Welcome! Need support, you got it. Or share youe ideas and experiences.
Return to K&K Audio / Lundahl Transformers
72.235.7.37
Hello, i am thinking of building a custom stereo amp using the Dynaco A420 opt, with your ST70 upgrade kit. The pt will be an original st70 unit. One of my concerns is that the a420 was intended for cathode bias. I have no problem with that but how will that work with your upgrade kit for the st70 which uses fix bias. Whatever help and sugggestions you can give would be most helpful. Happy Holidays! Dak
Follow Ups:
Unfortunately, the terminology can obscure the reality with "cathode-biased" and "fixed bias". The K&K Audio ST-70 Upgrade output stage actually falls under the "cathode-biased" label, since the current sinks are between the output tube cathodes and ground and the output tube grids are returned to ground through a grid leak resistor. In "fixed-bias" output stages, the cathodes of the output tubes are connected directly to ground and the output tube grids are returned through a grod leak resistor to a negative voltage, which is typically variable, so that the bias current can be adjusted for different output tubes. So "fixed-bias" is adjustable and "cathode-bias" is fixed. Funny, huh?
Kevin Carter
K&K Audio
www.kandkaudio.com
Hi Kev, thanx for replying. What i am trying to find out is will my Dynaco A420 OPT be ok for use with your Dynaco ST70 transformer coupled mods. In other words, should i use the A420 with fixed or cathode bias? If its better with cathode bias, will your ST70 upgrade mod work with a cathode biased EL34 power stage? thanx again
Yes, your A-420s will work fine in the upgrade circuit. You certainly can use EL34s, but aren't limited to them. 6550, KT-88, 6L6GC, KT-120 will all work.
Kevin Carter
K&K Audio
www.kandkaudio.com
Hi Kev, does your st70 upgrade kit support cathode bias on the power tubes? Or is there a need besides the bias resistors for other changes? thanx again, happy holidays, Dak
In fact, the only way the K&K ST-70 upgrade works is with cathode bias, but not with a resistor like most, rather with a constant current sink.
Kevin Carter
K&K Audio
www.kandkaudio.com
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: