On Jun 1, 2:54*pm, Tim Shoppa wrote:
On May 29, 2:15*pm, JIMMIE wrote:





On May 29, 10:59*am, 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.

During transmission (modulation peaks?) the VR tube glow intensity
will change and even shift pattern from one part of the tube to others
with some hysteresis. This is normal. Arcing is not normal. But note
that the glow and shadows might show some structure (kinda shadows and
shadow edges) shifting as the current through the VR tube changes and
maybe you suspect that is arcing?

Tim.- Hide quoted text -

- Show quoted text -

I put a neon bulb across the dropping resistor, well across a piece of
the dropping resistor. I used a photo transistor to couple to my
counter. Over a 12 hour peiod I got 10 to 15 hits. With the Zeners in
place I havent had any.. First noticed the problem when the plate
current meter on my amp went from 0 to full scale just setting on the
bench. Bias supply is set up so unless the amp is keyed it has the
full -75V on the grids cutting the amp off when keyed it drops to
about -45. When I caught it on my storage scope it was about -18v. I
like the VR tubes too, now that I am more certain thats whats causing
the problem I dont mind ordering a couple more VR tubes. Before I
order new ones I am thinking of trying the same monitoring setup again
with the tube in place but this time changing the bias adjust pot.
That would be the only thing I havent checked. I hate intermittent
problems like this.

Jimmie