tolerance in st70 bias

Sorry for the long post. It is as thorough as possible. I have a pair of st70's that I'd rebuilt a few yrs ago. Each had been completely disassembled, cleaned and rebuilt. Basically all new parts except for chassis, transformers and sockets.  Also installed, bias balancing kits from Dynakitparts. I remember them being rock solid in biasing. Each was used off and on for a year or two, then rotated out of the system. Toward the end, I tried using 6u8a's and 6gh8a's in them with adaptors from eb@@. Each probably ran a month or so, and I dont recall there being a problem, but didnt check bias before putting them away. A year or so later, I pulled them out and rotated them in. Bias on each became erratic, changing thru the day, and never having the same bias as where I'd set them, though not to the point of causing a problem. There is no hum, hiss etc. I went thru the routine of checking voltages, cleaning and checking sockets, etc to no avail. Removed the bias balance, checked resistors etc, again to no avail. Finally I was so pissed I completely dismantled one. The other got new coupling caps, grid, bias supply and cathode resistors,and finally new bias pots and sockets , just because. Still did not do it. I have several sets of driver tubes fairly well matched, and el34's (the latter bought new, now having low hours, and working fine before storing them), that made no difference. At the end of my rope, I biased w no driver tubes in and went back to 7199's, and the bias settled down for the most part, but  I still have some drift in the bias, maybe +/_.025v while running and on startup. Is this anything to be concerned about? Any ideas why this may have occurred? Again, the only change was going to 7199 subs.

Scrap the original boards and install VTA's with 12au7s.

Bias will change up & down depending on many things ( mains voltage, temperature etc). In your case
a change of 2.5% is nothing to be concerned of.

I'd suggest that you put the measurment aside and check one a month or so.

audiobill wrote:Scrap the original boards and install VTA's with 12au7s.

Yeah, what Audio Bill says. Or go for the octal board which takes 6SN7s.

bias variations of 2-3% are pretty common, it's a combination of your AC input voltage and the tubes heating,
not to worry about unless you have 10%+ variation

One thing, just my experience . . .the pots on my VTA ST-70 are tuned like a violin string. Very sensitive so it's not hard to throw a reading into left field with just a blades-width micro-turn, . So I give them a range, and that works. Weird part, I've not had this sensitivity in ST-120 and M-125 builds. You can easily dial them right in.

deepee99 wrote:One thing, just my experience . . .the pots on my VTA ST-70 are tuned like a violin string. Very sensitive so it's not hard to throw a reading into left field with just a blades-width micro-turn, . So I give them a range, and that works. Weird part, I've not had this sensitivity in ST-120 and M-125 builds. You can easily dial them right in.

That sounds strange. Are the circuit boards significantly different?

rjpjnk wrote:

deepee99 wrote:One thing, just my experience . . .the pots on my VTA ST-70 are tuned like a violin string. Very sensitive so it's not hard to throw a reading into left field with just a blades-width micro-turn, . So I give them a range, and that works. Weird part, I've not had this sensitivity in ST-120 and M-125 builds. You can easily dial them right in.

That sounds strange. Are the circuit boards significantly different?

No but the tubes.

A future retape might use 10-turn pot for better presition ..