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Substitutes for EL34

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aliveatfive

New Member
Just completed a new ST70 amp based around Roy Mottram's board. (Very easy to build, BTW.) I used a matched quartet of Sovtek El34s for outputs. The amp sounds very good, but I would like to do some tube rolling. I got another matched quartet of new-production Genelex KT66s. They bias too high in this amp. Dyna stated in the original manual that KT66s could be used in the original amp. Does the new board preclude their use in this amp? Any other suggestions for output tubes that won't cost an arm and a leg?

Thanks.
 
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6CA7 from what I understand are the "Americanized" version of the EL34. Or the KT77.

Hope this helps...............R1:music::yes:
 
I've seen some with KT88's - but don't expect any more power out of them. I think I have seen one conversion of a Stereo 70 that used a 6L6 (I THINK - It was a long time ago). See if you can get confirmation on that before you do however cause I'm always mixing up the 6x6 type tubes. The kt88's plug right in and I think the bias voltage remains the same.

Shelly_D

Update:

Ignore what I said above and read here:

http://www.audiokarma.org/forums/showthread.php?t=56780
 
SED =C= EL34s are quite nice, being warm and smooth and fuller than some of the other EL34s.
JJ KT77s are a substitute, but they have or had reliability issues.
Jack
 
i've used mullard el34's for a long time and when it was time for replacement, =C= was the only thing that i was content with.
 
You can plug 6L6's into that ST-70 with Roy's Board. I use them now and the Russian 1970's 6n3ce sound fantastic in it. I'm the guy that swore he would never use a Russian tube. A great deal at about $30 a quad. Bias em up at .40 and good to go! Just my 2 cents....
 
Thanks to all for the advice. I e-mailed Roy and he said to bias KT88s at 50 mv. Didn't say anything about KT66s. The lowest I can set bias on them is about 50 mv. BTW - for those inclined to experiment - he advises an upgraded power xformer for those who wish to use KT88s. I got mine from Triode.
 
To get more bias voltage for the larger tubes like KT66 and KT88
change the bias "bleedoff" resistors on the board, (R37 and R38)
from the standard 27K to more like 33k (yes you can experiment).

More resistance in the bleedoff resistors means more available bias
voltage and less resistance means more goes through to ground so
you have less available, you can play around with R37 and R38 to try
an find a happy medium so more tubes are in the bias range available
so you can plug and play without having to mod it all the time.

My favorite output tubes in a 70 with a good front end are the Sylvania
Fat Body EL34's and the new JJ KT77's work really well to!!
 
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