Valve in a valve?

[Yorkie]

Anyone tried it before? Am I getting carried away, daydreaming?

It would resolve the availability problem of valves like EBL31, sub in a pentode, say EL84, and 2 silicon diodes hidden in the base.

Sam.

The problem with EBL31 valves, is that all of those sold by the main UK valve dealers aren't EBL31s at all - they're EBL1's which have a side contact base and a 3/8" top cap, whereas  the EBL31 has a 1/4" top cap and a short octal base. The converted EBL1 valves simply have an octal base glued over the side-contact  base, which - if there's space to accommodate the taller converted valve isn't a problem, but given that EBL31s are used mostly in Ekco A22s,  the converted valves are useless. They can be converted properly and I've converted several NOS EBL1s by carefully cutting the side contact base and fitting a cut down octal base cross-wired to the valve, then I've removed the3/8"  top cap and fitted a 1/4" one. It's a tricky task which calls for a steady hand using a Dremel type drill with a cutting disc. I wrote up my experiences in a BVWS Bulletin article.

Before I found EBL1s to convert, as a temporary measure I made an adaptor using an EL84 valve, and two germanium diodes hidden in the octal valve-base, with a 'top cap' mounted on the side of the valve-base  That has the merit that there is no re-wiring of the valve-holder on the set.  Of course, it does look rather comical, but it was functional for some months till I got my hands on some EBL1s to convert properly. EBL1s don't seems as rare as genuine EBL31s, and not as costly. (I bought two new in box at £10.00 each from Golborne, and another three from the NVCF from a dealer who said "these can be converted to EBL1s you know - some bloke wrote an article in the Bulletin about converting them".  I didn't let on, feigned disinterest but idly said 'really - how much for all three?", and he said "£15.00".  So, in effect, with a bit of effort and skill, five potential EBL31s for the price of one useless 'Big Bertha'.

Can't comment on the 'valve in a valve' idea - If Gerry Wells had still be around and anyone had said to him 'It would be impossible to do such a thing' , he'd soon take the task in hand! 

I've attached a few pics. I originally thought there were two types of EBL31s - small and tall. I spoke to Gerry Horrox who soon put me right on that score - he said "yes, there are two types - genuine and fake".

Hope that's of interest, albeit rather off topic from the 'valve in a valve' idea.

You've set yourself a challenge Sam - go for it!

(Plenty of space in a valve-base to fit a couple of silicon rectifier diodes in a hard to find expensive rectifier valve).