Restoring/repairing a triode output valve

[Michael Maurice]

Following on from this thread Can someone advise me how can I tell if the getter is still ok?


How do I set up such a test bed? Can I use the Dynatron amplifier but say remove the good valve and set the grid using an external power supply?


At the moment when the amplifier is on, the bad valve gets very hot (not red plating) and there is a purplish glow from it.


The valve in question is a CV1168

[Boater Sam]

Sounds like it is gassy.
The getter is molten metal flash sprayed inside, very shiny, if its white and milky it is depleted and the vacuum is soft.
It cannot be reactivated. Sometimes the getter has not been fired properly, in which case with an RF heater it may be possible to re-heat it but the valve electrode assembly will out gas at the same time so its debatable whether it will work.

[Michael Maurice]

The getter looks like its intact. The top and bottom of the valve is dark and shiny.

[ukcol]

Perhaps try this lash-up to cook the valve whilst keeping an eye on the voltage across the resistor. It may get hot enough to fire the getter and soak up the gas. Worth a try.

[Michael Maurice]

I've been running it for quite some time like this, there is still a purple glow and a hum from the speaker.

The amplifier incorporates 10K grid stopper resistors. I have grounded the grid directly to chassis but I could leave the grid stopper in circuit which might let it fry a bit more, or even put a 1K resistor between the grid and ground.

Is that a good idea?

[kalee20]

Connecting 1kΩ between grid and ground is rather likely to severely kill the audio!

But, can you post a copy of the circuit segment? If the valve is gassy, a lowish DC resistance around the grid circuit may well enable it to continue to give adequate service. While (as above) a resistor alone may not do it, a high-value inductor (with a lowish DC resistance) may well do. I'm thinking of the primary of a small mains transformer, or the secondary of an ignition coil, maybe from a lawnmower, sort of thing, which you can tack in place on a temporary basis.

[Michael Maurice]

I'm not connecting the resistor permanently, In fact what I was thinking was to put a few positive volts on the grid WRT the cathode, to heat the valve right up to fire the getter.

Perhaps a 9V battery or lower via a power supply.

I could make up a jig but that may take some time

please see the diagrams.

Shouldn't the long line on a battery be positive?

[Michael Maurice]

Originally Posted by merlinmaxwell

Shouldn't the long line on a battery be positive?

Probably but it doesn't really matter as long as the grid is a few volts positive with respect to the cathode.

[ms660]

Positive grid WRT the cathode is bad news.

Lawrence.

[Michael Maurice]

Originally Posted by ms660

Positive grid WRT the cathode is bad news.

Lawrence.

I know that, but how else does one get the valve hot enough to activate the getter?

I really need some guidance on this

[Techman]

Definitely no positive volts on the grid.

I've seen a slight blue glow on the glass of these triodes that varies with the audio when viewed in a darkened room and I don't worry about it - no hum.

As to the audible hum, the first question I would ask is whether all the heater strings on the 'suspect' valve are fully 'lit' and are of exactly the same brightness level as those on the 'good valve'. This hum business on these Dynatron amplifiers with directly heated triodes is nearly always down to differences in heater performance which is easily sorted.

[Michael Maurice]

I shall carry on running it with the grid tied to the cathode for a few hours tomorrow or Friday and see what happens.

[Techman]

What about the heater string glow level compared to the other valve?

[Michael Maurice]

Will take a look and report back

[Techman]

It's worth measuring the resistance of the heater across the heater pins of each valve when the valves are cold - should be around an ohm, if I remember rightly. If the suspect valve measures a little higher than the good valve, you've nailed it.

[boxdoctor]

The only way to reliably fire the getter (in a new valve or a used one), is to use an R.F. induction heater.
It actually requires the getter plate/ring to be heated way above the temperature that can be obtained by doing "things" to the valve electrically.
I am not a valve expert per se, but I would imagine the chance of completely restoring the integrity of the vacuum is slight without also re-baking the electrode assembly to remove trapped gas particles. Tony

[GrimJosef]

What Tony says is true - re-firing the getter is difficult and, frankly, a last resort. However the getter film works much better when it's hot. So if, over the years, a bit of gas has built up in the valve then it is possible for the getter film to pump this away as long as you can safely run the valve hot for a while. This will also heat the electrode assembly above the temperature at which it normally operates - not by much, perhaps, but by enough to de-sorb much of the gas that might otherwise cause you trouble in normal operation.

Of course an old getter can't work miracles. If the valve is seriously gassy then the getter just won't have the capacity to deal with that. And in any case the gas may already have reacted with the 'oxide' coating on the filament, degrading the performance of that irretrievably.

Cheers,

GJ

[MotorBikeLes]

It seems to me that what you need is a sunny day and a magnifying glass. Must be an alternative to induction heating, IF you can see the getter pan.
Les.

What a wonderful solution Les.