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!!
I have recently purchased a very fun amplifier - a pair of 4-250's
that looks something like the stuff in the 1957 handbook, or perhaps
January '61 QST.
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).
The amp was initially set up for class C. I have already made changes
to the bias supply to run AB1 or Class C. I am now working on the
HV power supply and found an interesting problem.
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.
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).
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?
BTW, I am documenting my progress with this amp on the web. If you
are interested in following along, check out
http://web.jzap.com/n6tr/ampdiary.html.
73 Tree N6TR
tree@contesting.com
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