Re-claiming a leaky UL41

[kalee20]

After I took an interest in this thread, Allenmac kindly sent me his old UL41.

The fault was, big mains hum from the DAC90a in which it was fitted. As UL41's are a finite resource, I wanted a play with a bad one on the basis that there's nothing to lose yet some 'out there' could be saved. And I would have fun at the same time.

Fitting to my own DAC90a confirmed the fault. (Incidentally, my DAC90a has pin 4 isolated from the grid circuit, so the effects of internal leakage should be less).

I made some resistance measurements (cold):

Heaters - cathode 265k
Heaters - control grid 6.3M
Anode - control grid (very high >30M)
Screen grid - control grid 18M

The presence of leakage from screen grid to control grid would put positive voltage on the control grid (unless an absurdly low value of grid leak is used). Not good. The presence of leakage from heater to control grid would impress hum, as to a certain extent would leakage heater to cathode in the DAC90a (which, lacking a cathode bypass capacitor, has significant impedance cathode-chassis). Not good either.

The inside of the glass envelope looked very discoloured, with brown deposits on the inside of the glass. These are commonly held to be conductive. I thought I would try to vapourise these in a microwave oven. Putting the valve in, together with a cup of water, however caused bright flashes from the getter so I stopped immediately. Just as well, as the gettered area was hot and would no doubt have cracked the glass very quickly.

I then tried wrapping the valve tightly in aluminium foil, leaving the base bare, to shield the gettered area and electrodes. However, on switching on again, I got a bright orange glow from inside the base so again switched off fast. The total exposure to microwaves was thus about 3 seconds.

I then remeasured the leakages:

Heaters - cathode 325k (improvement)
Heaters - control grid 5.3M (worsened)
Anode - control grid still very high
Screen grid - control grid 15M (worsened)

So, not very conclusive! I considered making an RF heating coil, at lower power, to more gently cook the base. However, on reflection, the idea of vapourising the deposits en masse, whether by RF or microwave, without controlling or even knowing where they end up, is not really very sound engineering.

I therefore considered applying high voltage to selectively zap away the conductive deposit around the control grid pin only. This would still create vapourised deposit, but as there will be much less of it, I hoped that where it ended up would have lesser consequences. Therefore, I applied 2kV AC and then 4kV DC between heaters and control grid, and also screen grid and control grid, for a couple of minutes, with an insulation tester. I did NOT try to apply a high voltage betweeen heater and cathode, because I didn't want to break down the heater insulation! After about 2 minutes between each of the electrode pairs named, I stopped and measured:

Heaters - cathode 325k
Heaters - control grid very high (big improvement)
Anode - control grid still very high
Screen grid - control grid 24M (best so far)

This was taking a long time, so I strapped together all pins except the grid, and applied high voltage from a breakdown tester. At 8kV I was getting internal sparking which wasn't what I wanted, so I reduced to 3kV, and held the high-voltage lead about 2mm away from the grid pin so that a little spark continuously jumped the gap.

I let this run for about 5 minutes. I was rather hoping that the external spark would decrease, as the internal leakage reduced. This did not happen, so after about 5 minutes I disconnected and measured anyway:

Heaters - cathode now off scale >30M (can't explain this!)
Heaters - control grid off scale >30M
Anode - control grid off scale >30M
Screen grid - control grid off scale >30M

So, there is a definite big improvement! Replacing in my DAC90a, I powered up. Lo and behold, the valve still worked, minus the hum! In fact, the sound was now indistinguishable from the original UL41 in my radio. I let it run for half an hour, listening to Grieg's piano concerto. And it continued to work fine!

My conclusions thus are as follows:

* Microwaving is no good.

* Steady high-voltage is what is needed, not impulsive bursts.

* Voltage required is about 3-4kV, current limited to 1mA, applied between grid, and all other pins strapped.

* Exterior of valve must be clean and dry.

[Steve_P]

Interesting. I knew it was a problem with grot round the base inside but couldn't find out of a way to clear it.

Now - a voltage source....

Cheers,

Steve P.

[Sideband]

Alan Lord http://old.dundeecollege.ac.uk/Secti...taff/al_radio/

has already built a device to do this. It runs at about 3kV. Have a look on his website and see 'cool things to build yourself'.

I also tried the microwave and had no luck. I haven't built Alans gadget but something similar would probably do very nicely.


Rich.

[ppppenguin]

Some useful experiments well done.

If you want to build Alan Lord's design the easiest approach is probably to use the EHT transformer from an old CRT computer monitor. Usully diode split secondary so you don't need a tripler either. Drive it gently - it will produce a lot more than 3kV given a chance.

[paulsherwin]

High voltages certainly seem to be the way to go.

Anyone thinking of trying this should bear in mind that these voltages can be extremely dangerous. If you don't possess the appropriate skills and experience, it would be better not to use this technique.

Paul

[richrussell]

Indeed - I put my duff UL41 in a drawer and bought a shiny new CV1977 for £5 online

[kalee20]

Originally Posted by richrussell

I put my duff UL41 in a drawer...

Great! Can I have it please, (and a fault report)? I'll pay postage!

[Paul Stenning]

It is interesting that two independent tests, yours and Alan Lord's, have found the same results.

[richrussell]

Originally Posted by kalee20

Great! Can I have it please, (and a fault report)? I'll pay postage!

You're welcome. PM me your address and I'll pop it in the post. Does work, but came out of a Bush DAC34 with a leaky coupling capacitor - output transformer burnt out and rectifier suffering from the excessive current. Replaced the above and it hums badly, so probably a good candidate for reconditioning.

I never like to throw things like this away - after all, somebody might want it someday...

[Richard]

Hi

I must get around to trying this, have a box of about 12 old UL41,s some I know are very leaky, and an insulation tester that goes up to 30,000V so that should do the job...

Richard

[kalee20]

Originally Posted by Paul Stenning

It is interesting that two independent tests, yours and Alan Lord's, have found the same results.

Thanks Paul, in fact I had seen Alan Lord's website some time ago but I wanted to try (or at least consider) everything else I could think of.

Another option, for anyone lacking an insulation tester, would be to make a high-speed chopper (a kilohertz or so) feeding an old car ignition coil. Operating at a suitably low voltage, this should give the same results.

I've now been promised another UL41 by Richrussell in a post above - this one may be a bit more serious as it sounds like it's been seriously over-run for some time. The cathode emission may have taken a hammering, and I can also imagine extra anode dissipation having caused outgassing in which case the getter may be rather used-up. But, we shall see!

[richrussell]

The one I'll send you does work to some extent, so at least it's mechanically intact, just a bit unhappy.

Rich.

[kalee20]

I have richrussell's UL41 now. While the gettering is a bit dull, with plenty of rainbow colours at the fringes (indicative of prolonged high-temperature working), it measures rather well on leakage.

Grid to all electrodes reads >30M so I was comfortable enough to put in my DAC90a.

Switching on, the grid voltage is 30mV which I am very comfortable with. And it performs fine!

The only suspect pin is Pin 4, which is theorectically isolated. This measured 8.6M to heater. So, in a 'normal' DAC90a where this pin is used as a convenient grid-circuit anchoring tag, there would be significant hum injected. I've isolated Pin 4 in my DAC90a for this reason.

Anyway, as I seem to have a perfectly useable valve, I'm not qoing to zap it! But any unuseable ones, I would be grateful for - postage will be refunded!

[PJL]

Not having access to a 3KV PSU I tried connecting mine up to the gas cooker automatic ignition. It's an electronic type that fires with a pretty healthy pulse 1-2 times per second. So far I have recovered a UL41 and a PM24A both of which now have no measurable (200M range) resistance between pins and show minimal grid current on my mullard tester. Time will tell if this is a permanent fix.

[paulsherwin]

Originally Posted by PJL

Not having access to a 3KV PSU I tried connecting mine up to the gas cooker automatic ignition. It's an electronic type that fires with a pretty healthy pulse 1-2 times per second..

That's an excellent idea! I can hook mine up to a valveholder with just a pair of croc clips and some wire.

Again, I'd advise people to be careful though. The shock might not kill you but it could give a very unpleasant belt.

Paul

[richrussell]

I wonder if it hummed in my DAC34 because of the pin 4 issue? Ah well, maybe not all UL41s are as doomed as it seems.

Rich.

[kalee20]

Originally Posted by PJL

Not having access to a 3KV PSU I tried connecting mine up to the gas cooker automatic ignition. It's an electronic type that fires with a pretty healthy pulse 1-2 times per second.

That's the type of thing I'd decided against - the voltage is very high but brief - and I'd be worried that rather than zapping away the conductive deposits, the energy would be just dissipated in a direct spark discharge between the electrodes, possibly fusing delicate grid wires or picking away the cathode surface material.

On the other hand, the proof of the pudding... who am I to argue with something that's worked? Thank you for sharing your experience PJL!