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Heathkit sb-200 refurbishment
- Thread starter dozerman
- Start date
W
Also worth noting is that amp was optimized for SSB usage at 1200 watts PEP INPUT...OK that in mind...At 50% efficiency that is 600 watts max PEP OUTPUT on SSB on the best bands normally 80/40m...20m and up that is reduced, with 10m normally producing the least output.
NOW if looking at the specs, CW(carrier) mode is 1000 watts INPUT at 50% duty cycle...So with this in mind on 10/11m AM service MAX drive should produce at best 400 watts PEP OUTPUT...If the tubes are CHERRY RED your over driving the amp.
If you want those tubes to last any length of time.
The Bias is fixed by the supply board...MAX Plate current with ZERO drive should read about 100 mA on the meter...if the idling current is correct.
When driven correctly GRID current should not reach the the top of the GRID current scale except on Highest PEP peaks...normal voice peaks should be about 75% of scale...So keep this in range and the amp will give good service and clean signal. No one will hear the difference from 350-400 watts PEP and the 500+ PEP you are showing presently.
The tuned slugs are the INPUT tuning (SWR between radio and amp) and if showing good there as stated, there is no reason to adjust.
Try this operation and see if tubes run cooler and return to normal color very quickly(slightly Pink at most) after you unkey.
All the Best
Gary
EXTRA info:
The original Heath PA bias circuitry was routed through the antenna relay and the key line would ground this line with a small value resistor. This applied a high negative voltage to the tubes in standby(-120v...I think) and hopefully gave about 90ma of operating current in transmit. It was not variable. It also applied a high negative voltage to the key jack which is not compatible with the solid-state keying in modern rigs. I used the original bias winding rectified and filtered with a resistive voltage dropping network to give about -58 volts of standby bias. In transmit the relay on the control board switches the negative regulator in parallel with the standby bias with a 47 ohm resistor to ground. This shunts the -58 supply and the operating bias becomes just the adjusted output of the negative regulator. The regulator circuit has a pair of diodes in series in the output. This raises the minimum adjustable level getting it down close to zero volts. It also limits any backward current flow from the standby bias supply into the regulator.
http://www.crompton.com/hamradio/heath/sb200/sb200.html
https://forums.qrz.com/index.php?threads/sb200-amplifier-neutralization.242729/
And Wavrider is Dead on, in regards to the Grid resistors changing value!
Raining in biblical proportions here in Tennessee, so I sat back down with the Heathkit. Not being familiar with these 572b tubes and how they should look at idle or transmit. When I turned it on, tubes lit up and the plates got "pink". I back the drive down to a 12 watt pep radio. plates got cherry within 10 seconds. I'm gonna send this amp off to someone that knows heathkits. don't wanna mess it or tubes up. Thanks fellas!
One thing I just noticed, but this heathkit's bandswitch only covers 80-15 meters and only had those corresponding tuning slugs in the back. All the ones I've looked up cover to 10 meters. Is this one just older than the others? It does have a couple of variable trimmer caps in the back tied to the pins that would've been 10 meters.
This would be equivalent to the SB-201, no 10m after xx date, which are usually later versions due to the CB craze.
BJ radionut
Supporting Member and 6m addict
Well this would explain a few things. The original SB201 does not have 10m enabled.
Some early versions did not even have the band switch click for 10m (another click past the 15m) And did not have the input tuning coil. These parts had to be ordered from Heathkit after proof of license. Some had these parts but no instructions in the manual on hook-up. Then an addendum was sent after proof of license as to final completion.
I am wondering if this does not have these components, how you got it to even tune-up?
This needs to be identified.
All the Best
Gary
Some early versions did not even have the band switch click for 10m (another click past the 15m) And did not have the input tuning coil. These parts had to be ordered from Heathkit after proof of license. Some had these parts but no instructions in the manual on hook-up. Then an addendum was sent after proof of license as to final completion.
I am wondering if this does not have these components, how you got it to even tune-up?
This needs to be identified.
All the Best
Gary
that would explain the tubes turning red or pink, off resonance.
Would be nice to have an amateur rig and dummy load to test the other bands.
It seems you have a 201.
Working on 11 meters?? not with out some modifications.
And the 572b tube is not the best tube for AM use, grids tend to get hot and tubes do not hold up so well to abuse.
Would be nice to have an amateur rig and dummy load to test the other bands.
It seems you have a 201.
Working on 11 meters?? not with out some modifications.
And the 572b tube is not the best tube for AM use, grids tend to get hot and tubes do not hold up so well to abuse.