B9A Valveholder with no centre spigot

['LIVEWIRE?']

Would the lack of a centre spigot in a valveholder cause problems such as instability?

[G8HQP Dave]

In some circuits yes. In other circuits it might lead to more hum. Some will manage fine without it.

[John M0GLN]

None of the B9A valveholders I've seen on PCB's have had centre spigots but is that to suit the manufacturing process?

John

['LIVEWIRE?']

THe v/holder in question is a standard chassis mount type, John. The original holder has a spigot which is earthed, which is partly why I wondered whether the absense of this might be causing the instability. Back in my dim & distant student days I recall the instructor explaining that the earthed cetre spigot provided a capacitive coupling between the valve's electrodes and chassis.

[G8HQP Dave]

Grounding the spigot increases stray capacitance from each electrode to ground, but reduces stray capacitance from each electrode to each other electrode. Useful for RF or low hum circuits.

[turretslug]

Some circuits (LTPs, cathode followers, grounded-grid) could provide variations and debate on which part of the circuit is best to connect the spigot to.

['LIVEWIRE?']

The record player in which the above valveholder is used is now up and running, but there is an amount of hum which varies with the volume control setting.

[Diabolical Artificer]

Could you make a centre spigot out of thin sheet metal and pop it in the hole in the centre of the valve base?

A lead with croc clips on each end, one end clipped to ground/chassis the other clipped to a small piece of sheet metal, say an inch square or larger is good for diagnosing hum and interference. You just carefully move it about the amplifier whilst on to see if there's any change. Amp can be connected to speaker or scope on highest amplification. A variation is to use a small screwdriver instead of the bit of sheet metal; insert near/in centre of valve base to see if that improves matters.

Andy.

[Sideband]

I doubt that it will affect a simple record player at AF. The centre spigot was useful in keeping RF wiring short. The EF80 used as an IF amplifier is typically fitted in a valveholder with an earthed centre spigot. It provides convenient short earthing points and additionally provides capacitive coupling to the internal earth screen located in the base of the valve.

['LIVEWIRE?']

Thanks for the suggestion, Andy. Didn't think of that. What is happening at the moment is that the amp. works OK for maybe 20 minutes or so, then audible hum is present, which varies with the volume setting, and also with the setting of the treble control, which is a simple top cut circuit wired in parallel with the v/c.

[Boater Sam]

Heater to cathode leakage ?

['LIVEWIRE?']

That thought crossed my mind, Sam. I can borrow an ECL86 from my Silvertone Record Player to prove the point. In a way, I hope it's the valve, not the metal recifier. To replace that would involve dismantling the HMV again, which is fiddly, or, more precisely reassembling the 2022 is fiddly. If the valve is faulty, hopefully I can get a good ECL86 without paying silly money!

['LIVEWIRE?']

Sam, you were right about Heater-Cathode leakage. The valve from my Silvertone player is in the HMV2022, and has behaved perfectly even when run for 2 or 3 hours continuously yesterday, and again for a similar length of time today. I shall leave that in place, and hopefull get a NOS ECL86, or even better a pair of them for my Silvertone Autochanger