[ratbbb2]

Ok so I know I should have a grip on tubes if I own them but up until now I haven't had any thing go wrong (other than a bad recterfier valve)
I had Groovetubes EL34 (JJ, Penta...) in the amp and one of the quad went bad (started to glow and poped the fuse). I changed the fuse then played with the biasing and switched tubes to come to that conclution. ones I replace the old(er) EL34 for a set of 6CA7 (Ei big bottle) I can no longer get the recomended voltage of 1.56 in fact the highest it goes is 1.24v on one side and 1.21v on the other . Is there a reason for concern once I bias them at 1.15v or is it just a lower output tube?
Thanks Roger  

[avahifi]

Three likely possible problems here with a stock St-70.  First is a low voltage 5AR4 tube.  It is putting out much less than 420V DC (measured at the first lug of the filter cap) then it will not be turning the output tubes on hard enough to make proper bias voltage.

Second suspect, the original selenum rectifier in the bias circuit which become resistive with age.  This can be replaced with a 1N4007 diode.

Third possibily, hardware at the output tube sockets not tight enough, causing a high resistance path from the 15.6 ohm precision resistors to ground.

Frank Van Alstine

[ratbbb2]

avahifi
  Thanks for taking the time to reply to my post but as it turns out this is not a stock peice. The selenum rectifier was substituted with a AVA"type"set up buy a local tech  (no longer working). As far as the hardware and 5AR4, they both checked out good to go.
  I guess I'll be visiting you web sight soon.
 Thanks again Roger