Some surprising valve failures

[trh01uk]

I've been turning out my valve collection. I was surprised to find that I have 14 type 803/RK-28A/4069A (all equivalvents) big power valves, pentodes intended mainly for RF amplifiers, with 125W anode dissipation. Physically big brutes, being over 8" tall - see pics. These are directly heated, so they light up like bulbs when operating correctly.

No idea why I have 14! I only ever needed two - for a big transmitter refurbishment that never happened - and that only had two in its PA.

Anyway, time to move them on I thought. So I devised a test within the scope of my remaining test gear, involving picking a static spot on the anode characteristic curves and seeing if they met that. As I recall it was Va = 500V, Ia = 38mA, with Vg1 = 0, Vg2 = 300V and Vg3 = 40V.

What I got surprised me. Out of the 14, only 7 of them showed up with Ia = 38 to 56mA, in other words a healthy anode current suggesting the valve to be basically functioning.

The other 7 had a range of faults:

• Three had open circuit filament
• Two had excessive anode current and tripped the AVO I had in the anode circuit - presumably internal shorts
• One had Ia = 78mA, which I suspect is a bit too high to be healthy
• And most peculiar, one had a dim filament, and took no anode current at all

The one with 78mA anode current might have gone into spurious oscillation. I had no suppressors in this test set up - and only discovered them to be essential when I started on a batch of 813s.

The last one is quite weird - what possible fault could make a filament go dim? Increasing the filament voltage from 10V to 15V made it go brighter but it still took no anode current.

The other unhappy part of this experiment is just how many faulty valves I had! Either I bought from very dodgy sellers - entirely possible when these big valves are very hard to test - or they have sat on my shelves for up to 20 years, and literally failed from old age. Hard to imagine though why filaments would go o/c by themselves......


Richard

[M0FYA Andy]

Richard, check the soldering of the filament wires into the pins before you conclude the filaments are open circuit. This could also explain the dim filament.

Andy

[Chrispy57]

Hi Richard - I'd be leaning towards the dodgy-dealer/pulls scenario, but did notice that you seem to be testing them horizontally, could that be a factor?

Cheers
Chris

[trh01uk]

Yes, thanks Andy - I will check the soldering of the pin connections.

I noted an old thread that mentioned to not store valves horizontally because of the risk of grid elements sagging. As they have been horizontal for many years, its possible that sagging has caused these failures. I will retry some in a vertical position - though I suspect that won't rectify it.


Richard

[cmjones01]

A dim filament with no anode current might be due to air having got in to the valve. The air convects heat away from the filament/heater so it doesn't glow, doesn't emit electrons, and even if it did, the gassy interior probably wouldn't give them much chance of getting to the anode.

Does the valve get strangely hot when running? That's a good clue - the filament's heat gets transferred to the envelope by convection. I've seen this happen with indirectly-heated valves which have gone soft, but I'm not sure how a directly-heated one would react.

Chris

[m0cemdave]

Dead 4069A's make quite good ornaments, I have seen a couple mounted as a Radio Club trophy.
But to avoid inadvertently perpetuating the "dodgy sellers" scenario they shoud be clearly and indeliby marked as duff...

[trh01uk]

Originally Posted by m0cemdave

Dead 4069A's make quite good ornaments, I have seen a couple mounted as a Radio Club trophy.
But to avoid inadvertently perpetuating the "dodgy sellers" scenario they shoud be clearly and indeliby marked as duff...

If they are irretrievably duff, and anyone wants them, I will be happy to give them away.......so hopefully we won't get into the "dodgy sellers" area!


Richard

[Aub]

Strange the way they intend that the suppressor grid draws current by putting +40v on it. Different from the usual set up of grounding it. Wonder if this is because of the direct heaters?

Aub

[trh01uk]

Originally Posted by M0FYA Andy

Richard, check the soldering of the filament wires into the pins before you conclude the filaments are open circuit. This could also explain the dim filament.

Andy

Sadly, none of these failures responded to new solder on the filament pins. In one case, I can actually see the break in the filament near the top of the valve - clearly a hopeless case!

Richard

[bigfathairyvika]

Parts of the filament appear to have vaporised from my 813 I disassembled.

[M0FYA Andy]

Originally Posted by trh01uk

Sadly, none of these failures responded to new solder on the filament pins.

Richard

Ah well, it was worth checking!

Andy

[Joe_Lorenz]

Reading all this made me scratching my head! I ought have a close look at some of my flower pot sized tetrodes, me thinks!

Joe