Can a valve be "boosted"?

[Tractorfan]

Hi,
I've read on here that ECC85's and their bretheren tend to have a relatively short lifespan and I was wondering if a valve such as this could be persuaded into a further lease of life (even temporary) by upping the heater voltage by maybe 20%? It works for telly tubes, so why not?
I've got a couple of "dead" UCC85's and an ECC85 so I'll probably give it a go anyway. Nowt to lose.
My KB radiogram is dead on FM and I want to try another valve before delving further. I'm thinking of shoving a known good UCC85 & feeding it from a separate tx. via its RF isolating system.
Cheers de Pete

[G8HQP Dave]

My guess is yes. You could try running the heater very hot for an hour or so, to burn off any contamination from the cathode or bring new emissive atoms to the surface. Then normal heater voltage may be OK. I think there was a thread about this recently, so do a search.

[SPCh]

This certainly used to work with ECL80s, PCL82,s, even PL81s. The results were variable, but were often successful - though rarely worth the effort commercially.
The method is as GH8PQ Dave suggests, but 20 minutes at +20% would usually indicate if you were likely to be successful or were just wasting your time.

[timohaveri]

I've tested some bad ECC85's with my Sencore Mighty Mite valve tester which has a very sensitive grid leakage or gas test (100 Mohms according to the manual). Almost every time an ECC85 that doesn't work well in a radio shows excessive grid leakage on one or both triodes. The emission is usually still on the green part of the scale. Has anyone else had similar results?

[kalee20]

I would give it a try. You don't have much to lose.

There could be a distinction between 'flashing off' the existing cathode coating with a highly boosted heater supply, followed by a medium boost to diffuse more active material to the surface; and operating long-term at a slightly boosted heater supply to compensate for failing emission near genuine end-of-life.

One slight caveat with highly boosted heaters, you may indeed flash off the existing cathode coating, but it has to go somewhere! And the control grid is very near. So, you may end up with a slightly emissive grid, with peculiar grid current characteristics.

[dominicbeesley]

I did an experiment on a load of old telly "pulls" PCL82's. It effected a cure. I've not measured how well the cure has lasted but the ones that went into my Pantry amplifier seem to have held up well after plenty of use over the past couple of years so well worth a try. I just plonked them in a valve tester and "abused" them for a few minutes with a high heater voltage and a bit more current than normal.

I think it depends on what has caused them to drop, is it cathode poisoning or cathode stripping...

Dom

[jay_oldstuff]

I did it with some EF80's just to see what would happen a while back, i had a pair that performed rather poorly in a philips radio (vhf front end) I used my AVO two pannel tester and boosted the heter by about 20-30% for about 20 minutes both came up very well for emission and are still working in the radio now.

Jay

[XTC]

I've tried this by winding up the heater voltage by 20-25%, grounding the grid and attempting to make the valve conduct at least its max rated anode current for five minutes.

With an ECC85 in which one section had always had heaters powered, but with the anode disconnected, the used section improved, but not by enough to be useful in the set, the unused section showed a slight improvement, but I assume it was thoroughly poisoned anyway.

With mixer valves there was sometimes a tremendous improvement, back to as new.

I only tried a couple of power valves which were down and there was no worthwhile improvement.

My view is it's worth a try but the results are very variable. I'm not sure how long the improvement lasts. In my experience it works best with valves which never conducted a lot of cathode current and had lowish anode voltages.

Pete.

[Peter.N.]

This used to be standard practice on clapped out valve TV's. you would set the mains voltage selector to 220 volts (or sometimes even to 200 ) and the set would take on a new lease of life.

Peter

[Tractorfan]

Thanks for your replies People.
I'll give it a bash when I get a chance. Looking after wifey's gammy leg at the moment. Hadn't thought of grid leakage shortening the useful life of these valves. Just thought it was the usual falling emission. Learn something new every day .
Cheers de Pete

[DangerMan]

Hi all,
I have done this a few times, sometimes with quite surprising results. The attached article was taken from Practical TV magazine a few years ago, and seems to work... it is a bit more scientific than the hammering I inflicted on some of my early patients, but along the same lines
One thing that is important, is that the heater voltage in operation is correct... i.e. it may be temporarily boosted during reactivation, but the valve should not afterwards be run continuously like that.

I must add, I wouldn't dream of using a reactivated valve in a Leak or Quad amplifier, for example, or anywhere else it could cause damage, but it can provide a quick "get out of jail" solution or be used to prove a point.

This process can cause release of gas within the valve, and the results are often down to how much activity is left in the getter so as to absorb this gas.

TTFN
Pete

[kalee20]

Now that's interesting - the OP was asking about valves being 'boosted' like old TV tubes, where the technique was to use a transformer to up the heater voltage slightly, on a permanent basis. Then we moved to discussions about flashing off the existing cathode coating, finishing with Pete Dangerman's exhortation to not run continuously above rated heater voltage!

Any reason why not, Pete, or is your thinking just on the basis that a CRT has a much bigger envelope and can cope with gradual evolution of gas, whereas in the small envelope of a valve a bit of gas has so much more effect?

[Peter.N.]

It depends on the cause of low emmission, some CRTs will respond to a relativly short burst of high temperature operation because it has burnt the impurities off the cathode, others fail due to the deterioration of the cathode coating, these will only respond to a permamant temperature increase.

I use to have about 150 TVs out on rent, so believe me I tried everything!

Peter

[DangerMan]

With a crt there is usually good cooling area available to the grid cylinder, a small area of cathode adjacent, and low beam current to cause self heating. On the other hand a valve usually has an enclosed grid with relatively poor cooling surrounded by a possibly hot anode. The damger then is that the grid will itself emit electrons itself if it gets hot enough, and this can cause the valve current to increase and the valve to possibly run away. Bearing in mind the effect of gas is also to promote run away, it's not a desireable state of affairs in something like an output valve... and unless you have to, it would seem best not to do it. Excessively increasing the cathode temperature of even a new valve is usually a way of shortening its life.

As Peter says, it depends on the cause of low emission. a bad vacuum or contamination may cause emission to fall away again unless the cathode is continuously run hotter.. You pays your money and takes your chance.
Pete

PS
One reason I've seen given for Mazda crt's having short lives was because they had poor traps between their mercury diffusion pumps and their crt's, leading to some mercury contamination of the cathode... and the need for continuous heater overrunning to drive it away.

[XTC]

Thanks for the article Dangerman. The possibility of rejuvenation must have been looked into very seriously when TV tubes cost a small fortune and there was a sizable regunning trade.

I've always looked at valve rejuvenation as one of those things like putting a pinch of epsom salt into each cell of a failing car battery. It works a bit sometimes and it might get you a useful month or two extra life out of the battery, but there are reasons why the battery is in that state (loss of material from the plates etc) and epsom salts won't reverse them.

If a valve is going gassy, any sort of treatment along the lines discussed will probably make it worse.

Pete.

[DangerMan]

Absolutely Pete, I couldn't agree more. Whatever the process it is unlikely to give any lasting cure, but faced with a dead valve and no chance of a replacement, it might have a use (albeit limited).
Pete

[kalee20]

I've learned a few things too, thanks DangerMan!

[Tractorfan]

This is getting very interesting. I'm not expecting a long term cure and with it being a signal double triode it's unlikely to "run away"
Should be interesting to see what happens (probably nowt, knowing my luck )
Cheers de Pete

[kalee20]

Might do, all the same... if the cathode is tied down to deck via a low DC resistance (or direct), the anode is taken to HT+ via another low DC resistance (RF choke or transformer?), then potentially a high current can be passed by the valve. And if the grid has a high resistance to cathode (AGC decoupling, with signal capacitively coupled), then you have conditions for runaway to occur. If the grid starts emitting, it'll lose electrons, so go more positive. That'll cause increased anode current, the anode'll get hotter, and radiate more heat to the grid (which already has a hot cathode on the other side!), which therefore also gets hotter and emits more electrons.

But if the grid-cathode resistance is small, then even if the grid does emit electrons, it can't affect its potential much. So have a look at your circuit. And have fun!

[glowinganode]

Originally Posted by Tractorfan

My KB radiogram is dead on FM and I want to try another valve before delving further.

If the sets totally dead, boosting the valve may not help. Have you checked for heater - cathode leakage whilst hot? An h/k short or even a few K's could prevent a cascode amplifier from functioning.
Rob.