Awakening the Sleeping Beauty - Reviving a GZ34 rectifier

[Hartley118]

I've recently been working on one or two Leeds-manufactured 25-30W valve amplifiers - typically Linear Conchord and RSC A10 models otherwise known as the 'Birdcage'. Together with suitably robust speakers, I find them excellent to lend out if a friend wants to amplify their iPhone audio to a cheerful party level. Unlike more modern solid state jobs, my experience suggests that they're unbreakable.

One amplifier, which had probably been out of use for for half a century, seemed completely dead, with no HT supply. Despite the GZ34 being lit and hot to the touch, it was outputting no DC. The mains transformer was OK, providing its 450-0-450 AC.

Not believing that a hot cathode could be emitting zero electrons, I plugged the valve into the VCM Mk4 tester. Sure enough, no shorts, and heater continuity OK. But, even with visibly glowing cathodes, there was no current flow on either anode (see the photo). Must be an internal disconnection I thought, but odd that *both* anodes could have lost their connections.......

With nothing to lose, I increased the heater supply from the rated 5V to 6.3V and saw a glimmer of anode current. A further increase to 7.5V produced a measurable current.

Not wishing to misuse the valve tester as a burn-in supply, I returned the valve to the amplifier to meet its full 450VAC input on the anodes. Slowly it began to perform, as did the amplifier - just a couple of watts at first, but after around half a day 'burn-in' I was seeing the full 30W output.

The valve now tests 100% good! (see the other photo),

So what's been going on in this valve which had probably 'slept' unused for many decades?

The amplifier had no <Standby> switch which could have kept the heaters alive but the HT off, which is recognised as a reason for poisoned cathodes. It's also interesting that, after 'waking up', the valve appears restored not just to marginal emission, but 100% on the VCM 160mA setting. So were the cathodes really poisoned, or just sleeping?

Suggestions will be most welcome.

Martin

[Diabolical Artificer]

That's a puzzler. Recently I've been testing small signal triodes with zero emission and have tried increasing heater voltage. This increased anode emission a bit but nothing like your GZ34. Maybe like some of us old un's it needed a kick up the artichoke to get us/it out of bed in a morning, once up it's not too bad, but rarely do I feel 100% though. Would that I was your GZ34, a few volts in the right place and youth restored, no more aching back and clicky knees : )

Sorry, not a technical answer. Andy.

[GrimJosef]

Originally Posted by Hartley118

... Must be an internal disconnection I thought, but odd that *both* anodes could have lost their connections ...

Good to hear that this very nice valve is working again . For a moment I wondered whether there might have been a single disconnection to the cathode. But although the valve's symbol only shows one cathode there are, of course, two separate cathodes in reality. They're both connected to one of the common heater rails and you know that that rail must be good because the heaters are alight. So again you'd need to have had two cathode connection failures.

I wonder if the cathode coating was somehow exotic and therefore capable of recovering its emission ? The Philips/Mullard datasheet for the GZ34, at least this one http://www.r-type.org/pdfs/gz34.pdf, is unusual in that it has two curves for the current needed to generate a given forward voltage drop. The first curve, dated 1.1.1954, shows a drop of 20V or so when the current's 150mA. The second one, dated 6.6.1958, shows the drop having fallen to 11.5V for the same current. I'm not aware that the electrode structure changed size or shape hugely between 1954 and 1958, so presumably the performance improvement is due to a more emissive cathode coating ? If so then some pretty effective development work was clearly going on.

Cheers,

GJ

[Hartley118]

Thanks for your comments. I can relate to them all, both technical and medical.

I wasn't going to bother to pursue this rather interesting experiment until I saw the typical price of a new GZ34!

A little more information:

The GZ34 in question is branded Tungsram, but has a Mullard Blackburn date code B5H2.
This suggests that it was made by Mullard/Philips in 1965 (or possibly 1975) in the second week of August. If it was an original fit in the amplifier, 1965 is the more likely.

That could well confirm GJ's suggestion of a rather advanced emissive cathode coating resulting from Mullard/Philips development work.

Martin

[toshiba tony]

Waking up a rectifier? New one on me, still why not. Cathode poisoning?

[trobbins]

Cathode interface resistance has been known to get quite high in triodes (eg. 6SN7) operating with very low duty cycle. Typically they 'came good' once some current can be forced to pass. It could be plausible that after 50 years, metallurgical processes have slowly caused a significant interface resistance, as although such processes have been seen to require operating cathode temperatures - that is in the context of a few years operation, and chemical processes don't stop even at room temperature.

[kalee20]

Well yes, but chemical reaction rates approximately double for a 10deg C rise. So, a reaction which progresses slowly at operating temperature (700 deg C maybe?) is going to progress 2^68 times slower at room temperature (20 deg C, so 680 degrees colder than operating temperature).

So if the valve poisons in the absurdly low time of an hour at operating temperature, it'll still take 33,600 billion years at room temperature (English billion).

There's a flaw in this reasoning obviously, but I can't put my finger on it, or suggest an alternative mechanism.

[GrimJosef]

I suspect the flaw is the assumption that a 10 degree temperature rise leads to the doubling of the reaction rate. The full story is covered here https://antoine.frostburg.edu/chem/s...ion-rate.shtml. As they amusingly point out, doubling with a 10 degree rise is a rule of thumb, so "thumbtimes it works, and thumbtimes it doesn't" .

Cheers,

GJ