• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Report: One-Electron PRC as input choke

Well, since nobody seemed to have tried it, I got a couple of One-Electron plate chokes (PRC-2) from Antique Electronics to try as choke-input power supply chokes.

The power supply in question is a choke-input, ~300VDC @ ~110mA supply, using a 5V4GB or 5AR4 rectifier tube. The power xfmr secondary is 350-0-350, so there's close to 350V RMS AC across the choke. At 10H current ripple is fairly small, about 25mA (so peak current is about 125mA).

This is for a headphone amplifier project which I intend to sell (in small quantity).

Anyway, I compared several chokes:

#1: James 12H 200mA potted choke (not intended for choke input)

#2: Old stock Thordarson swinging choke, 8H - 20H @ 180mA

#3: Custom-wound power supply choke, requested spec 15H 110mA, for choke input service

#4: One-Electron PRC-2 parallel connected (12H, rated for 150mA as a plate choke)

Bottom line: The One-Electron works perfectly.

Details:

#1: The James choke performed pretty well, even though it is not (explicitly) designed for choke-input use. I know from a previous experiment that at ~180mA it buzzes in protest.

It is rated for 200mA, so one might expect it would be OK at 110mA. It did not make any audible noise, even at power-up, and did not vibrate significantly during operation. It exhibited a little bit of ringing (observing the rectifier cathode) but I wouldn't say it was ugly.

#2: This was an old choke designed for input service. Pretty big for it's rating; only the One-Electron PRC was bigger.

It was dead quiet and did not vibrate. Unfortunately I didn't bother observing the waveforms on this one because it's one-of-a-kind; even if I wanted to use it I've never seen another like it.

#3: Big dissapointment here. I asked a custom winder to make a choke for this exact application. It did not work out well. It vibrated significantly during operation - I think enough that I'd be worried about coupling noise into the tubes via microphony. It also made a nasty sound when the amp powered up - almost certainly it was saturating. When I asked the winder about it, the response was something like "choke input chokes always do that". :mad:

#4: The One-Electron PRC-2 was physically the largest of the lot, even though rated about the same as the others. Not the prettiest thing, but very well impregnated in varnish, which probably helps matters; it's dead quiet and no vibration. And the waveform observed at the rectifier was absolutely textbook perfect - no ringing at all. :)

Sort of makes sense, if you think about it... this choke was designed as a plate choke, so attention was no doubt paid to the winding arrangement. So from an AC standpoint it's pretty darn good.

Based on what I see I suspect that you could push the current up even more.

So, I'm going with the PRC. It's price is good ($60 from AES) compared to the other few "choke-input-specified" options (Sowter, Tamura, custom...) and it's performance seems perfect.

Pete