On May 29, 10:59*am, Tim Shoppa wrote:
On May 29, 8:23*am, JIMMIE wrote:

I want to replace the vr 0A3 tube used in the bias circuit of a 6146B
amp with a 75V zener string. Can I just directly replace the tube with
the diodes or *will I need to make other modifications to the circuit?

Jimmie

Typical OA3 is idling with between 5 and 40mA of current flowing
through it, so make sure the zener string can dissipate the power.
(75V * 0.040A = 3 watts worst case, but I'd expect less idle current
in a grid bias regulator.).

If this is a major redesign you might want to use a solid state series
pass regulator (e.g. TL783 or the like) and probably get rid of that
old wirewound pot in the bias circuit.

0A3's are not unobtanium, rare, or expensive. I kinda like seeing VR
tubes around the PA, nice purple/pink glow dims and brightens as I
key, and if they're not glowing then something's wrong.

It draws about 30ma calculated by measuring the voltage drop across
the series dropping resistor. Once in a while it arcs, I think. I
tried another tube and that seemed to fix it for a while but it
started up again. There is a posssibility this is not the problem but
for now it seems most likely. If I can easily replace it with a zener
string I could be more sure that it is or isnt the cause. Plan for now
is to put 5 15V 1 watt zeners on a tube base I can plug into the
existing socket and see what happens. If that is the cause I will
readily invest in another tube because I think they look cool too.

Jimmie


Jimmie