Checking the input tuning of a SB-200, or other amps as well, I guess.

[KC2ZPK]

Can I use an antenna anaylser, such as a MFJ-269, to check the input of a SB-200. Can I hook up the antenna port of the MFJ-269 to the RF in on the amp, and then key the amp up with a straight key, and use the controls on the amp to get the lowest SWR reading on the MJF-269?

[K1ZJH]

Can I use an antenna anaylser, such as a MFJ-269, to check the input of a SB-200. Can I hook up the antenna port of the MFJ-269 to the RF in on the amp, and then key the amp up with a straight key, and use the controls on the amp to get the lowest SWR reading on the MJF-269?

Remove ALL power from the amp.  Manually hold the antenna changeover relay in the TX position.
Add some low inductance carbon resistors from each of the tube cathode pins to ground--these
should be equal to the input impedance for the tubes. Check this site for a list of common
impedances:   http://home.earthlink.net/~wd7s/TU-6B.htm  A pair of 572B tubes will be close
to 110 ohms. So, a 220-ohm carbon resistor on each cathode to gnd will do it.

Now you can use the analyzer to set each input coil for lowest SWR. Leave the tubes in the
socket, since their internal capacitance will affect the tuning. Again, the amp must be OFF,
UNPLUGGED and the capacitors DISCHARGED! Once the coils are set, remove the resistors.

Pete

[KC2ZPK]

Thanks, I will try that.

[KC2ZPK]

Resistor wattage doesn't matter in this case, or does it?

[WB2WIK]

Resistor wattage doesn't matter in this case, or does it?

No, it wouldn't.

But it's a lot easier to just use a transmitter and SWR bridge on the input of the amp, with a dummy load on the output...takes 10 seconds per band to make the adjustments.

[W1QJ]

Although the analyzer method can get you close it will never be as good as adjusting them live as the amp is working.

[KC2ZPK]

Although the analyzer method can get you close it will never be as good as adjusting them live as the amp is working.

Would I tune for lowest SWR on the radio, or max power out on amp? They would be the same? Yes? I would figure if the SWR is lowest, we have the most getting to drive the amp.

[W1QJ]

Well yes, you would assume that minimum swr would be maximum output.  In theroy that is true but for some reason max power may indicate a small swr on the radio meter.  I am sure that either way would be fine but I would tend to tune it for maximum output. The 2 should match up but not always depending on the swr circuit in the radio.  I am sure it would be so close either way that the difference in the output of the amp won't make a bit of difference on the receiving end.  I guess it comes down to what makes you feel all tingly inside.  Does a flat swr on the radio meter fancy you or does the extra 5 watts on the Bird meter fancy you?  take your pick. 

[KI4WCQ]

I have tried both approaches to tune the 10m and 15m inputs on my SB 200 and neither will fix the high SWR to the rig.  Using an antenna tuner between the righ and the amp will lower it somewhat on the 15m input, but still won't permit me to get full  drive--still can only get about 200 watts out.  On 10m, even the antenna tuner won't lower it to  below 2.5:1, so 10m is basically unusable to me on the amp.  I think the max output I can get even with a tuner between the rig and amp is only 130 watts out, which is no  better than running barefoot.

I'd like to figure out the problem to be able to use 15m and 10m to get enough output to bother fooling with those bands on the amp.  The upside is that 20, 40, and 80m will put out about 600 watts.

[W1QJ]

Depending on who may have used that amp in the past they could have mistreated the amp with too much input and they may have heated up the caps which do the matching.  Since there are only 3 or 4 mica caps for 15 and 10 meters, it might be a good idea to invest about 2 bucks and change them out with new ones and see what happens.  The last SB-200 I worked on had the 80 meter mica cap go from 680pf to about 200pf.  Something drastically made the value change.  Hence the swr was off the wall.  IF THE CAP(S) ON THE INPUT COILS ARE OUT IN LEFT FIELD, ALL THE INDUCTOR ADJUSTMENT IN THE WORLD WON'T DO A DAMN BIT OF GOOD!!!

[KI4WCQ]

WQ1J:  Thanks for the answer.  I have already checked the values and they test almost exactly to the values they should be.  I was wondering if the problem might be the slugs have been overheated or for some reason just aren't doing what they are supposed to do.  Any adjustment of the slugs on 10 and 15m has virtually no effect as you mentioned.  Do you think that the problem could be the slugs?  On this particular SB200, both the 10 and 15m inputs are L networks, with a 68pf on the 10m coil and a 75pf on the 15m coil, so there would be only two caps to replace.  I can go ahead and put new ones in as I think I have those values here somewhere.