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Re: [Amps] SB-220 Re-Build

Subject: Re: [Amps] SB-220 Re-Build
From: Jim <>
Date: Mon, 13 May 2019 06:45:49 -0500
List-post: <>
What is a useful bias?

You have lowered the Q of the tank circuit by raising the plate voltage 500 volts which makes it more efficient, but lowers the harmonic suppression.

What power output level were you running in RTTY when you blew up the original transformer?

Jim W7RY

On 5/12/2019 4:54 PM, Joe 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.

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

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.


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?


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.


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