EL34s with metal base-shell.

[G6Tanuki]

In the 1970s I bought (for a pittance) a job-lot of used vibration-testing gear, most of which was variable-frequency power-drivers each of which featured twelve EL34 valves in a parallel-bridge arrangement. These had been used at one of the UK's larger aero-engine manufacturers' sites in the East Midlands for research/testing of turbines.

I bought them for the mains-transformers, which soon found new homes in the guitar-amps I was building [keeping bassists happy with zero-bias class-B 807s needed a couple of hundred milliamps at 750V] but have recently discovered a few of the EL34s that I kept.

Several of these have spun-metal bases rather than the usual moulded-bakelite. The spun-metal base is akin to the types used on the old "Loktal" valves or the likes of the little QV04/7 RF valve used in the WS62 transmitter and 1950s VHF two-way mobile sets.

Which leads me to ask - why were some EL34s made like this?

[GrimJosef]

I think it was the very earliest ones that had the metal bases. I don't know if they were easier to make in small-ish quantities which might have been more appropriate for the start of production. There are similar GZ34s too.

Cheers,

GJ

[snowman_al]

Yes the very first Philips EL34s were derived from the EL60, which has the 9 pin Loctal base. https://www.radiomuseum.org/tubes/tube_el60.html
There is lot s of EL34 'family' history here http://www.jogis-roehrenbude.de/EL34...EL34-Story.htm (in German, but Google translates it fine.) Page 2 shows various forms of the metal / plastic bases. http://www.jogis-roehrenbude.de/EL34...ory-Seite2.htm

They command a good audiophool price too.

[G6Tanuki]

Heh! I hadn't realised there were so many variations of the EL34!

These metal-shell ones are cosmetically rather poor - having been stored in an attic for 40-odd years the writing's rubbed-off the glass (so nobody would believe they were made by Mullard) and the metal base-shells are tarnished - definitely not audiophool-material. I _do_ have a Wodem UM1 transformer somewhere though - combined with a couple of EL34 it would make a nice modulator.

Interestingly, my metal-shell ones all have the getter in the form of a flat metal plate that is almost the same diameter as the inside of the envelope, so the 'fired' getter material is neatly confined to the top end of the bottle; the bakelite-base ones have 'ring' getters which means the getter-material is rather more promiscuously spread.

[snowman_al]

There should be a 5 or 6 digit (Philips/Mullard) code pressed into the metal base. It will tell you where and when it / they were made.

Philips / Mullard plastic base ones should have a hole in the centre of the spigot and the code is 'etched' into the glass somewhere...

[G6Tanuki]

My metal ones are stamped

6Y0
57A

and yes the bakelite-based ones have a little hole in the centre of the spigot.

[Maarten]

Could that be SY0?

57A probably means they were manufactured in january 1957, in an unknown factory (probably Eindhoven).

[G6Tanuki]

Yes looking closely at it, it could be SY0 not 6YO - the stamping is not perfect (it must be hard to stamp on to a curved surface!).

The only etched marking I can read on one of the bakelite-based valves [which still has some of its white Mullard 'shield' logo visible] appears to read

Xf1
68H

though the '6' looks rather like the odd Russian '6' - or is it a B?

[Maarten]

Likely a B, indicating it was indeed made in the Mullard works, with 8H pointing to august 1958.

[Restoration73]

Not quite the same thing, but I do have two 6CA7s with a metal base.