Heathkit SB-200

This is a set of notes on the Heathkit SB-200 amplifier. It’s mostly for me to make sure I don’t operate the thing incorrectly.

General notes

It’s rated at 600W CW and 1000W PEP SSB. Anything between 500-600W is good.
Don’t overdo it! It’ll not be acting linearly.
A well maintained rig will output 700-800W on the lower bands but the grid current is pretty high. Don’t do it - limit drive output to whatever you need to be able to peak/load at 600W output.
The HV should be around 2300V - if it’s not then (after carefully discharging everything) check all of the meter resistors for correct values and replace accordingly

Note the ALC voltage is .. very negative, and it could damage your radios. Kenwoods for example don’t need more than around -9v for ALC to shut down transmit.
So either build a circuit to clamp the ALC at -9v (and realise it could swing quite a bit more, so be careful on your choice of zener) or don’t hook up the ALC line and be double careful.
The relay circuit is also -110V DC; that’s fine for a TS-440S but other radios using transistors will be unhappy. Make sure you install the soft-key module.
Check the service bulletin section

This includes a note about ALC and using an 8v zener.

From the manual:

Set load at minimum (left), set tune at the centre of the band
Set low output from exciter, look for around 200mA plate current on the meter
Adjust Tune to maximise output power - this should be dipping the plate
Adjust power up a little
Re-adjust Tune to maximise output power - should dip the plate again
And start adjusting Load to peak power; should be dipping plate each time
Repeat until you hit 600W output power
If grid current peaks above the white section (ie, 100mA) - increase load to back it off
If plate current peaks above 500mA then decrease exciter power

Limits:

Aging tubes can manifest as not being able to load down on 80 and 40m input - the load cap is fully counterclockwise for max power and can’t dip.
BUT - the amp also doesn’t present a 50+j0 impedance on all bands. So although tube transmitters will just output full power, solid state transcievers will begin reducing power above 1.5:1 and thus you may not be outputting 100w.
I’ve had to use my tuner on the TS-440S for 15 and 10m to stop the SWR to be terrible, but it’s touchy - the SWR at the exciter changes as tune/load happens so the autotuner needs to be kicked in and re-run a few times during tuning.

Tune the amplifier up at 100W CW, like any other mode
Then reduce AM carrier power down to the SB-200 outputs 100W AM carrier
Use this; don’t key down for more than a couple minutes at a time and all should be OK.

Look at what it’ll take to make a proper ALC reducing circuit - see if ALC can be asserted above 600W to keep the amplifier from being overdriven.
Add HV fuse, glitch resistor.
Maybe add a series input resistor to limit the input power - ie, 100W exciter but only 50-60W into the amplifier input tank.
Write up something on checking the metering resistosr to ensure that they’re still within spec.