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Old 26th Sep 2022, 8:14 pm   #1
Colin Boggis
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Default Faulty Telefunken UL41's

I recently purchased 15 brand new boxed and sealed Telefunken UL41 at what I thought was a bargain price - £100. Despite them being new I thought I ought to check them. Not having a suitable radio to hand I rigged up a rudimentary test - 170 volts for anode & grid 2, variable negative volts for grid 1 and 45 volts for the filament and cathode plus G3 grounded. Just a milliamp meter in the anode circuit. Initial G1 volts set at -10.4. The first valve warmed up, went pop, no anode current, blue glow.
The 2nd started to conduct, reached 15 mA, then dropped sharply to zero. Increasing G1 towards zero changed nothing - no emission. 3rd, 4th 5th valves all same. To check my test method I tried a used Mullard valve, got 48 mA anode current, increasing as G1 made less negative, so tester OK. To cut a long story short, all the remaining TFK valves exhibit same behaviour.
I'm not sure what is wrong with them - some kind of leakage when hot? anode or cathode goes OC? Have any of you experienced a similar fault or have any ideas as to the nature of this fault?
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Old 26th Sep 2022, 8:27 pm   #2
Robert Gribnau
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Default Re: Faulty Telefunken UL41's

My first thought would be: Demand that the seller takes them back and get a refund.

Are there any additional marking/codes on these valves? If so, could you post them? Maybe they are not UL41's?
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Old 26th Sep 2022, 8:43 pm   #3
paulsherwin
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Default Re: Faulty Telefunken UL41's

This is odd. The German Navy used equipment using UL41s until relatively late, which is why Telefunken (and subsequently Tungsram) made so many late production examples. They're not known to be unreliable.
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Old 26th Sep 2022, 9:16 pm   #4
Half a Mullard
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Default Re: Faulty Telefunken UL41's

Odd indeed. I bought this NOS example a few years ago. I've kept it as a spare but it has had some use in a couple of DAC90A's without any problems. I think I would also be inclined to take this up with the seller.
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Old 27th Sep 2022, 12:24 pm   #5
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Default Re: Faulty Telefunken UL41's

What is the heater resistance when cold?

Could they be EL41s with the wrong markings?
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Old 27th Sep 2022, 1:59 pm   #6
Mr 1936
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Default Re: Faulty Telefunken UL41's

Hi

I was told recently that some NOS valves develop air leakage around the glass to metal seals of the pins during storage, and a whole batch can end up faulty.

The problem may be due to a change in materials or manufacturing processes over the years, with valves from the 1930's and 1940's using lead-out wires seemingly less affected than more recent (but still decades old) B7G, B9A etc examples which have formed pins.

Maybe this is manifesting itself only after warmup ? A blue glow in a low power valve suggests it has gone "soft".
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Old 27th Sep 2022, 2:06 pm   #7
Colin Boggis
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Default Re: Faulty Telefunken UL41's

Heater resistance when cold is 62 ohms (I checked several). With 34v DC the current is 100mA. A UL44 I have reads the same so I assume they are correctly rated and not EL41's. Manufacturing code is B7202709 which I think means made in Berlin on 27th July 1962.
I've attached pictures, showing markings and box.
And yes, I got my money back (and still have the duff valves).
The seller was an antique dealer (not a radio buff) and claimed they were her grandfather's - why he would have 15 duff valves she couldn't say. He wasn't into radio or audio. It is a mystery.
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Old 27th Sep 2022, 2:11 pm   #8
Colin Boggis
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Default Re: Faulty Telefunken UL41's

I have noticed the pins have some green Verdigris indicating damp storage, so maybe this has somehow penetrated into the envelope. Only the odd one or two glowed blue most remain clear. I cleaned the pins before testing.
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