Tube exchange question; EL34 with 7 or 8 pins.

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Posts: 2
Joined: Sat Jan 13, 2018 3:51 pm

Sat Jan 13, 2018 9:23 pm

I purchased a new Blackstar HT Stage 60 MKII that came with a bad power tube, no big deal, tubes were replaced by vendor.

While waiting for the replacement tubes, I put a set of Electro-harmonix EL34 tubes in the amp.
No glow, no sound...nothing (including none of the fizz/static!!!)
Put the stock Ruby EL34BHT tubes back in and everything was back to normal (including the fizz/static).
Replacement tubes sent by vendor are Groove Tube EL34. The new tubes fixed the noise problem.
(Performance wise, the Groove tube are a little "spongy"compared to the Rubys. I'm happy with that.)

What I didn't previously notice was that Electro-harmonix EL34's have only 7 pins. Pin 6 is not extending below the base of the tube (internally connected only). The Groove tubes and the Rubys both have 8 pins.
An internet search has turned up nothing other than a document that showed EL34 pin 6 is "normally "connected internally only.

Has anyone ran into this before?
Does anyone know the "technicalities" of tube construction to amp requirements?

Posts: 1
Joined: Wed May 30, 2018 2:52 pm

Wed May 30, 2018 3:27 pm

Before seeing ds60's post, I bought EL34EH tubes for my HT Stage 60 MKII 212 without realizing they only had 7 pins. However, the amp fired-up just fine with the new tubes and was easy to bias.

Is there any reason I shouldn't be using Electro-Harmonix EL34s in this amp? I read somewhere that the stock Ruby BHT tubes are designed to withstand higher plate voltages, but I've found no documentation showing the plate voltage requirements for this amp.

Hopefully someone here knows the answer.

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