SB220/221

[Mark Graalman WB8JKR]

  Hi John,

    I guess I'll have to start by saying that I don't agree
with Measures on his parasitic theories, and cures.
 About the same thing could be accomplished by simply
pushing the suppressor turns a bit closer together.

 That being said, here is what I would do to a SB-220/221
if I had one and planned on using it:

1) I would paint the inside of the RF compartment around
the tubes flat black, this will reduce internal tube temperature
considerable.
2) replace the stock HV supply diodes with 1N5408's and use
no resistors or caps across them.
3) place back to back silicon diodes (hep170's or 1N5408 etc.)
across both meter movements.
4) replace resistor R5 with a rear panel mounted 50K pot, this
will allow you to have adjustment of the ALC threshold
5) disconnect R27 from relay lug 3 and reconnect it to lug 6
6) drill two new holes in the chassis either side of and RIGHT NEXT
to pins 2, 3, and 4 of the tube sockets and ground the grids
directly to the chassis using SHORT copper strap at each grid
pin, remove the 200 pf caps and the 1 mh chokes.
7) IF I was having problems driving the amp with a solidstate
rig, I would do Measures mods to the input circuit to get the
Q up, if I wasn't having trouble I wouldn't bother with it.
 However if I did do it, I think I would mount a two lug terminal
strip at each coil location and use T-68 size cores in place of the
original coils so the unit could be returned to stock as these
coils are no longer available. I would use a #2 mix for 80-20 meters
and a #6 mix for 15 and 10.
8) A stepstart circuit wouldn't be a bad idea, but not absolutely
needed, more important to not turn it on in the SSB position.
9) I would NOT change the zener bias, no reason to.
10) I would install a EBS system, contrary to Measure's opinions
it CAN be done with minimal distortion problems. I know, I've
done it. Its worth it because it reduces the average temp of the
amp quite a bit. If you're interested in my EBS system I can
give you the info on it.
11) Replace RFC2 with a 10 ohm 5 watt wirewound resistor

 As far as the metering resistors are concerned, just using modern
MOF 2 watt resistors will take of that, the problem really lies in the
old carbon comp style resistors. I would follow Rich's comments on
bleeder resistors as well.
 I've never modified a fan for oiling, but I don't think it would be a
bad
idea to do that at all.
 I also agree with putting a 100-200 ohm resistor in series with the
relay
keying line and a diode across the relay coil.

 As far as the amp that blows the breakers at turn on, I would
first disconnect the HV transformer secondary and try it, if
its ok then check the HV diode strings, if they are ok, then check
the filter caps, then C6 and/or C7, assuming of course the tubes
are ok.

Hope this helps you somewhat. The thing is not to go hog wild with
mods on these things, a lot of these amps have played fine for 25
years as designed from the factory. Nothing perfect, but it doesn't
make a lot of sense to butcher up an amp just for the sake of mods
that may not be needed anyway. I wonder how much trouble with
these amps is because people just try to make them do what they
were never designed to do, they were meant for 600 watt out CW
and 1 KW output PEAK SSB in amateur service, period.

73,  Mark  WB8JKR


On Tue, 30 Apr 2002 10:34:23 -0700 John C Owens <jcowens3 at juno.com>
writes:
> Mark:
>
> I have documented some facts and thoughts in the attachment re the 2
> amps which hopefully you will read and comment on. Your help would
> be appreciated to help sort through all of this. Thanks again for
> offering to help.
>
> John Owens (N7SEJ)

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