PX25 fault?

[Radio Wrangler]

I'd probably go for 807/6L6/KT66 triode strapped. Their running voltage/current characteristics will be a reasonable fit to the load line imposed for the PX25s. You can still find NOS 807 and 6L6 at friendly prices, and if you want KT66 there are new ones out of China.

An add-on subchassis with a filament transformer (or auto transformer as Ed says) and a couple of valve holders will do the job without ruling out future restoration if you ever find another PX25.

David

[thyratron]

Thanks for all your esteemed comments. I have now obtained a replacement valve which involved a long drive. The postal service in this country is designed for EF50s not PX25s.

[thyratron]

A couple of pictures of the new valve in place.

[Peter.N.]

I would put that in a glass case if I were you.

Peter

[Techman]

Well, at least that's one valve that's been saved from being sent across the water into 'audiophoolery stockpiling' never to be seen (or indeed used) again! Well done for putting one of these valves to use in a genuine vintage repair.

When the heaters go like the one you had it often causes hum due to the imbalance with them being directly heated. Often it's a high resistance connection within the base pins, but in your case with such bright filament runs in the centre with nothing on the outer ones it seems like a genuine internal failure.

[Herald1360]

The half dead PX25 may still be of interest/value to someone out there.......


For an application where lower output doesn't matter, perhaps, or a single ended use.

[thyratron]

Indeed the valve in question was in fine fettle before losing half its burner. I have seen an example with much lower emission from an intact filament so it's certainly mightily useful. I don't suppose anyone ever designed equipment using a single PX25 unlike PX4; did they?

[Radio Wrangler]

You've got to wonder about non-invasive repair like laser welding?

David

[Synchrodyne]

Originally Posted by thyratron

I don't suppose anyone ever designed equipment using a single PX25 unlike PX4; did they?

In fact it was done, the Lowther B5F amplifier being an example. It was the post-WWII version of the pre-war B5. It was mentioned in the book "Audio! Audio!", page 50. It was only of 5 watts output, but that was evidently enough when it was used with the relatively efficient Voigt horn-loaded speakers.

Lowther mostly used triode outputs (PX4 and PX25) until 1956, when it swung over to "Lowther Linear", its own development of the Hafler & Keroes ultralinear circuit. (It did later use conventional UL with output valves that did not have accessible suppressor grids.)

Cheers,