI.. at 120V youre good up to 30watts of total current draw. (90% efficiency, 33.3 watts at 100% efficiency)

For small amplifiers this may be ok, but I would personally use a dedicated variac.

????

you do mean 30 AMPS, yes? 4KW at 120V would be 33.33A.


--pete

Title: Re: Cheap "variac". To good to be true?
Post by: PRR on June 10, 2019, 11:31:20 pm

Just by the size, this can't be a 3000VA autotransformer.

The fit of screws/holes does not promise precision construction.

I think it is a lamp-dimmer. Incandescent lamps, simple heaters, etc. "Some" wall-power motors can be run on a dimmer. (Others burn-up and fill the attic with smoke....)

Title: Re: Cheap "variac". To good to be true?
Post by: chocopower on June 11, 2019, 08:07:10 pm

Quote from: sluckey on June 10, 2019, 08:38:41 am

Quote

I found this analysis (in English)

Not my English.  :icon_biggrin:

That is the same link as in your original post.

Sorry....


https://owenduffy.net/blog/?p=11745 (https://owenduffy.net/blog/?p=11745)

Title: Re: Cheap "variac". To good to be true?
Post by: PRR on June 11, 2019, 11:58:55 pm

It is a triac lamp-dimmer. And not done well.

Aside from electrical shock, this will BURN a power transformer.