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Old 13th May 2017, 10:56 am   #1
ssaunders
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Default Valve sockets and internal screens

Dear wise ones....

A couple of queries regarding valve construction practice:

1) What is the significance of the centre spigot on a B9A socket - I've noticed that most are connected to chassis and wonder why? Are there circumstances where one absolutely should or should not utilise the spigot?

2) My project uses an EF80 which I notice has a connection to an inner screen - should this be bought to chassis, left floating or what? (And why?)

Thanks

Simon
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Old 13th May 2017, 12:17 pm   #2
kalee20
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Default Re: Valve sockets and internal screens

The EF80 is designed with copious screening in mind.

The anode pin and the cathode pin are well separated. Internally, there's a metal strip between them, running right down to the bottom of the bottle. It's connected to the mesh screen, so if you ground this, you'll have virtually no capacitive coupling between grid and anode (in the active area, of course, you have the screen-grid between them which is also at signal ground).

Outside, there's the capacitance between the pins and valveholder tags, but again, if the valveholder has a central spigot, you can ground this and again the anode pin and grid pin will then have negligible capacitive coupling between them.

There'll still be the direct 'view' of one to the other through the glass of the base, but that's only a few millimetres, and by utilising the other features you can make it as good as it gets!
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Old 13th May 2017, 2:27 pm   #3
G8HQP Dave
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Default Re: Valve sockets and internal screens

Normally you ground the central spigot. I suppose there could be rare situations where you might use it as part of a guard - effectively bootstrapping it to reduce leakage currents. You should never leave it floating, as then it would aid feedback from anode to grid.

One interesting question is whether it should go to chassis or the cathode. Won't make much difference below VHF anyway.
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Old 13th May 2017, 11:41 pm   #4
kalee20
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Default Re: Valve sockets and internal screens

That's a good point Dave!

The EF80 has two cathode connections anyway, so you can return the input circuit to cathode via one pin and the output circuit via the other pin. This way, there is almost no length of wire which is common to both input AND output (inductance of any common cathode lead is bad news).

The screen-grid should be decoupled to cathode, so that the cathode-screen voltage is constant. And it should be to the output-related cathode lead, because any signal-frequency currents flowing through the screen-grid decoupling capacitor must not cause any change in the input (which could happen if you took it to the input-related cathode pin).

That being so, I'd argue that the spigot should also be connected to cathode. But which pin? If taken to the output-related pin, then any signal voltage on the cathode, due to non-zero inductance of the wire from cathode itself to this pin, would put the spigot at a different RF voltage to the cathode itself, and you could get coupling to the nearby grid pin. If taken to the input-related cathode pin, then any signal voltage on the cathode would also appear practically unchanged on the input-related pin because hardly any signal current flows through this leadout, so there would be much less coupling to the grid pin.

That's my thinking - unless you get clever and deliberately add coupling effects to neutralise something else! Comments Dave?
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Old 14th May 2017, 11:15 am   #5
G6Tanuki
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Default Re: Valve sockets and internal screens

In RF designs I've seen, the central spigot is used as a common-earth point for the decoupling components: typically the leads of the components are fitted into the open end of the tubular spigot before soldering: this "single-point-earthing" makes sense as a way to reduce the stray-inductance of connecting leads by making them as short as possible.

From an RF perspective there's something to be said for connecting cathodes direct-to-ground and providing a negative-bias supply, rather than using cathode-resistor bias. Look at something like the AR88 receiver as a practical example!
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