Uses of large choke

[qualityten]

I have recently obtained a large, high quality, varnish-dipped choke. It measures 1.9H and has a DCR of 18 ohms. Would this be suitable for use in a power supply for a valve amplifier? I attach a pic, with a pen giving an idea of size.

[G6Tanuki]

Back in the old days the traditional smoothing-chokes for valve HT supplies to amplifiers/receivers/transmitters were more like 10H.

But that was back when the only electrolytic/paper-in-oil smoothing-capacitors were of really low values so choke-input, swinging-choke or C-L-C HT smoothing was the only practical way to do it.

The world's moved on in the last 60years: now we have good high-capacity high-voltage electrolytics and semiconductor diodes, so the need for chokes in HT supplies has passed.

Your choke's too-low an inductance to be useful in recreating an old-fashioned valve amp power-supply. There are much better, choke-free ways to deliver a few hundred Volts to an amplifier these days. Treat your choke as an amusing museum-exhibit rather than trying to build an amplifier around it.

[AC/HL]

With those values it was probably used in a low voltage supply rather than high, although a Henry is a Henry whatever the use.

It has only five time fewer Henrys than a traditional HT supply choke, simply up the smoothing cap by five and it will do a good job. Great for a huge class A stereo amp, shame not to use it. Considerably less resistive loss than a resistor, in this green world any watts saved must be a good thing.

[Argus25]

Originally Posted by merlinmaxwell

Great for a huge class A stereo amp, shame not to use it.

Yes a very good idea.

It is a beautiful part "they don't make em like this any more"

In fact it could be used as the collector output choke if you wanted to make a beautiful mono amplifier that operated from 12V with a large germanium output transistor in class A, something like a 2N174, or ADZ12 running a 4R speaker. That transistor driven by a matching transformer and an EF98 12V HT operated valve and you could add another EF98 as a pre-amp and series the two heaters.The DCR might be a tad high, you could run it from 24V possibly

If you have a look in the early Philips transistor manuals, they give example circuits with values for these sorts of amplifiers which were used in hybrid car radios.Normally the choke would have a lower DCR but I think with the right setup it could be made to work with a modest collector current and power output.

[qualityten]

Thank you for the diverse responses. It was the low inductance and resistance made me wonder what it was originally used for. The seller had no idea.

Thanks to MerlinMaxwell for the suggestion of a huge class A stereo amp! Hmm. And to Argus25, whose suggestion came in as I was writing this reply.

It's good to know it's useful. It's beautiful to look at, but too good for a paperweight.

[Radio Wrangler]

Um, you'd need to find another one, just like it, to make a stereo amplifier...

David

[Diabolical Artificer]

Looks a nice choke and from it's size it might do for a stereo HT supply. What's the core size?

"so the need for chokes in HT supplies has passed." So what do you use to filter a HT supply instead?
AFAIK the only other way to suppress ripple is regulation, and chokes are still used in HT supplies as they are better than a plain old resistor.

Andy.

[G6Tanuki]

Originally Posted by Diabolical Artificer

"so the need for chokes in HT supplies has passed." So what do you use to filter a HT supply instead?

Masses and masses of capacitance.

In the big valve linear-amps I used to design, typically they had at least 1200uF of smoothing [ series-parallel networks of electrolytics, with voltage-balancing and bleeder-resistors **everywhere** ] - this was a lot lighter than anything involving chokes, and provided a good low source-impedance for the valve to draw current from on peaks.

Chokes were OK in times when high-value high-voltage capacitors were expensive/unreliable and (valve) rectifiers couldn't handle surges of tens of amps without damage.

[Jon_G4MDC]

OP's choke looks like it came from some piece of Pye gear.
Maybe a T30AM or T55AM I'm thinking...

In fact one just like it,with 400V on the tags, bit me when I did something stupid with my U450 TX...

[Radio Wrangler]

The inductance value would be good for a modulation choke with say, 300Hz cutoff. in an AM transmitter for speech.

David

[Herald1360]

Make a good doorstop for the shack..........

[qualityten]

Originally Posted by Radio Wrangler

Um, you'd need to find another one, just like it, to make a stereo amplifier...

Can you say why David? Because I need a separate power supply for each channel?

[qualityten]

Originally Posted by Herald1360

Make a good doorstop for the shack..........

That's what it's doing ATM! It weighs 2 Kg.

Andy, the lamination stack is 38mm thick. Not sure what dimensions to measure for the core size.

The only markings on it are a small white label marked 79042 and a stamp on the frame added before the varnish, 17665. If only these things could say, 'I was formerly employed as a ...'

[joebog1]

Looks like a fine choke to me !! I still use chokes as a "rule of thumb" in all valve amplifiers. With guitar amps I only use a choke for the input stages and phase splitter, I reckon it STILL reduces hum. As far as value goes it sounds perfect to me if using sand rectifiers and some big cappies. 100uF. 1.9H, 100uF or more, and thats the perfect power supply. My Golden ears amp uses one choke per channel, and they are only 1 Henry. I am using fast rectifier diodes with 100uF 1 H choke and 200 uF. My chokes are also ex equipment, namely a 200 watt base station from a Royal Flying Doctor Service. They weigh 2.3 kilos each, but are potted so I figure they will be the same size as qualityten's choke
I give it the thumbs up as a good idea


Joe

[Skywave]

I can't put my finger on exactly why, but somehow, looking at that choke, I get the feeling that it was commercially made item for fitment into a piece of test gear or radio comms. equipment for use by the military or government service.

Al.

[Radio Wrangler]

The quality of the varnish impregnation looks expensive, but that spirenut on its mounting foot says it's from more economical gear.

David

[Skywave]

Ah yes! I'd spotted that - and the same thing did cross my mind, too. It's just that there's a certain familiarity about the appearance of that choke . . . from somewhere, sometime, something . . . Perhaps it's time to have a rummage through my box of salvaged HT chokes . . .

Al.

[Skywave]

Alas! Nothing similar found. But that choke has a certain 'Pye' look about it: Ex-Pye 697 CTV?
I'm really tapping into very distant memories now!

Al.

[Diabolical Artificer]

The core dimensions would give you a rough idea of it's current handling , we have 38mm, now you need to see if you can get an idea of the other side.

A better way is to connect it up under load and push it until the output current changes, it buzzes or gets warm to find it's current capability. At 2kg it is fairly hefty which may mean it could filter a supply for two amplifiers.

If you say an EL84 amp needs about 120mA call it 250mA for a stereo jobbie with an HT of 350v you'd need about a 1k5 load of 100w say. Better a 1k and a 500 ohm each rated at 100w. If your using those ali clad jobbies they need a heatsink or you need to keep an eye on how hot they're getting. A voltage follower and a few big mosfets on a heatsink would do the same job.

Andy.