Buying a PX4

OK, so you've decided that you are going to try a PX4, or two for a stereo pair, so where are you going to find one?

First of all, ask yourself 'Why do I want a PX4?'. Possible answers include:

  1. I want to build a PX4 amplifier.
  2. I want to restore an old radio which uses PX4's.
  3. I want to build a PX4 collection.

If your answer was iii) above, then it probably doesn't matter too much how well the valve works, or even if it works at all. You probably want one of the early balloon-shaped valves, just because they're beautiful.

If your answer was ii), then you need a working valve, but the highest performance is not necessary, or you could use one of the cheaper PX4 equivalents. This gives you more flexibility in your choice.

If you want to build a PX4-based amplifier, then presumably you want the highest quality - otherwise why bother? - and you'll want one of the later higher-powered designs, not a balloon however nice it looks.

Now decide what your budget is. Keep to it. If possible, make sure you can see a valve, and its test results, before you buy.

Possible Sources

Specialist Dealers

There are a number of these dealers - in England we have Billingtons, Chelmer Valve, and quite a few others. In theory, these guys may offer a large choice, and offer a warranty. The price is that they charge a lot.

In practice, with something as rare as a PX4, these advantages may not apply. Once I bought a PX4 from Chelmer Valve, it was supposed to be a NOS (New Old Stock) valve. It was very expensive, and when it arrived it was in a new plain white box, not a genuine original box. This was a little worrying, but not a show-stopper.

With eager anticipation, I put the valve into one of my single-ended triode amps and switched on. Sound quality was poor, and intermittent. I was gutted, and my feelings were not improved when I rang Chelmer.

On a close inspection of the valve, I found that one of the internal wires was not soldered into its pin on the valve base, any electrical connection was entirely fortuitous. I put this right and tried again - hey presto! - I had a working PX4, not the best I've heard but pretty good. I suspect this valve was some kind of reject and had little use although it probably wasn't NOS.

Don't accept a valve as NOS unless it's in its original box, and you have some accurate test data.

Vintage and Amateur Radio Shows and Swap Meets

You'll often find PX4's at these shows, and you'll come across gentleman valve dealers like Jim Fish of Wilson Valves.

Junk Shops and Car Boot Sales

These sometimes have old valves, radio sets or amplifiers which run PX4's.

eBay logo

You'll usually find some PX4's on eBay, once you've filtered out the Korg PX4 guitar effects processors. Unfortunately, not all the offerings are genuine PX4's - there's a tendency to offer, for example, Mullard AC044's as PX4's - and people get carried away in their bidding, hiking the price to extraordinary levels.

Things to watch out for

Thanks to Rod Burman for these tips:

  • With the grey anode versions of the PX4, patches of discolouration on the anode may indicate that the valve has been over-loaded.
  • Sometimes dealers end up with a PX4 where half the filament has gone. Whilst no good for serious audiophile use, these are OK for old radios and may be bought for around £ 20 each.

Remanufactured PX4's

If your main objectives are audio quality and reliability, KR Electronics of Prague have re-manufactured the PX4. The quality is good - sound at least up to the standard of a genuine NOS PX4 - and there's a one year warranty.

In the UK, the KR PX4's are available from Valve and Tube Supplies at a cost of £ 220 per pair. They come in a smart box, with individual test data.

At this price, it isn't worth messing around with used valves of unknown provenance, unless you're collecting of course.

If you know of someone supplying PX4's in your country, please let us know and we'll publish contact details here.