SB-200 issue

[W0TNM]

I recently acquired a very nice SB-200, installed the three key Harbach mods, installed a glitch resistor and meter protection, and did some other work on it as well. The amplifier works beautifully and easily performs to spec -- with one exception: on the low end of 80 meters (up to 3.75 Mhz) I can not get much more than 300 watts out without exceeding 500 ma of plate current, and once the amplifier is keyed, the output (as measured by my power meter) and the grid current drop quite rapidly while the plate current and plate voltage remain constant. This is true whether using a low SWR antenna or a dummy load. I have never keyed it beyond two or three seconds in this condition, but it appears the power output would drop to near 0 watts if it was keyed longer. I also find that I must use maximum load capacitance (variable caps fully meshed) to achieve much output. The padder cap on the load capacitor checks out OK.

If anyone can give me some some suggestions about what I am dealing with here, I would appreciate it.

[W1QJ]

If the amp works well on other bands and makes enough power, then perhaps you have the wrong combination of tune and load settings. I know there is a setting you can find that will make power but not correct. Try giving the amp an initial 30 watts of drive with the load control at about 2, then tune for maximum output only with tune control, then tune the load for max output. Then add about 70w drive and again tune the load for max output and then tune. You just may have caught the wrong settings. I've seen this before with this and some other amps. it is unlikely that the amp performs well elsewhere and not there. Is the input swr low on that frequency?

[W0TNM]

Thanks for the reply.

I have tried tuning as you suggested starting with the load control from 2 all the way to 10. In all cases, to maximize output, the tune control needs be readjusted to 1. At that setting I can drive the output to 500 watts, but only if I drive the plate current over 500 ma, which I do not like to do, and the output drops fast after the amp is keyed. If I do not readjust the load control, I get about 280 watts output with 500 ma of plate current and the output does not drop after the amp is keyed. So, the bottom line is that I can not find another combination of load and tune settings that help the problem.

Now, I need to mention something else. I am now starting to see output dropoff at higher frequencies on the band as well. For example, at 3.87 Mhz I can easily load the amp to 500 watts output (50 watts drive, 420 ma plate current, input SWR 1 to 1, load setting 4, tune setting 2/3 of 80 M band marking), but am starting to see output dropoff after keying the amp, but at a slower rate than at lower frequencies. I still see no signs of power output dropoff on any of the higher bands.

If I were seeing power drop on the higher bands, I would suspect a bad tube or tubes, and that still might be the case, but it would seem strange to me to have the problem on the 80 M band only. I should point out that on all of the higher bands, I easily get full output with slightly lower plate current than I do on 80 M. Input SWR on all bands is very low. I should also point out that I have a damping resistor in the input to the cathodes which causes the drive requirement to be marginally higher than it otherwise would be.

[KM1H]

If the power doesnt drift down on the other bands Id suspect the 500pf fixed padder, they have a history of developing leakage especially the old square case style. They are also the cause behind most 80M loading problems. The input circuit could also cause power drift if a cap went bad or the coil core overheats.

A bit over 500ma Ip is no danger either, thats just to limit you to the old 1000W DC input power rule.

Carl

[KH2G]

Might try forgetting about all peaking and go for the dip and load tune technique then you don't have to worry about false power readings. Also keep that grid current down unless you want to be buying new tubes.
Dick

[W1QJ]

Tuning for maximum forward power is always better than anything else. The dip is not always the maximum output setting. it is usually close or right on, but not always. if you tune for maximum output it is always better since the more power delievered to the load the less the plate dissipation. PLATE DISSIPATION IS THE KEY ELEMENT THAT RUINS TUBES. Tubes do not get ruined because you might exceed 500ma on an SB-200. Depending on circumstances you could load to 650ma with grid current at a safe level with an output of 750 watts and have less plate dissipation than another circumstance at 500ma and 450w output. Plate dissipation is power input minus power output. It all has to do with plate efficiency which could change over the length of a given band.

[KH2G]

No argument but I like using the dip and load to get it to the top and then I might check the peak.
Regards
Dick

[KM1H]

Dip and load is for old dips still in the Class C world; on many linears the dip is too shallow to be useful.

The only true indicator is to peak the grid current in a GG triode amp. You will also likely find that the maximum grid current AND power out does not coincide with minimum plate current on 10-15M. The SB-200 is NOT completely neutralized.

Carl

[AG6K]

Dip and load is for old dips still in the Class C world; on many linears the dip is too shallow to be useful.

The only true indicator is to peak the grid current in a GG triode amp. You will also likely find that the maximum grid current AND power out does not coincide with minimum plate current on 10-15M. The SB-200 is NOT completely neutralized.

Carl

•• Indeed Carl, It is not possible to completely neutralize a grounded-grid amplifier because it is not self-neutralizing above the resonant frequency of the grid. Also, it is not possible to neutralize a grid-driven amplifier at other than the operating frequency.

Murphy was right: Things are usually more complicated than they first appear.