Valve covers
 

Is there any merit in using one of those push-down latching metal valve screen covers on a valve used in a VFO? Is it likely to help stabilise the valve temperature (and hence drift) more quickly, or is any variable capacitance between it and the valve electrodes likely to do more harm than good?

Thanks.

Ian

Re: Valve covers
 

I always thought the screening cans on LO was mainly to reduce oscillator radiation although the black ones reputedly radiated the heat way better than plain cans.

Re: Valve covers
 

Yes, I think you're right on both counts. More for screening than anything else, but also a heatsink - maybe! I can remember seeing them on most (all?) of the valves on an RA17 chassis, and/or a BRT400.

Re: Valve covers
 

Is it easy to try it and see?- or would it involve changing the valve socket for a skirted type? I suspect that the best way to minimise temperature rise in a valve is to leave the envelope exposed- plain, i.e. bare metal cans were criticised for keeping heat in and blackened outer reduced the problem rather than being better than no can. The ex-professional use kit that abounded in skirted sockets and screening cans probably did so for retention and screening reasons rather than cooling- it's noticeable that hot-running valves such as rectifiers and small PA valves tended to have spring or fabric string retainers even when other valves were canned.

Also, valves like metal octal and many B9a pentodes are screened anyway, but not usually B7g types. As an empirical example, the B7g EF91 LO in my Eddystone 750 has a plain metal screening can as standard (this valve having an unscreened anode as outermost element), lifting the can changes frequency slightly (and unsurprisingly) as it represents stray capacitance to anode, I would expect a B9a type with outer mesh screen to be less affected. Also, spraying the originally plain can with a single thin coat of matt black paint made a very marked difference to warm-up drift.

Probably hen's teeth now, but some expensive kit like Marconi's TF144H had oscillator valve fitted with rather posh, sturdy cans that had an inner tensioned copper braid girdle that pushed snugly round the valve, the can locked down to the socket with several turns of a fine brass thread- I expect this type of screening can genuinely would keep the valve cooler than being open to the air.

Re: Valve covers
 

What's the VFO valve? As others have mentioned, stray capacitance between the anode and its surroundings can be a problem and the screening-can does at least provide some consistently-close, grounded metalwork.

Re: Valve covers
 

The VFO valve is an EF80. I'm making a Codar AT5 copy to work on 60m. I've got the prototype working quite well, but the original AT5 VFO operated on 1.9 MHz and as these things go is pretty stable at that frequency.

My prototype has some drift and chirp at 5 MHz, hopefully only because of the appalling mare's nest layout. A stable VFO at 5 MHz shouldn't be too difficult and I'm not looking for the stability required for SSB. The VFO HT supply is stabilised, so I'm hoping the noticeable CW chirp is probably caused by poor screening/RF feedback - maybe I'm clutching at straws. (Neither the VFO nor buffer are keyed.)

The VFO inductor is in a screened can as in the original, but I'm putting most of the other VFO components in a screened box around the VFO valve pins under the chassis. I'm minded to keep the anode load and scree-grid resistors outside the box in case they get warm. In the prototype they only get slightly warm.

Incidentally, the one side of the heater (pin 4) uses a separate earth connection on the original chassis, when pins 1, 6 and 9 are already earthed. A good idea to keep them separate even if only by an inch, or just ease of assembly?

Ian

Re: Valve covers
 

I thought the main reason for using them was to hold the valve in, black for the hotter ones. For a VFO mechanical stability is important, one of these may help.

Re: Valve covers
 

I'm not sure about the influence of the valve covers on VFO stability, but the temperature coefficient of the caps in the tank circuit are often an issue. The inductor and the variable cap will have significant positive coefficients so some compensation in the form of ceramics with negative coefficients will help to improve things. LF AM transmitters of the AT5 era often seemed to get away without such corrections.

B

Re: Valve covers
 

At least an EF80 has the mesh screen round the anode so it's less likely to be affected by extraneous variations in anode-to-the-rest-of-the-world capacitance. And if my memory's correct the AT5-style osc is an "electron-coupled" Vackar-type design where the valve's screen-grid acts as a 'virtual anode' to the oscillator itself, which should also give greater isolation from real-anode-capacitance fluctuations.

[Oscillators can be fickle beasts - in another thread I narrate the hassle of trying to stop the 'ant trim' control of my Trio 9R-59 acting as a VFO-shift. This uses a triode [half an ECC85] and the anode-to-earth capacitance definitely does have an influence on oscillator frequency in this case! From the factory it has a close-fitting screening can [a tube with a slot cut down the side to provide springiness] and while this is needed to stop the anode-to-world capacitance varying it does make the valve run rather hot]

Heater wiring - I'd say that you should earth any pins that are not-used and are also not part of any 'internal connection' to the valve. And decouple both heater-pins to earth with 0.01uF if you're using 'balanced' heater wiring rather than one side of the heaters being returned to chassis. The only reason I can think of for a separate earth to one end of the heater rather than commoning it with the cathode and suppressor-grid and mesh-screen is that there's a *vague* possibility of any common impedance in a shared earth [corrosion between earthing-tag and chassis?] resulting in a squib of heater-current AC being injected into the RF side.

[My AT5-alike transmitters always kept the heater wiring to each valve 'floating' above ground - though thoroughly RF-decoupled, because it made 6/12/24V supply-switching a lot easier]

Re: Valve covers
 

Indeed: The Codar AT5 lists one capacitor in the VFO - C5 - as "10/40pfd temp comp" and the blurb says "Temperature compensating capacitors are used to obviate frequency drift".

Can you still get the old "N750" cylindrical ceramic capacitors in low values?

Re: Valve covers
 

The sort of screening cans under discussion are certainly extremely common in aircraft equipment from the cold-war era, but I suspect the main reason may well be to act as valve retainers.

Re: Valve covers
 

There were cans intended to reduce valve envelope temperature.

They were rather like wound up strips of spring steel you opened them up and let them close to grip the glass of the valve. the outside was blackened, the inside grippedthe glass,

David

Re: Valve covers
 

That's the kind I have in my Trio 9R-59.

Though they're not blackened on the outside.

Re: Valve covers
 

Probably NOS components still around. If I recall, don't polystyrene caps also have neg coeffs?

B