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Old 15th May 2009, 06:40 PM   #1
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Default Advise for nervous EL84 user

Hi Newbie here,
I have a Tube Technology Unisis and need some help on valves. I've swaped the signal valves no problem to Brimar's.

Problem (might just be me) I have is with EL84's. It takes 8 of these and I'm currently running JJ's with no problem. I ordered some Ei valves and subsequently the amp went up in smoke, one valve seat flashed over, a few resistors taken out and a fused pin welded into one of the sockets. So I'm extremely cautious about trying anything else....I've been offered some used NOS Mullards , the Genalex mullard re-issues seem to be well respected. I'm not a valve expert and the amp needs to be biased manually...

My questions are really how do I know which valves to match the amp...I know the JJ's work so what valves work on the same spec..I know an EL84 should be the same as any other but some of the higher voltage models ...how do i make the decision?...I've seen NOS Tesla's on offer too but really dont want all the fireworks and fuss all over again. You see my dilema? I want to improve on the JJ's which I though the smaller 12AX7's were a bit thick and chesty...but I really dont want to throw away on NOS valves and then the amp goes up etc...

Any help much appreciated.
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Old 16th May 2009, 01:43 PM   #2
Funker is offline Funker  Germany
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Hi ,
you got probably a huge problem.
Before anyone can really help you to find the fault put the drawing of the amp to your thread.

But I can you give you some useful hints without seeing the drawing. I assume you got an 2x30 watter with 8 EL84 Ultralinear PP.

Pull out all valves an repair/ replace the damaged parts.

You mains voltage should be in the range of +- 10%. If you have permanent overvoltage in your sockets due to have the power transformer in close neighbarhood, connect the mains selector to 240 or 250 volts ( England turned from former 240V down to the new european 230V).

Check all voltages on the tube pins with a Multimeter. Compare the indicated voltages with those in the schematic. they should be 15 to 20% higher without valves.

Check the values of the kathode and grid resistors with an Ohmmeter. they shoud be + - 10% , and at best if they are all the same.

So if you have no voltages given in the schematic, look to the valve data sheet.

http://tubedata.tigahost.com/tubedat...129/e/EL84.pdf

Thats all I can do for you by now,


good luck !


regards from Hamburg
Wolfgang
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Old 16th May 2009, 01:59 PM   #3
Enzo is offline Enzo  United States
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Just my opinion, but I think you are confusing a tube failure with tube "compatibility."

Your new set of EI tubes had a bad one which burnt up. It was not due to some incompatibility, it was just a bad tube. You could just as easily have gotten a bad JJ tube. Or NOS tube for that matter.


For what it is worth, I got too many defective EI tubes in the past, so I no longer will use their products. (I operate a pro audio repair shop.)


It isn;t a matter of matching the amp to a particular brand. (Other than that you might prefer the sound o0f one over the other.) It is a matter of not having a defective tube in the bunch.
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Old 16th May 2009, 02:31 PM   #4
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Quote:
Originally posted by Funker

you got probably a huge problem.


Or maybe not. As Opusover21 is not asking about the repair we should probably assume this is already taken care of.

I agree with Enzo that you probably had a defective tube. And i find it cheaper to buy NOS or used Mullards and Teles instead of cheap crap. As a tube hobbyist you should definitely invest in a good tube tester and make sure you test all tubes before plugging them in. This will also allow your own matching instead of relying on someone's word.
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Old 16th May 2009, 03:49 PM   #5
pointy is offline pointy  United Kingdom
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may be try <http://www.langrex.co.uk/sales.html>

they should be OK about testing etc.....
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