0.75A variac suitable for vintage radios? - UK Vintage Radio Repair and Restoration Discussion Forum

SurreyNick · 17th May 2015, 7:07 pm

With the price of enclosed variacs being so expensive I am considering assembling one myself and I have found a design which includes a useful built-in voltmeter and all-in should cost just £60-70 to assemble. The spec uses a 0.75A open-type variac and I was wondering if this is suitable for use with vintage valve radios?
Thanks
Nick

Ed_Dinning · 17th May 2015, 8:42 pm

Hi Nick, if the radio is rated below 180 Watts it should be fine to use.
Slight overloads for a short time should nor be a problem as it has a pronounced thermal lag.
Note that the rating refers to the Wiper current, not the input current as the carbon brush is the limiting factor.

Ed

glowinganode · 17th May 2015, 8:48 pm

Hi Nick, 0.75A sounds a bit on the lean side, but it depends what sort of radio you intend to use it to power. If you are going to power an AC/DC set with it, I'd suggest 2A is a good compromise between cost and durability.
Rob.

kalee20 · 18th May 2015, 6:25 pm

It is a bit on the lean side, yes, but should still be within its ratings even with an AC/DC set using a 0.3A chain.

If the choice is between having a 0.75A Variac or no Variac at all, take it and use it! Just put a fuse in series with the wiper output.

Phil G4SPZ · 18th May 2015, 11:08 pm

For my bench supply, I use a small 1 amp variac in conjunction with a CVT-type isolating transformer that is only rated at 120VA. This saturates at anything over 0.5 amp and the output voltage just falls away, which is quite a useful safety feature. I also monitor the AC current drawn by the receiver under test, and most 'normal' vintage valve radios draw little more than 150mA if working properly. So for most radio purposes, I'd say that your 0.75 amp variac will be more than adequate.

Perhaps if you were supplying larger devices such as TV sets etc you would need something a bit bigger.

broadgage · 19th May 2015, 11:36 am

Yes, a 0.75 amp variac will be fine for almost any domestic radio receiver.
I would regard 0.4 amps as about the highest mains input current likely for a transformerless AC/DC set. That allows for a 0.3 amp heater chain (many are less) and for 0.1 amp HT.

A set with a mains transformer will probably be less unless of most improbable size.

SurreyNick · 19th May 2015, 6:34 pm

Well all of that is really good news and I'm very happy as I have managed to pick up everything I need to assemble the unit, including the AE variac, for just £43.20 and that even included postage!
Should take just an afternoon to assemble it too

Many thanks
Nick

FrankB · 21st May 2015, 5:30 am

Just remember to de-rate the .75A rating if you are using it to put out a very low voltage.
After a chat with one of the variable autotransformer company engineers, A number of years back; I found out the rating is across the entire winding, not just part of it.
This explained why the transformer I was using on quite low output voltage let out all the factory smoke.
Here at the hamfairs one can usually find a nice one rated for 5A for $25-30.00 USD.
Often much less if you can find an old piece of equipment with one of them in it, and buy the equipment for the spares in it, and pull the trans. out of it.
I always use a variable auto trans. with an appropriately rated isolation trans. also.
Its not fun getting fried by a hot chassis radio or TV set. Been there, done that.
Most of my var. TX. setups I built are 5-10A and fully metered and fused.

I like the idea of a CVT setup, and will try to find one here to use.

broadgage · 21st May 2015, 9:42 am

I fail to see why the current rating of a variac would be reduced at low output voltages.
Surely the current rating of say 0.75 amps is determined by the current carrying capacity of the winding wire or of the carbon brush, neither of which is affected by the output voltage setting.

The output capacity in KVA does indeed reduce at low voltages, and variacs used to be rated in VA/KVA.
If a variac is rated at 2.4KVA at 240 volts, you certainly can not draw 2.4KVA from it with the output set to 24 volts as that would be 100 amps. But I believe that you could draw the 10 amps continually at any voltage from a 2.4KVA variac.

Mr Moose · 21st May 2015, 11:05 am

Hello,

This document ,although technically only referring to Claude Lyons Variacs, is a good introduction to their use.

http://www.claudelyons.co.uk/pdf/tech.pdf

It states that "Rated current can be drawn at any brush setting"

Yours, Richard

SurreyNick · 22nd May 2015, 6:02 pm

This question is slightly off topic but I didn't think it worth starting a new thread, so I hope you will forgive me.

My question is....how hot is the variac likely to get?
I need to decide whether it is necessary to drill ventilation holes in the enclosure.

Nick.

Boater Sam · 22nd May 2015, 7:57 pm

FrankB is in USA, 115v so twice the current!
You will be fine with minimal ventilation, you are not running it at full output with a radio, and not for hours on end either.
Regard a variac as test equipment. Short time use intervals are all you need for repair work.
I have used 0.5A variac on battery chargers at near full current and never cooked one yet.

17th May 2015, 7:07 pm	#1
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	0.75A variac suitable for vintage radios? With the price of enclosed variacs being so expensive I am considering assembling one myself and I have found a design which includes a useful built-in voltmeter and all-in should cost just £60-70 to assemble. The spec uses a 0.75A open-type variac and I was wondering if this is suitable for use with vintage valve radios? Thanks Nick

17th May 2015, 8:42 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: 0.75A variac suitable for vintage radios? Hi Nick, if the radio is rated below 180 Watts it should be fine to use. Slight overloads for a short time should nor be a problem as it has a pronounced thermal lag. Note that the rating refers to the Wiper current, not the input current as the carbon brush is the limiting factor. Ed

17th May 2015, 8:48 pm	#3
glowinganode Octode Join Date: Apr 2009 Location: Aylesbury, Buckinghamshire, UK. Posts: 1,522	Re: 0.75A variac suitable for vintage radios? Hi Nick, 0.75A sounds a bit on the lean side, but it depends what sort of radio you intend to use it to power. If you are going to power an AC/DC set with it, I'd suggest 2A is a good compromise between cost and durability. Rob. __________________ We have done so much, for so long, with so little, we are now qualified to do anything with nothing.

18th May 2015, 6:25 pm	#4
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,088	Re: 0.75A variac suitable for vintage radios? It is a bit on the lean side, yes, but should still be within its ratings even with an AC/DC set using a 0.3A chain. If the choice is between having a 0.75A Variac or no Variac at all, take it and use it! Just put a fuse in series with the wiper output.

18th May 2015, 11:08 pm	#5
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: 0.75A variac suitable for vintage radios? For my bench supply, I use a small 1 amp variac in conjunction with a CVT-type isolating transformer that is only rated at 120VA. This saturates at anything over 0.5 amp and the output voltage just falls away, which is quite a useful safety feature. I also monitor the AC current drawn by the receiver under test, and most 'normal' vintage valve radios draw little more than 150mA if working properly. So for most radio purposes, I'd say that your 0.75 amp variac will be more than adequate. Perhaps if you were supplying larger devices such as TV sets etc you would need something a bit bigger. __________________ Phil Optimist [n]: One who is not in possession of the full facts

19th May 2015, 11:36 am	#6
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,130	Re: 0.75A variac suitable for vintage radios? Yes, a 0.75 amp variac will be fine for almost any domestic radio receiver. I would regard 0.4 amps as about the highest mains input current likely for a transformerless AC/DC set. That allows for a 0.3 amp heater chain (many are less) and for 0.1 amp HT. A set with a mains transformer will probably be less unless of most improbable size.

19th May 2015, 6:34 pm	#7
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: 0.75A variac suitable for vintage radios? Well all of that is really good news and I'm very happy as I have managed to pick up everything I need to assemble the unit, including the AE variac, for just £43.20 and that even included postage! Should take just an afternoon to assemble it too Many thanks Nick

21st May 2015, 5:30 am	#8
FrankB Heptode Join Date: Oct 2005 Location: Olympia, Washington, USA. Posts: 664	Re: 0.75A variac suitable for vintage radios? Just remember to de-rate the .75A rating if you are using it to put out a very low voltage. After a chat with one of the variable autotransformer company engineers, A number of years back; I found out the rating is across the entire winding, not just part of it. This explained why the transformer I was using on quite low output voltage let out all the factory smoke. Here at the hamfairs one can usually find a nice one rated for 5A for $25-30.00 USD. Often much less if you can find an old piece of equipment with one of them in it, and buy the equipment for the spares in it, and pull the trans. out of it. I always use a variable auto trans. with an appropriately rated isolation trans. also. Its not fun getting fried by a hot chassis radio or TV set. Been there, done that. Most of my var. TX. setups I built are 5-10A and fully metered and fused. I like the idea of a CVT setup, and will try to find one here to use.

21st May 2015, 9:42 am	#9
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,130	Re: 0.75A variac suitable for vintage radios? I fail to see why the current rating of a variac would be reduced at low output voltages. Surely the current rating of say 0.75 amps is determined by the current carrying capacity of the winding wire or of the carbon brush, neither of which is affected by the output voltage setting. The output capacity in KVA does indeed reduce at low voltages, and variacs used to be rated in VA/KVA. If a variac is rated at 2.4KVA at 240 volts, you certainly can not draw 2.4KVA from it with the output set to 24 volts as that would be 100 amps. But I believe that you could draw the 10 amps continually at any voltage from a 2.4KVA variac.

21st May 2015, 11:05 am	#10
Mr Moose Octode Join Date: Jul 2008 Location: Gloucestershire, UK. Posts: 1,043	Re: 0.75A variac suitable for vintage radios? Hello, This document ,although technically only referring to Claude Lyons Variacs, is a good introduction to their use. http://www.claudelyons.co.uk/pdf/tech.pdf It states that "Rated current can be drawn at any brush setting" Yours, Richard

22nd May 2015, 6:02 pm	#11
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: 0.75A variac suitable for vintage radios? This question is slightly off topic but I didn't think it worth starting a new thread, so I hope you will forgive me. My question is....how hot is the variac likely to get? I need to decide whether it is necessary to drill ventilation holes in the enclosure. Nick.

22nd May 2015, 7:57 pm	#12
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: 0.75A variac suitable for vintage radios? FrankB is in USA, 115v so twice the current! You will be fine with minimal ventilation, you are not running it at full output with a radio, and not for hours on end either. Regard a variac as test equipment. Short time use intervals are all you need for repair work. I have used 0.5A variac on battery chargers at near full current and never cooked one yet.