"Steve O'Neill" wrote in
:

Hi:

You note that the 6BQ5 spec sheet shows 130 ohm cathode resistor for 2
tubes, class AB PP. Be aware that this is for designs at approx max
ratings of 300V A-K: the AA151 runs the 6BQ5s WAY over max plate
voltage at something like 360V A-K (similar to Dyna ST/SCA35). It
seems reasonable to assume that if the original design used 100 ohms
for four cathodes, equivalent for two pairs of two cathodes ea would
be 200 ohms and this is still running the BQ5s pretty hot. I'd be
inclined to run more like 220-250 ohm @10W ea.

A real benefit of separating the common K resistor is that a matched
quad of BQ5s is no longer required, just two matched pairs. Heath
(and Dyna) don't really do a good job of alerting the user to this
issue. Individual K resistors reduce the need for matched pairs but
if it were my amp, I'd still go for them.

--
Steve




Good point! I was wondering about the reduced current sharing the
resistor 4 ways...