Zero emission?

[ITAM805]

On the bench I have a rare 1973 Marshall Master PA, an attic find in a bit of a state. Testing the 4 Mullard EL34's I was surprised the three had zero emission, one had gone to air.

So to check that my little homebrew tester wasn't misbehaving I tried a pair of Edicron's that I bought new around 1990, that I use as reference, one tested as having 50% the other 0%! Oh no, my tester must be broken?

So I got out my stash of used 34's that I had marked the emission on when I put them away, and they all tested exactly as marked? So I retested the Edicrons and yes, they appeared to gone duff?

So what's going on, how could they loose all their go simply sitting in boxes, the getters look OK in both so I'm at a loss. I used them a year or so ago and I noticed that one had slightly lower emission than the other, which had 100%, ironically the one that now has 0%

[Craig Sawyers]

A connection problem between the leadout wires from the valve envelope and the solder joints on the octal pins?

Does the heater light up?

Craig

[ITAM805]

Hi Craig, yes heater are bright and HT is getting to the anode and screen pins on the tester

[MotorBikeLes]

If your tester will handle it, leave a "zero emission" sample to "cook". It may recover.
Many valves and CRTs that are left unused lose emission, but recover it in use. Never tried a zero emission one though.
Les.

[ITAM805]

Thanks Les, I did actually cook it for an hour but no signs of life? I turned the bias down to -1 volt (from -20V) but still nothing so I shall try re-flowing the pins, which I think Craig was alluding too

[kalee20]

Originally Posted by ITAM805

Hi Craig, yes heater are bright and HT is getting to the anode and screen pins on the tester

That's not quite what Craig meant as I see it - is the power getting to the actual pins on the base of the valve, and from there to the leads disappearing into the bottle?

I've seen the occasional dry joint on an octal valve base.

[ITAM805]

Hi Kalee, yes I know, I meant with the meter probes on the valve pins in the tester socket. I reflowed the pins but still it seems to be dead, mysterious ...

[G.Castle]

When there's nothing to loose.

For a start, run the heaters for a few hours with grid and cathode shorted, and no other external supply, re-test.

Then if no luck.

WITH NO HT USED.

Run the heaters at 150%
Apply + grid voltage, (current limit may be wise, say 50mA) 10-20volts and watch for grid current.
When/if current starts to flow as cathode wakes up, switch off heaters and allow to cool with +bias applied.

If nothing happens the valve is internally shot.

[Heatercathodeshort]

The EDICRON EL34 on the right appears to have suffered overload possibly due to a biasing problem.
It is possible that either the screen grid or anode may have been raised to a red hot condition releasing gas molecules that the getter finds it impossible to mop up.

I had a similar problem with an amplifier I built a few years ago using 807s in the output stage. I accidently removed the anode voltage from one valve [top cap]to see the screen grid glow red hot followed by a blue fog in the valve envelope as the gas was liberated.

All seemed well after replacing the T/C and the blue fog reduced to zero after about 5 mins. Two weeks later the valve failed completely giving almost zero emission possibly caused by contamination of the cathode.

Just a possible answer. John.

[ex seismic]

I have had the same issue with the valves in a WS18 that had sat for many years. I tried all the above mentioned potential cures but to no avail.

[ITAM805]

G Castles, thanks for the detailed info, I'll give that a try

"Just a possible answer" well John, you have jogged my memory. Last year I had an old Marshall JTM on the bench that under test would intermittently and quite suddenly cause one of the 34's to light up like a beacon before returning to fully working, it destroyed 2 new valves and I never got to bottom of the fault before giving it back. So it's quite possible the my Edicron was sacrificed in that amp, perhaps suffering the burnout while I was doing something else, me not realising what had happened?

[Philips210]

Hi.

I know this may sound far-fetched but is it possible for helium gas to enter through the slightly permeable glass envelope and therefore ruin any useful emission? I recall reading something similar in the past.

Regards,
Symon