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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Nov 2014, 12:14 pm   #1
G6Tanuki
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Default "Zapping" a hummy UL41

If you've got an unserviceable UL41 [or even a UL84] where there's excess leakage between the heaters and the grid causing hum [which goes away if grid is shorted to earth] here's something to try.

You'll need:

A traditional valve-radio mains-transformer with a 250-0-250V secondary winding or similar, and a 6.3V heater winding.
A current-limited 12-volt power supply capable of giving 4 or 5 Amps.
A B8 valve-base.
Some lengths of wire.

Proceed as follows.

Link together all the pins on the B8 valvebase except for pin 6.

Connect two wires - one to pin 6, the other to the linked pins.

Connect the other ends of these wires across the outer ends of the HT winding of the transformer.

Connect the -ve output of the power supply to one end of the transformer's heater winding.
Connect the +ve output of the power supply to a length of wire, and switch on!

Then repeatedly 'flash' the end of the +ve lead on to the other heater-winding terminal and pull it away quickly You should only leave the lead on the grid for 1/10 second or so. Bright blue/purple sparks are likely to be produced as you do this.

Do this 20 or 30 times. Then switch off, and try the UL41 in your radio. You should find the hum-level has reduced - if not, repeat the 'flashing' process a few dozen more times.

Basically, the high-voltage pulses produced when the magnetic field in the transformer collapses (as happens in a car ignition-coil) will vapourise whatever conductive stuff exists between the grid of the valve and the other electrodes/base-wires.

I've successfully 'zapped' the hum-plagued UL41 that came with my Eddystone 840A to the point where I'm not going to bother converting it to use a UL84 as originally proposed.

[Disclaimer: seriously defective valves may be destroyed in the process. High voltages are involved - take care to use good insulation on the connecting wires. Sparks may be generated - do not undertake this process in a dusty or flammable atmosphere.]
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Old 16th Nov 2014, 12:35 pm   #2
Nickthedentist
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Default Re: "Zapping" a hummy UL41

That's a very useful trick. I'm glad I've not been throwing away defective UL41s.
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Old 16th Nov 2014, 12:56 pm   #3
G6Tanuki
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Default Re: "Zapping" a hummy UL41

I'm not saying it will work with _every_ hummy UL41 - or that the hum won't come back later - but it's surely worth a try.
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Old 16th Nov 2014, 1:35 pm   #4
mark pirate
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Default Re: "Zapping" a hummy UL41

Sounds like a good tip, I have managed to revive a couple by zapping in the microwave, although very hit & miss!

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Old 20th Nov 2014, 4:57 pm   #5
AC/HL
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Default Re: "Zapping" a hummy UL41

Two posts moved here: https://www.vintage-radio.net/forum/...d.php?t=106204
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