[R-390] OA2 replacement

[Tim Shoppa]

kc0lwn asks:
> Can the OA2 be replaced by simply putting in a 150
> volt 10 watt zenner in the appropriate socket pins?

Pretty much.  But a real OA2 has better performance
(lower dynamic resistance, less noise) than a high-voltage
Zener.  (There's a reason they use Zener's in noise generators!)

And there's already enough heat on the audio deck -
my gut feeling is that it's better to put an OA2 there (which
dissipates heat over several square inches) than to put a
Zener that'll run mighty warm in the vicinity.  Yeah, I know
that the total power dissipated is the same, I just think it's better
to spread it out.  Most of the heat dissipated by a 10W Zener gets
conducted out through the leads, which is passable on a PCB with
big pads but tube sockets aren't so good at conducting out heat,
realistically you'd have to derate a 10W Zener to just a few watts
in that environment.

Is there a shortage of OA2's or something?  They happen to
be a lot more readily available to me than 10W Zeners at the
moment... but if you were in a pinch and couldn't find an OA2
I think it'd work OK as a short-term substitute.  Better would
be to use a low-voltage Zener as a reference for a transistor
shunt regulator, it's easier to heat sink the transistor and you'll
get a lot lower noise.

Tim.