Tube reactivations

[Guest]

Hi,

on Neumann web site I find the following post. Can somebody explain how to made circuit for that:

Have you ever heard a U47 which makes some strange noises, which could be described as crackling, crushing and swishing, all at the same time? Sounds funny, but is only usuable if you look for special effects. Chances are you hear the 'crushing-' and 'twinkling-effect' (Schrot- und Funkeleffekt), as described in old tube manuals. Andreas Grosser from Berlin told me so. This effect is caused by underheating the tube over a long perdiod, or by using a new tube which has not been burnt in. (I am no technician, so I hope I got this correct.) The Neumann U47 gives the VF14 a heating of 35 Volt, however, the tube is constructed for 60 Volt. So there is an unterheating of 42%, altough Telefunken only recommends 5% under- or overheating. This may cause the cathode only glow at the tip and electrons escaping arbitrarily, causing the crushing and swishing. Now, there is a solution for this. The tube simply has to be slightly overheated (5%) for at least ten hours, with 200 Volt at the Anode. This is a simple circuit, or it can be done with a tube tester. My technician has done this for me, and now the U47 works fine. Many thanks to Andreas Grosser!
I had the same problem with an UF14 before after using it about two weeks and which also was underheated. But now I stay with the VF14, altough I think, an UF14 works just as fine as an VF14.
New VF14 tubes often show the same effect, probably because there is some dirt at the cathode from production, which vanishes when the tube is burnt in. Probably, Neumann installed the VF14 in the underheated circuit of the U47 only after the tubes had been burnt in.

I forgot to mention: The reactivating should be done in AC. This is faster and safer. A more detailed explanation, written in German, can be found here.

I forgot to mention: The reactivating should be done in AC. This is faster and safer. A more detailed explanation, written in German, can be found here: http://us.f1.yahoofs.com/users/13dd71e6/ bc/U47/VF14+Reaktivierung.htm?bc1F.28Aw_ MiR8Gs.

 

[CJ]

Very interesting!

OT, I read in an old book that if usuing DC for heaters, the polarity should be flipped from time to time to keep the tubes from suffering an early death. How often> I don't know.
Another good reson for using ac on the heaters if you can keep the hum down.

 

[PRR]

> underheating of 42%

A coated cathode is a complicated thing. Thorium in the bulk of the coating has to melt and spread over the surface of the cathode in a monoatomic layer. At normal heat, this happens easily. 42% underheat might be too cool. Because the tube starts with plenty of Thorium on the surface, it might work under-heated for decades; I suppose that is what these good engineers were thinking when they picked such a radical underheat. Tubes were everywhere, so a 10-20 year life before replacement was not seen as a problem.

Since this is a very low current application, simply bringing the heat up to full rated temperature for a while should bring enough fresh Thorium to the surface so you are good for another few decades of underheated operation. I doubt it even needs 10 hours at 105%, but that won't hurt anything.

> The reactivating should be done in AC. This is faster and safer.

If this is a heater-cathode tube, I don't see how it matters.

> I read in an old book that if usuing DC for heaters, the polarity should be flipped from time to time to keep the tubes from suffering an early death.

Again: not applicable to heater-cathode construction. Heat is heat, whether AC or DC or a little gas fire.

In filament tubes, where the heater IS the cathode, AC heat hums bad and DC heat upsets the grid bias which makes one end of the filament do more work than the other end. Most small tubes with coated filaments have so much reserve emission that this is not a problem. Big transmitter tubes with uncoated filaments do need their filament-ends swapped from time to time to equalize wear. (High voltage transmitter service is VERY hard on filaments: some of those 30,000 volt electrons bounce off the plate and bombard everything inside the tube, bashing hell out of the surface of the filament, so coated filamants can't be used.)

 

[Guest]

[quote author="lampas"]I forgot to mention: The reactivating should be done in AC. This is faster and safer. A more detailed explanation, written in German, can be found here.
here: http://us.f1.yahoofs.com/users/13dd71e6/ bc/U47/VF14+Reaktivierung.htm?bc1F.28Aw_ MiR8Gs.[/quote]

Forbidden.
It s pitty for me, but something from me:
But when I was young school - boy, there was many
poor picture tubes. I reactivate them by overheating and appliing high
grid current. Then outer part of cathode emission film was
removed from cathode (and by picture tube manipulation and
optionally by condenser charged removed from the cathode - grid space)
There was effect of this to the cathode emission and it
enlarges tube life by one or couple of year. Sometimes this can by
repeated.
Today, japan picture tubes have shorter moral to phisical living period.
I have one at home for a sixteen years, and no solarisation ....

This was effect of regeneration to the cathode emission, I don t know effect to the
cathode - related noise. If is, it is fantastic.
Only there can be problems with cathode - grid space
and then grid isolation resistance.
If part of cathode film (after this regeneration) shorts G1 to cathode,
there may be problem to remove it. Grid in the tube is more subtle
as in the picture CRT, and usually used charged cap can destroy grid.
Only mechanical way. Vibrate vith tube.
And I sugest make diode from tube (connect plate to grid) and
then regenerate it by regulated current limited power supply between cathode and plate.
And do it on the overheated tube (overheat it by second regulated supply,
heather voltage must rise slowly, elsewhere heather wire break)
I do not know values, someone can test it on dead tubes?

xvlk