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

On 05/29/2011 08:23 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
If you use diodes rated at the same current as the 0A3 (40ma) then it
should be a direct replacement, at least for a voltage regulator
application.

On 05/29/2011 08:23 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
Forgot one thing, with zeners the CATHODE of the diode is wired to the
tube ANODE and visa versa (zeners work in the REVERSE mode!).

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.

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.

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

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"JIMMIE" wrote in message
...
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

Don't know if I read all the responses, but I hope someone explained the
need to provide a series current limiting resistor in the zener string.
Something that would keep the no load current up near the top of the zener's
operating range would be good.

Old Chief Lynn, W7LTQ

On May 30, 7:20*pm, "coffelt2" wrote:
"JIMMIE" wrote in message

...

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

Don't know if I read all the responses, but I hope someone explained the
need to provide a series current limiting resistor in the zener string.
Something that would keep the no load current up near the top of the zener's
operating range would be good.

Old Chief Lynn, W7LTQ

Hey OM:
I take a DC supply from the filament voltage and use that to power a
programmable zener TL431. and use a FET to replace the OA3 driven by
the TL431.

I've replaced OA2's this way, all it takes is three 1/2 watt resistors
to set the voltage.

73 OM

de n8zu

On May 30, 7:20*pm, "coffelt2" wrote:
"JIMMIE" wrote in message

...

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

Don't know if I read all the responses, but I hope someone explained the
need to provide a series current limiting resistor in the zener string.
Something that would keep the no load current up near the top of the zener's
operating range would be good.

Old Chief Lynn, W7LTQ

Hey OM:


That resistor is part of the OA3 circuit. All you need is noisy zeners
that can handle the current. You would need 3 watts worth of zeners.
Three 25 volt 1 watt zeners, or five 15 volt 1 watt would give you
more head room. Or 15, 5 volt 1/2 watt zeners, or 1/4 watt. Put 15 SMD
5volt 1/4 watt zeners between the 2 pins on an octal plug, pin 2
connect the anode of the zener string and pin 5 connect the cathode of
the zener string. If you are lucky you can find a 5 watt 75 volt zener

73 OM

de n8zu