On 05/29/2011 10:59 AM, Tim Shoppa wrote:
On May 29, 8:23 am, 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.
The OA3 glows a bright orange since at 75 volts it would be using a pure
Neon gas mixture. The other tubes in the series add some Argon, with
the 0D3/VR150 being pure Argon. Argon glows blue to purple so the 0B3
and 0C3 tubes glow with a color somewhere in between orange and purple.