Unusual preamp valve screens in WEM Dominator 25

[Stratfordade]

I’m looking at a working (though will need new supply electrolytics) WEM Dominator. Want to identify/test all the valves and the 2 EL84s and one of the preamp valves all have same Mullard Blackburn 1978 date. Probably original to amp as this model first appeared in WEM’s 1978 catalogue.

I can’t inspect the other two ECC83s as instead of the usual sprung screening can twisting onto lugs, they are in a tight fitting thin metal sheath. Interestingly this sheath is connected to a grounding ring around the valve base via a soldered wire.

Has anyone come across this? How can I get the valves out the screens without risking damage to valves or inconvenience of breaking off the grounding wire?

Thanks for any advice.

[trobbins]

A 'spring' roll of that format was often seen around the microphone input stage of PA amps in australia (Fig.33 in hum document). Even such a 'spring roll' can be difficult to remove, but yours seems harder still - perhaps the roll may expand just enough if heated to make it easier to lift off.

[g1]

It looks to me like the ground wire is long enough so you can unplug the tube from the socket with the shield still attached?
Then you would fight with the spring roll as stated above. Is there a slit in the opposite side of the shield that we can't see in the picture?

[Jon Snell]

Unsolder the ground solder blob and remove the valve. Test as normal with your AVO CT160 or equivalent. No need to remove the can for testing.
I expect it will read between good and excellent.
This system was used extensively in that era.

[Stratfordade]

Unsolder the ground solder blob and remove the valve. Test as normal with your AVO CT160 or equivalent. No need to remove the can for testing.
I expect it will read between good and excellent.
This system was used extensively in that era.

Thanks Jon I’ll do that though my ‘ear’ tester suggests they’re all fine. The V2 was 0.9/1.2mA and 1.8/1.9mA/V so as you predicted.

I can just see the top of the Mullard logo on the screened valves. Was just curious if they were all same date.

[Stratfordade]

It looks to me like the ground wire is long enough so you can unplug the tube from the socket with the shield still attached?
Then you would fight with the spring roll as stated above. Is there a slit in the opposite side of the shield that we can't see in the picture?

Yes plenty of slack to unplug the valve. The screen actually overlaps very slightly on the other side.

Fortunately I’m almost certain all the preamps/PI are good. But if one did need replacing I would replace these with collars around the base and the usual twist lock cans.

[g1]

Garnet amps used a similar tube shield. With age they kind of seize on to the tube. If you work something thin into the seam, and apply pressure against the tension of the 'spring roll', they will eventually let go.

[Stratfordade]

Good idea thank you. There’s nothing holding the shields on there except habit!

[Stratfordade]

g1's tip worked. Worked a tiny jeweller's screwdriver around the inside of the screen while prying very gently, and it soon broke the hold and slid off. Nice Mullard ECC83 inside!

[Mick Bailey]

I've come across this screening arrangement in radio and lab equipment, though never concluded if there was any advantage over regular sprung cans.

[Stratfordade]

I think there may be some saving in cost and time in manufacture (no need to install bases to attach the sprung cans), but from a repair/replacement point of view they are ridiculously fiddly!

[Helmholtz]

It might be a Mu-metal shield against magnetic EMI.
That wouldn't be a cheap solution.

[Stratfordade]

It might be a Mu-metal shield against magnetic EMI.
That wouldn't be a cheap solution.

Good point, thank you. Also in normal use someone is only going to be replacing preamp valves once in a blue moon, if ever. I just enjoy fiddling and measuring, even when no repair necessary!