Hello all, my first post here so be gentle.
I've got a friend with a Bob Latino built ST70 and he's having trouble with the choke buzzing. I've been tinkering with audio for about 35 years but somehow never manged to own one of these amps so I figured I'd ask those who do.
Anyway, he brought it by last week to have me install a new choke to replace the buzzing one. This is the second choke it's had, the first replacement was installed by the builder when he sent it back for repair. I installed the new choke and fired it up on the bench and it was dead silent mechanically, which of course was great. Hooked it up to my system and quiet electrically as well, no hum or buzz and it was sounding quite nice. After a bit he asked me to check the bias and to set it at 50mA per tube. He's running GEC KT-77 reissues. The bias was at about 44mA and as soon as I turned it up to 50mA it started buzzing again and of course wouldn't stop when I dropped the bias.
Looking up the specs of the choke from dynakit parts it seems to be rated at 200mA and at 50mA per tube plus the input/driver tubes this is of course somewhat over the 200mA rating of the choke.
My question is this, has anyone had trouble with the chokes in their ST70's or is he just wanting to bias too hard? 50mA seems pretty high to me but I tend to run tubes pretty conservatively. Not really any room in the chassis for a higher current choke. Since I don't have any real experience with this amp or this tube I'd love some input on what the best course of action is. I'm guessing 40mA would be a much better bias current for this tube but I'd prefer some more data points.
Thanks in advance for any thoughts, always appreciated.
Mike