Erratic bias

knowledge base for the classic Dynaco ST70

Erratic bias

Postby dmag » Mon Oct 28, 2019 3:23 pm

Sorry for the long post. It is as thorough as possible. I have a pair of st70's that I'd rebuilt to stock 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 epay. 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.
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Re: Erratic bias

Postby WA4SWJ » Wed Oct 30, 2019 6:12 am

Hi dmag,

You don't state how much the bias was moving around (voltage change) before you put the 7199's back in. I assume you're measuring across a small value cathode resistor or something like that. If it is moving 0.025 volts that is a very small move and I would not be concerned about that at all, but it might depend on how you are measuring the bias. I'm not familiar with the Dynakitparts circuit. I might look it up. Line voltage variations could be the reason for that kind of move or just minor moves in the operating points of the EL34's during the day. I wouldn't worry about that. I hope this helps a bit.

Ed
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Re: Erratic bias

Postby dmag » Wed Oct 30, 2019 12:13 pm

Thanks Ed. The board is the stock 7199 circuit. Bias is checked via a 10 ohm resistor off the cathode. Ill monitor the wall voltage, but it looks like a non issue.
Steve
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