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JandG 18th April 2009 12:55 PM

520v in PP UL...?
I have re-freshed a pair of el-34 Ampex monoblocks with SS voltage doubler PS & B+ & g2 is about 510-520'ish range w/ 300ohm cathode resistor & 100 ohm bias pot. I am just wanting a reliable output tube that will handle 520v in UL without problems. I am seriousely thinking of fuseing the HV somewhere to protect the OT's. This is a ef86/12au7/el-34 affair, that sounds pretty good, but red plates my old mullard el-34's. I need good tough toob.If I need to change cathode resistor that I will do .

wfmali 18th April 2009 07:53 PM

You might want to try KT77 - they are supposed to stand the > 500V g2. I personally would not run El34s UL at that voltage level if you are looking for something reliable...

DigitalJunkie 18th April 2009 10:25 PM

If you can use 6L6GC's,some of the Chinese tubes are pretty robust.I've some running at ~640V(!) in UL,and they haven't melted down....yet....They are able to withstand a bit of spec-busting.
Unfortunately I don't have a good idea how they sound yet,I haven't been able to listen to the amp much yet,as everything is in storage atm.

JandG 18th April 2009 11:59 PM

Nice........amp. What the heck ya got your bias set at..? I got some 5881 popped in one block & they seem to do o.k.. Nothing bad yet. I am not pushing my luck till I get proper bias resistors. They have a 300ohm resistor in there now from 1959. They still had to be running about 500v, this puts everything in the very hot department. I think I will up that cathode resitor to something a little more sane. KT-77's I will try.I really like 6l6 tubes & might experiment with some. That is a great pictorial of your the red plate gig..

DigitalJunkie 19th April 2009 03:11 AM

LOL,yea..Doing some Tubelab style 'testing'.
The bias was roughly set to about 45ma in one channel,and 55ma in the other for initial testing,as you can see,the tubes get pretty roasty at the higher current. They seem to cope with 40ma bias 'okay' at this voltage.Like I said,I haven't been able to do much testing with the amp yet,I need to tweak a couple of resistors to get the bias range a bit lower..I'm also looking into lowering the B+,even if it involves a new/custom power trans.
Those Chinese 6L6GCR's are pretty tough tubes. ;)

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