Can valves have 0% emission?

[Pete_kaye]

Several times recently I have tested old valves and there has been no deflection on the Gm test , using Avo Mkii. Is this quite possible or could it be bad pin contact or valve needing more time. Others gave 'normal' readings.

[Alan Stepney]

Quite possible, but,
Dont always believe the tester.

Some time ago I had a couple of "dud" valves, that showed zero emission, on a Taylor, AVO Mk3 and almost zero on a VCM 160.

Strangely enough, they worked. Not much output, but they did work.

[Peter.N.]

The most likely cause of a genuinely 'no emission' valve would be an internal disconnection, or in the base if it has one.

Peter

[Silicon]

Are they Triodes or Pentodes?

[Skywave]

Not wishing to state the blindingly obvious - and certainly not to insult your intelligence - but are the heaters open-circuit? (An easy thing to overlook in the heat of the moment).

Al.

[Herald1360]

If the valve's in a low power circuit and normally runs a mA or less, it may still "work" even if emission is down to 10uA or less. That would probably look like 0% on most testers.

If the filament/cathode still gets hot, presumably there will always be some emission, even if the special coatings are shot.

[Pete_kaye]

The most recent one was an EL37 and an OT220 before that on a early Avo. I am not sure whether it is a Mk1 or 2 or neither.It is in 2 parts and fold sout with valve bases on the left and meter on right . It isn't mine. And the heaters were intact.

[GrimJosef]

I'm afraid that finding EL37's with good emission has been difficult for a long time. Something about the production process seems to have made them unusually short-lived .

Cheers,

GJ

[Radio Wrangler]

No deflection on the Gm test doesn't mean no emission.

No emission would mean zero anode current, and nothing of anything else consequently.

Some anode current but zero Gm suggests that the valve is in saturation. Every electron that can leave the cathode is doing so and waving the grid voltage around a little for the Gm test makes no difference to the anode current. If you play around with the grid bias voltage setting, you may find a region with some grid influence and see the Gm lift a little. However, the bottle doesn't sound like a promising candidate for much.

David

[Station X]

I experienced this very problem yesterday when testing a pair of "pulled" ECL83's on an AVO CT160.

No sign of being down to air, no inter-electrode shorts and heaters intact/light up. However at the specified grid voltage there was zero anode current in either section. By reducing the grid voltage to zero I was able to get a few mA's of anode current to flow in the pentode section.

Suspecting a tester fault (it wouldn't be the first time), I tested an ECL80 which has the same pin out. It tested fine.

So it looks like I have a couple of duds. All I can think to do is clean the heater pins in case dirt on them is putting resistance in the heater circuit. If that fails I'll try increasing the heater voltage. There's nothing to lose.

[Herald1360]

Originally Posted by Station X

I experienced this very problem yesterday when testing a pair of "pulled" ECL83's on an AVO CT160
.................
Suspecting a tester fault (it wouldn't be the first time), I tested an ECL80 which has the same pin out. It tested fine.

But they don't....... ECL80 has its infamous common cathode, ECL83 is more user friendly with separate ones.

[Station X]

Agreed Chris, but the two valve types are tested on a CT160 using the same selector switch settings. This means that the two cathodes of the ECL83 will be connected together. My aim was to show that the CT160 was working correctly.

[Herald1360]

That works. Never driven a CT160 myself- just responding to pin out at face value.

[ionburn]

Logic would tell me that a valve that is physically Ok as per working heater and reasonable vacuum would always have some emission even if the cathode were heavily contaminated. Just likely to be out of spec and measurement range of whatever tester is being used.

Admittedly I have not got a commercial tester, but I do prefer to go back to first principles and my 'plug together' modular setup as it does tell and teach me far more about what is going on.

[julie_m]

It's just about feasible for a valve that has been subjected to long-term abuse to fail with low emission.

If you strap the ECL83 cathodes together, it has the same final pinout as an ECL80 with the suppressor grid strapped to the common cathode, so that would explain the similarity of the tester setups for the two valves.

[Station X]

It turned out that the valves I was testing were PCL83's with 12.6V 300mA heaters rather than ECL83'S with 6.3V 600mA heaters. When tested with the correct heater voltage they showed some emission, but nothing like the specified figure. I put that down to age.

I mentioned earlier that these were "pulls". They were in a large number of pulls which came into my possession. There was no visible lettering on the glass, but someone had stuck on labels saying ECL83. I became suspicious when I noticed that they lit up very dimly when compared to a NOS boxed PCL83. Applying a variable voltage to the heaters whilst measuring the heater current showed that they were indeed PCL83's

[Peter.N.]

They had quite a hard life in TVs as audio or frame output valves, the glass became discoloured internally if they had been worked hard so if yours were that's probably why,

Peter

[Station X]

They're definitely discoloured Peter. I don't throw valves away unless they're down to air though.

[Peter.N.]

I'm the same, that's why I have several boxes of them in my garage loft, I stripped all the valves from the last of the valve/hybrid TVs I scrapped, I have a couple of CRTs up there as well, a 14" and a 17", no labels but I think they are Mullard electrostatic focus.

Peter