[Amps] SB-220 Re-Build

[Richard Solomon]

Looks like you also had to rotate
the Filter Cap bank 90 degrees.
Was that because of the size of
the toroid ?

73, Dick, W1KSZ

On Sun, May 12, 2019 at 10:02 PM Joe <nss at mwt.net> wrote:

> I have had quite a long discussion going on the Heathkit Amps E-Mail
> Reflector, and thought for those of you here not on that list might like
> some of the information I have done with this amp. Especially if you are
> in the need of a new High Voltage Transformer.
>
> OK Heres the whole story,
>
> After running RTTY at full output in a Contest it lasted 6 or 7 hours
> then the HV transformer blew.
> Yeah I know it says to run at like 50% But the tubes were remaining
> cool, so thought I was OK.
>
> This old amp needed new stuff anyway, so Installed the Harbach soft
> start and the RM-220 meter board.
>
> Re capped the HV filter stack with brand new caps and resistors.
> Found a kit someone was offering that the caps were the same diameter
> and voltage yet were more than double the amount of "C" cool.
>
> Then went shopping for a new HV transformer.
>
> Found one for $300,, and later someone provided a link about someone
> else but that was even HIGHER at like $400,
>
> Then I found this article from a Silent Key so cant ask any questions.
> But it peaked my interest.
> https://www.zl2al.com/1807/heathkit-sb-220-modifications/
>
> And I went with the giant Doughnut transformer.
>
> WOW! this thing is HEAVY!!! 20 pounds,  I was able to mount it slightly
> different from the article, just a hole  to lower it some. Here is what
> it looks like installed
>
> http://i65.tinypic.com/1553xom.jpg
>
> It is running up near the top 3400/3500 volts, and pretty solid, at
> 1500W+ OUT
> The voltage only drops at the most 100 volts!
>
> It is just at that high voltage, the idle current is at 210 or so is
> making the tubes glow even when it is not doing any transmitting. Almost
> 800 watts of happening but not doing anything useful.
>
> so need to drop it down some either via a harbach board modification,
> (adding more diodes to the string) or that active bias thing if I can
> find it again.
>
> The amp had some design of this like active bias thing where at zero
> watts drive when in transmit mode it drew no bias at all. but it had
> Bias when it was needed. I wish I could find that again. Man the AMP ran
> soooo cool with it.
>
> Oh yeah, that 20 pound doughnut only cost $100 bucks!
>
> OK now with Bia at a more reasonable level the tubes remain gray at
> idle, and only change color when hitting it hard with a long
> transmission running CW or RTTY.
>
> GREAT!!
>
> Yet still good gettering a long CQ at full output will get them to color
> up.
>
> I see how someone said it would need more drive with the lower idle
> (higher Bias) to get same out, and it does.
>
> And the HV drops a bit more now too, Before no drive vs full drive and
> it only dropped say 100 volts or so in the HV 3500/3400  down to 3400/3300.
>
> Now it's 3500/3400  and at full out it's 3200/3300
>
> Little confused on the efficiency tho.
>
> The watt meter was calibrated against a Bird, so I know it's good.
> Yet it says This girl is putting 1500 watts OUT dead carrier on 40 meters.
>
> 3300 volts at 600 ma = 1980 input,  1500 out = 76% efficiency, that
> seems a bit high doesn't it?
>
> 3400 X 600 =2040 input 1500 out =74% efficiency
>
> One thing that was weird, and I wonder if it has anything to do with the
> much higher voltage than stock.
>
> I wonder if I need to add some more "C" to the load cap?
>
> Reason is, I had the controls pre set where they used to be, within next
> to nothing,, I have vernier drives with readouts on the two caps. so
> it's almost like a no tune amp when band-changing I can dial it in so
> close.
>
> Anyway had it pre set, applied a little drive till I saw the plate amp
> meter rise a little,  then dipped the plate. and tried to peak the load
> while watching the watt meter.
>
> Max out was fully meshed.
>
> applied more drive, dipped plate, peaked load still maxed out fully meshed.
>
> More drive peaked plate for max out, tried load still fully meshed.
>
> finally when rig was almost at full drive did the load control have a
> peak on the output. and not want to be fully meshed.
>
> Right now the amp technically will not be in tune unless running near
> full output 1KW+ anything less and it wants more "C" on the load control.
> Will the higher voltage have caused this to happen?
>
> Thoughts?
>
>
> Otherwise the Doughnut transformer is working great! So far I would
> highly recommend it to anyone that wants to spend 1/4 or 1/3 rd the
> amount of a drop in replacement.
>
> And have higher voltage too he he he.
>
> Joe WB9SBD
>
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