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A kick up the cathode?

Famed V-Ratter Registered

We all know that a CRT may sometimes have it's cathode emission improved when treated on a CRT rejuvenator, or by a small DC current passed through the electrodes by alternative means.

So, I thought that I would try an experiment with a couple of very old Mullard PCC84 VHF tuner valves that were low gain, especially on the band III frequencies. I really don't know why I kept them, but I hate to chuck things out. They both had been run for a while in a set to see if things improved.... they didn't.

The experiment was really just for fun, I simply connected only the heaters to a power supply and over-ran them for for a few moments. I got them up to around double their rated voltage, in this case it was 14 volts. About a minute later, I reduced the voltage to 7 volts and left for another minute.

After testing each valve in turn in my Bush TV53, I was quite surprised at the results. No snowy picture and plenty of contrast on Band III. Both valves seem restored to normal operation with pretty much the same gain on all frequencies. They in fact compare with a new valve.

Whether or not the results will last is not important, as these valves may be had fairly cheaply on places such as ebay and they are quite plentiful. It was just a bit of fun really, but it is quite possible that overheating the cathodes for a time has improved the emission.

Topic starter Posted : 12/10/2020 10:00 pm
peterscott and freya liked
Noble V-Ratter Registered

I've actually hooked up my leader to a couple of thrashed EL84's once to give them a blast... they lasted a little while.  

Posted : 13/10/2020 9:30 am
Katie Bush
Illustrious V-Ratter Moderator

This reminds me somewhat of an old EKCO TV set I once came across, with its HT tapping on the main dropper correctly set for 240V, but the heater tapping set for somewhere around 210V. It became apparent as to why, once we'd corrected what looked an error. Turned out that the CRT was pretty low, but the extra heater voltage bumped up the picture quite nicely. - I wouldn't like to comment on the state of the valves, but that set actually lit up the wall behind it, not excessively obviously, but noticeable if you looked at similar sets in a darkened room.

Posted : 13/10/2020 8:58 pm
Illustrious V-Ratter Registered


This reminds me very much of a story one of my workmates told me many years ago, of someone who asked if something could be done cheaply about the ailing picture on his telly. This was the black and white era. 

The engineer visited and this chap noticed all he did was move the tap on the dropper resistor in the back. So off the engineer went, and when the picture started to fail again, he knew what to do - and sure enough, moving it on to the next tap worked! Feeling very satisfied with himself, he did this yet again and by now, the valves were glowing brilliantly but he still had a picture! 

Until the final time, when he duly moved the tap yet again, and switched on... to be rewarded with an almighty BANG! and a cloud of smoke and tinfoil. 

He'd found the DC tap on the dropper, which bypassed the rectifier...


Posted : 13/10/2020 9:44 pm
PYE625 liked
Active V-Ratter Registered

Has anyone seen "On the buses - The new telly" Jack Harper does this to Stan's old telly with a worn out tube (a Rediffusion set) so he can sell it to Blakey and buy a new colour set (Jack was on radios in the RAF). With hilarious results of course.


Posted : 14/10/2020 3:45 pm
Famed V-Ratter Registered

I went on to try the same thing with a couple of tired and worn out PCF80's, both giving very low contrast pictures when used as a video amp and cathode follower. One it made no difference whatsoever, the other was actually worse afterwards.

My conclusion is that it is probably dependant on several variables. For example the general condition of the valve to begin with, the valve type and the application for which the valve is used.

Topic starter Posted : 14/10/2020 5:14 pm