> From: Larry Tyree <n6tr@teleport.com>
To: <amps@contesting.com>
> Date: Tue, 15 Jul 97 19:36:41 +0000
> Hi everyone! I am a new member of the amps reflector and wonder if
> anyone has any good modifications for an SB-220??
> :-) Just kidding!!
Welcome to the warm friendly group. ;-)
> It has always been a dream of mine to have something like this,
> and I could not pass this one up (got it for $575 with a spare
> set of new tubes). It has some really nice components in it (like
> a B&W 850 coil/bandswitch).
Neat.
> The supply uses negative lead filtering (something I wish Henry had
> done when the HV choke shorted to ground). It also has a 50 Ohm
> resistor from B- to ground, with a one amp fuse in series with it.
Oh oh, I wouldn't put a fuse in the B minus line. Safety resistors
are also not good, unless augmented by diodes. If the resistor ever
opens and you solely depend on it to keep the negative rail clamped,
you might fly up the big 160 meter duct.
> The RF deck uses chassis ground from B-, so this fuse will protect
> things if something in the RF deck shorts to ground. Normally, this
> cicruit works just fine - until the fuse is blown.
>
> Then, the B- seems to float with respect to ground and I start hearing
> arcs in the amplifier (there is a current sense resistor at the "bottom"
> of the bleeder stack which is used to generate voltage for a relay to
> turn switch the bias voltage to the tubes - the arcs are at the base of
> this relay because the pins are isolated from ground well enough for
> HV).
Hmmm. That sounds dangerously backwards Tree. It is better
to use the BIAS to turn on the HV, than to use the use the HV to turn
on the bias.
The best way is to use the blower to turn on bias and filament, the
bias to turn on HV, and the HV to turn on screen. Then if you loose
one of the mess nothing explodes or cooks inside the PA.
> It seems to be the best fix for this would be a high impedance resistor
> across the fuse to drain whatever leakage current is pulling up the
> B-. I was wondering if anyone else had any good ideas on a cure for
> this?
Yes, let the negative rail stay grounded, or near ground and FIRMLY
clamp it there. Put the fuse in the HV, where it belongs. Be sure to
use a HV type fuse!
By the way, you missed a good discussion about bias, screen
regulation and IMD. You might check the archives.
Be real careful what you do in the negative rail, or you may be
documenting the progress on http/fallen.tree.
73, Tom W8JI
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