Variac recommendations - not entirely boaty

Skylark

Skylark

Well-Known Member
Joined
4 Jun 2007
Messages
7,667
Location
Home: North West, Boat: The Clyde
Visit site
I have a mid 1980s radio which includes a valve power amp. Until recently, it hadn’t been used for more than 30 years but it seems to work ok.

I’ve been told that it’s “kinder” to the set to power it up slowly using a variac. I believe the primary concern is related to the capacitors of that generations.

Max power is shown as 300w within the spec.

Given the wide range of knowledge and experience on the forum, can I ask if anyone has any recommendations for a light use variac.

Places like Amazon and eBay show quite a diverse range from cheap and cheerful Chinese to used, old-school heavy as hell that granddad owned.

Anyone care to make a recommendation?

Thanks in advance.
 
Interestingly one of the items being restored in this weeks " The Repair shop " programme on BBC 1 was an old valve radio and amp ( homemade !)

The chap who does the electronics repairs powered it up slowly using a Variac Maybe this is the reason for your question if not maybe youd be interested in watching
The Repair Shop - Series 6: Episode 8

Sorry dont know where you'd get a Variac

RS have them at a price . Dont know any school physics teachers by any chance. The last one I saw was in a school physics lab. (Variac that is as well as physics teacher)
 
Last edited:
Last edited by a moderator:
A bloke who used to be my manager used to power his flymo via a variac, so he could turn it up to 110%, to compensate for the long, probably under-spec'd, mains lead needed for his long garden.

If you want to limit the inrush current, the easiest way is an NTC (negative temp coefficient) resistor.
Inrush Current Limiting using NTC Thermistor

Available from the likes of Farnell, or foraging in an old computer power supply perhaps!
 
VicS said:
Interestingly one of the items being restored in this weeks " The Repair shop " programme on BBC 1 was an old valve radio and amp ( homemade !)
Click to expand...
Thanks, VicS, we saw the program, it’s compulsive watching in our house.

My radio is a fairly high quality HF transceiver.


Rogershaw said:
A variac is basicly a variable auto transformer.
Several on E Bay
Click to expand...

Thanks Roger, I’d already searched eBay to find plenty of them. I’m hoping that sometime can tell me that the low cost Chinese ones are either a bag of rubbish or good value, based upon experience rather than perception.

Thanks to all posters, much appreciated. Still looking for someone with hands-on experience?
 
I must have powered up hundreds of dodgy bits of kit in various electronics labs, and I've never seen a variac used for this.
But then we were never in the TV entertainment business.

If you are going to manually ramp up the volts that slowly, you need to understand the circuit, and know whether it's actually a safe thing to do.
Some things do not 'like' being run at the wrong voltage. Less is not necessarily safer.
Some of these things have circuits designed to turn on in a certain order, and ramping the volts by hand over a few seconds or more might defeat that.
Running at half or 3/4 mains volts is probably not something the designer would have considered.
 
I’ve just sold one! I used to work in electronics (many years ago) and it’s standard practice to power up old valve equipment gently using a variac. It’s to ‘reform the capacitors’. I used to check various readings and insulation and then remove all valves, then ramp up the voltage watching for stress (smoke usually!) starting with the rectifier valve you insert it and try again checking the HT line voltage. If all is well add the some of the valves (starting at one end of the line up or the other. A signal generator and scope will tell you what’s happening. Watch out for leaky anode to G1 capacitors that leak high voltage into exactly the wrong place!

One can usually find a data sheet with ‘typical voltages’ round the valve pins and at critical points to check against. I’ve found using a traditional AVO gives more reliable readings against the charts/diagram than taking the readings from very high impedance digital multi meters.
 
From my experience from way back the biggest destructive force on valve gear on turn on is the application of high tension on valves where filaments are not up to temperature. It was standard practice in transmitters to warm up the valves before applying HT. So perhaps a relay on a timer to delay application of HT for 10 seconds might help.
If you are concerned about electrolytic capacitors forming you could also fit a time delay to reduce HT for a period. Perhaps via a high powered resistor (5kohms) which was bypassed (shorted) after a time. I would not suggest a variac on the AC input for the effect on filament voltage especially on old sets using a valve rectifier.
As an asside at a TV transmitter I worked at using hundreds of valves they reckon they increased the life of all valves (not just big ones) by cooling (air condition) the input air to cabinets so reducing temperature of the whole equipment. (however this in Perth West Oz where ambient temp can get quite high. ol'will
 
William_H said:
From my experience from way back the biggest destructive force on valve gear on turn on is the application of high tension on valves where filaments are not up to temperature. It was standard practice in transmitters to warm up the valves before applying HT. So perhaps a relay on a timer to delay application of HT for 10 seconds might help.
If you are concerned about electrolytic capacitors forming you could also fit a time delay to reduce HT for a period. Perhaps via a high powered resistor (5kohms) which was bypassed (shorted) after a time. I would not suggest a variac on the AC input for the effect on filament voltage especially on old sets using a valve rectifier.
As an asside at a TV transmitter I worked at using hundreds of valves they reckon they increased the life of all valves (not just big ones) by cooling (air condition) the input air to cabinets so reducing temperature of the whole equipment. (however this in Perth West Oz where ambient temp can get quite high. ol'will
Click to expand...
All agreed. Although perhaps I didn’t make it clear that the variac is only used the first time you resurrect the radio after being ‘off’ for so long. I was playing with an AR88 a couple of years ago and had to replace the old main smoothing capacitor(s). The original ones were paper oil filled from when it was made in Canada in WW2. The oil was leaking out and it was a very messy business. Lots of little ‘bath tub’ capacitors got changed too. But I drift the thread, so my apologies.
 
Many thanks, John and ol’will.

My concern relates to the HV caps and the cathode resistors which, notoriously, don’t always awake from 30+ years in hibernation. Standard recommendation does seem to be power-up slowly using a variac.

My set seems ok. With some trepidation I turned on the heater. I first put low power (tune mode) into a dummy load before loading an antenna.

I’ve yet to wind up the carrier to full power but so far, so good.

Most of the advice I’ve received is from hybrid radio user groups but most are American so not able to recommend a suitable variac.

Brilliant responses to make reference to an AR88 and an AVO8 in the same thread ?

I listened to HF from an AR88 as a teenage. I still have a AVO somewhere.

So, back to my question, I will likely only use the radio infrequently so I’m still interested to know how a low cost, Chinese variac compares to, say, an older heavyweight, used one from either eBay or John’s garage ?
 
I am another that has a donkeys years old British made variac. I actually have two, my big weighs a ton 8A one and a smaller 2A one that I would use for this job.

I always thought with old capacitors you should bring the voltage up very slowly over a period of minutes, not seconds.

Another tip for testing dubious mains powered kit, particularly something like an inverter, is power it up via the variac but include an old school 100W filament lamp in series with the supply. If anything breaks down or has a short, instead of blowing anything, the lamp will just light up full brightness.
 
Skylark said:
I believe the primary concern is related to the capacitors of that generations.
Click to expand...
I'm no expert - my most advanced electronics project was a mains adaptor for a transistor radio 40 years ago - but I've watched a few restorations on of old electronics on Youtube. One thing they're often leery of is a capacitor in the power supply. Apparently, if it goes bad, it can make the chassis live. Would it be worth finding out if you have such a capacitor and, if so, replacing it with a modern one? If I understood what was being said, modern ones fail safe in that application, the old ones don't
 
You can bring the electrolytic capacitors back to life if they are not totally knackered by running the set up in series with a light bulb.
Starting with say a 40w bulb for an hour then work up to 60w, 100w then maybe 2 bulbs in parallel. Much cheaper than a variac and better if something is going to maybe go dead short, will do no damage.

I restore vintage radios as a hobby, always run them up first on a bulb dropper even though I do have a variac.
There will be lots of variacs going second hand I would imagine from theatres now that they are going over to LED lighting.
 
Boater Sam said:
You can bring the electrolytic capacitors back to life if they are not totally knackered by running the set up in series with a light bulb.
Click to expand...

That's good and reassuring, Sam. The set has been working in total now for a couple of hours including low power transmission into a dummy load and rf out to the always-present Russians and Eastern Europeans :)

No smells, no strange noises - are you suggesting that I can now rest easy or are there other, sensible precautions to take?

As I wrote in #10, the HV caps and cathode resistors are known to fail after a long period of hibernation but it's by no means all of them.

thumbnail_IMG_1366.jpg
 
Just because it's got a few valves at the back end, it's not like a WW2 relic with ancient type caps.
It also has a lot of solid state in it, you might want to consider what effect running that at half volts might do.

If a few HV caps are going to fail, then replace them.
If you don't know exactly what the rest of it consists of, you might be risking that running it from a low supply.
A few HT caps is few quid to replace with better parts, damaging the solid state part of it could reduce it to junk.

It's not like working on just a valve circuit.

The sensible thing might be to remove and test the likely suspect caps, depending on what they are , you might replace them with better.
But 1980s caps are possibly pretty reliable, unlike 1940s items.
1980s stuff is more likely to be killed by time at volts and temperature than years in storage.
 
You must log in or register to reply here.

Share this page

Top