EF86 screening

[Phil Y]

Hello all,

Just wondering, if an EF86 is in a skirted base with a valve can on, would that make it unnecessary to connect the internal screening or would it be still worth doing?

Thanks, Phil.

[John Caswell]

Hi Phil,
Simple answer is yes. When using hi gain small signal valves - EF40/86, 6BR7, ECC81/82/83 etc I always use the best low loss base, with a centre spigot if possible plus a screening can, and earth where necessary. Often saves a lot of trouble later. If the gain is very high then an "anti-microphonic" base will also often help.

John

[Richard]

Hi John, Phil, can I ask if you'd connect pin 2 to chassis mains earth or signal ground 0V? I'd guess the former but don't know for sure.

[John Caswell]

Hi Richard,
Interesting question.
The skirted valve base and therefore the screening can are automatically earthed to chassis, as the base is bolted to chassis
Normally pin 2 would be connected to chassis as opposed to signal earth, however (and there is always an however), we tend to have "earth lift" components so ideally it should go to signal earth or often to the cathode of the valve as this is generally decoupled to signal earth. However using a valve base with a central spigot I have always tried to connect this to chassis earth - although I did spend a lot time head scratching where I had wired the central spigot to the cathode to find the skirted base was made with the central spigot connected to the skirt, therefore the cathode and screen (not g2) of the EF86 were shorted out to chassis. Much cursing and blinding and rewiring ensued
Overall I think I would connect to chassis, just taking care not to short out the earth lift components, but I don't think it will make a great deal of difference , just as long as it is earthed some how. It is going to depend very much on the construction on the valve base.

John

[Phil Y]

Hello John,

I will connect the screen to chassis earth then.

Many thanks, Phil.

[John Caswell]

Hi Phil/Richard,
I forgot to add that if you are dealing with RF, say an ECC88 in an FM tuner, then any screens, metal or valve type or often screens across the valve base itself should be strapped to earth/chassis in the shortest way possible.

John