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Re: [Amps] SB-220 Output Network

Subject: Re: [Amps] SB-220 Output Network
From: Jim <>
Date: Fri, 13 Aug 2021 19:20:29 -0500
List-post: <>
Do the math Joe....

Watts      dBm
500 =      57 dBm
600  =     57.8 dBm
700 =     58.5 dBm
1000 =   60 dBm
1500 =   61.8 dBm

I run 1500 watts on RTTY because I can. BUT, now that I'm moving to a city lot from 13 acres, I'll concentrate on running 100 watts. Perhaps as much as 400-600 watts with an SB-220.

A pair of 3-500s is fairly marginal at 1500 watts output. (That is only 1000 watts of plate dissipation). Thats why I build my own amplifiers with an 8877 and 2ea 3CX800As. They both do an easy 1500 watts on RTTY with 25 to 40 watts drive, depending on which band.

The 8877 is one of the best and easiest tubes to build a 1500 watt amateur linear amplifier ever made! And, their not that expensive. Although, the prices are going up... (because of the stupid Democrats)... :(     (Have you checked the inflation rate lately)?

Good luck in your SB-220 endeavor.

Jim W7RY

On 8/12/2021 11:57 AM, Joe wrote:
Hopefully some of you here may remember my adventures with this SB-220.

I'm the one that ran RTTY at full power in a RTTY Contest and literally melted down the HV transformer.
And in the process toasted a LOT of other stuff with it.

So a complete re-build and change of the HV section was needed.

So after looking at what a Pair of 3-500Z's can actually do, I decided to try to get all I can from them.
Think like a Big Henry Amp.

I looked at the Eimac data sheets. And found a suitable HV transformer.

This amp now has 3800 volts at rest on the plates and at full load 3600 to 3700 volts.

Problem is, running this much higher voltage has changed the values of the output network components needed to be to obtain a match now.

It was discovered that on EVERY Band, until the amp is making 1400 watts or MORE the load control wants to be fully meshed, max amount of "C" available.

Once it gets above 1400 watts, then you can get a peak on output power when adjusting it.
But it is still a very small less "C" like 5% less than maxed out.

So a simple thought was add more "C" so added a nice variable in parallel with the load cap. This cap was about 1/2 the value of the main load cap, so thought this should be plenty.

I was amazed to see that if it made any change at all, it was very minimal.

So with discussions with many on this group and elsewhere, it was decided that because of the higher voltage, this amp needs more "L".

That is a major re-do for sure so I set that on the side. So continued to use the amp at 1400 watts or more.

Thats been geez 2 years now? The AMP is still running great! But often I did not need or want to run 1400+ watts, So I bit the bullet and decided to try to add the More "L"

Now I "CAN" just do a poke and not do anything Scientifically.

Like on the 10/15 coil, just move the 10 meter tap 1 turn longer.
looking at the 80/40/20, coil, It actually looks like I can add in the coil form up to 7 or 8 more turns! The grooves in the form are there.

Give like 15 meters one more turn,

20 meters 1 turn
40 2 turns
80 3 turns

And just see what happens?

Or is anyone here great with all that math stuff that I probably learned when I took my extra exam in 1980 or so, but have long forgot how to do this kind of work to calculate the needed components.

But thinking Ok here is a stock SB-220  a pair of 3-500Z's
Stock voltage is what? 2500 & 3000 volts

With stock caps and the pair of 3-500Z's How much "L" is needed on each band.
I would assume that is how much "L" was designed in the Kit.

Now raise the voltage to 3600 to 3800 volts.

Now how much "L" is needed with the rest still stock?
for each band?

and using the stock 1.75" coil form, how many turns should be the best chance of working for each band segment?

Anyone willing to try a gander at this?


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