Q: Heathkit W6 Cathode Follower Driver for 6550 PP

[LinuxGuru]

Hi,

Heathkit W6(A-M) uses DC-coupled 12BH7 cathode follower to drive 6550 output tubes. Some variants have 12AU7 instead of 12BH7. In order to bias output tubes, driver's cathodes connected not to the GND, but to -180V negative bias supply via 10K resistors. At power on, when 12BH7 is still cold (warm-up time is about 11sec), voltage between plate and cathode is well over 600V DC - with plate current almost zero, 10K cathode and 12K plate resistors have no significant effect. Data sheet states max plate voltage for class A amp about 300V (450V for oscillator which is not our case).
Is this design flaw based on assumption that 12BH7 (12AU7) can tolerate startup over-voltage for quite long time, or I'm simply missing something?

Thanks.

 

[Tom Bavis]

Looked at GE data sheets. Plate curves go past 650V for 12BH7 - and absolute maximum peak plate voltage is 1500. 12AU7 has a peak rating of 1200V, but curves aren't extended past 480V or so.

 

[gadget73]

If its not flowing any current, the voltage isn't a massive issue. Seems sketchy but its more dissipation that causes failures. Its also less of a problem if it settles out to sane voltages once its actually running.

Then you have things like this, which just look wrong but apparently works fine. McIntosh MC40, but others are similar. North of 500v while running across a 12ax7



if I'm being honest the socket itself kind of scares me more than inside the tube. 9 pin sockets just aren't that big, and for both a 12bh7 and a 12ax7 the plate and grid pins are neighbors. One of them is also next to a heater supply, which are often grounded or referenced to a not extremely high voltage. This is absolutely not where I'd be using crappy sockets and lead dress would be very carefully considered. No stray wires or bent over terminals.

 

[LinuxGuru]

Original GE 12BH7A can stay alive with 1500V peaks, yet it was industry benchmark in terms of quality. Today's JJ and EH explicitly rated at max plate=300V in their data sheet, with plate curves drawn up to 600V. They may survive, (or may not) 660V plate to cathode cold start.
So my proposal is to slightly modify W6 schematic (attached):
1) Use 1N5371B 60V 5W zenners, 4 total.
2) Use 20K 10W resistor from plate to GND. Original 12K plate resistor in W6 need to be changed to 10-11K.

 

[gadget73]

I might go slightly more old school here, 3x NE2H neons in place of the zener diodes. They strike around 85v and would be essentially invisible once voltage drops below strike. The H variant is higher current.

and/or just use old production tubes here. 12BH7 isn't exactly unavailable.