tl922 tunning circuit padding capacitors.

[KF4TH]

Hello fellows ,can any one tell me where to find 10 to 500 pf axial lead 7.5 kva npo capacitors suitable for the tunning padding caps in a tl922 amplifier. I have searched the net with out any results. Kf4th Franklin

[AG6K]

Hello fellows ,can any one tell me where to find 10 to 500 pf axial lead 7.5 kva npo capacitors suitable for the tunning padding caps in a tl922 amplifier. I have searched the net with out any results. Kf4th Franklin

 NP0 7500v barium-titanate dielectric "doorknob" caps seem to be limited to c. 100pF per unit. In commercial amplifiers one sees parallel metal plates with as many NPO capacitors as are needed to achieve the target C. I use a 16-cap version of this arrangement in my 160m antenna tuner. N750 doorknob caps are also suitable for use in tuned circuits and they are available in a 200pF, 7500v rating from HEC.
For tuned-circuit padding applications where the frequency is below 5MHz and more than c. 200 pF is needed, mica dielectric caps are another way to go. Cornell-Dublier makes 2500VDC mica capacitors.

[KF4TH]

Thanks Rich for this information . I am looking to parrallel some caps with existing tunning caps on 160,80,and add acap on 40 mters.My load control cap on these bands is running out of capacitance on tunning (ccw to the stop and still not loaded properly)
Franklin Kf4th

[AG6K]

Thanks Rich for this information . I am looking to parrallel some caps with existing tunning caps on 160,80,and add acap on 40 mters.My load control cap on these bands is running out of capacitance on tunning (ccw to the stop and still not loaded properly)
Franklin Kf4th

 You are welcome Franklin.

When a 922 Tune-C is against the stop on the low bands it's usually a missing/vaporized padder-C contact on the bandswitch. When the Load-C is against the stop it usually high SWR.

Are you tuning the Tune and Load controls for max RF out with full drive - using the Tune-CW position on the 922?

[AG6K]

You can also try to find HV mica caps which work a little better than door knob caps. Both transmitting micas and ceramic door knob caps have current limits because of ESR. But mica caps have better characteristics than ceramic caps do. One characteristic that works against the use of caps for padding is that the ESR is highest at the lowest frequencies which is where you most likely need a padder. As frequency increases, the max current rating increases. ESR drops as frequency increases. Also, small value caps (50 or 100 pf) have less current carrying ability than higher value caps (500 - 1000pf).

... ... ....

 A 500pF ITT-Jennings doorknob capacitor is rated at 2.5A @10MHz.
A 100pF ITT-Jennings doorknob capacitor is rated at 7.3A @10MHz.

[AG6K]

And what are these caps rated at 2 Mhz?

 ITT-JENNINGS ALSO RATES THEM AT 1MHZ AND 30MHZ - BUT NOT AT 2MHZ.


the 1MHz ratings:
100pF: 3.4A
500pF: 1.1A

The mfg states that the 500pF and 1000pF units are not for use in tuned circuits - rather they are for bypassing and coupling applications only -- a fact which Dick E. seems to have missed. .

[KF4TH]

I have looked my band switch very carefully and it is in good shape ,no discholoration are no evedense of arching. I am tunning into a new oil filled ameritron dummy load on these tests and the high bands work fine(10,15,and 20) Yes I am tunning with max out put from a ts440 exciter after starting with about 30 watts in and tunning then increasing drive till I reach 650 ma on the plate ip meter with the amp in the ssb mode. I am using a mfj tone generator for my input with the ts440 set for cw. and at max drive to get 650 ma on the ip meter I have to turn the load control to the ccw stop,if i turn it cclock wise from stop it decreases the output as i rotate it that way,In other words i cannot peak the out put with the load control.
I ordered 2 100pf 7.5 kva npo door knob caps from henry radio and changed out c43 padding cap on the 80 meter band and it did no good so i put in both 100pf caps and still no help. This amp from new in 1989 never did tune 80 meters very well,Is it possible the coil tap is wrong?
I am going to make some continuity check tomorrow,It acks like that the c43 cap is not in the circuit at all. after i put the two 100pf caps in I had a strange thing going on the ip and ig current were ig 120 and ip was 600ma but the watt meter was reading only 350 watts into the dummy load when it should have been a lot higher? Thanks for your help I was out of town or would have replied earlier. Franklin KF4TH

[AG6K]

 A quick way to confirm that nothing is wrong with an output matching Pi-network - a.k.a. "tank circuit" is to pull the mains power plug and couple a GDO/dipmeter to the tank coil. If all is well the dip should be about 10% lower in frequency than the band it is set for.
- note - Pi-networks are unlike a parallel-resonant L/C circuit in that they are a Z transformation device -- so they could rightfully be called a semi-resonant circuit.

[NN7B]

Any new news yet? I have the same problem on 75/80 M with my TL-922A. I purchased a 2kW tuner to dial in my SWR from 1.6:1 (gap challenger antenna) to 1:1, hoping that it would help, and I still hit the stop CCW on the load control before peak RF out was reached.

[NN7B]

Are you using a separate calibrated watt meter? I'm assuming your not using the RF meter on the amp, unless you've calibrated it against another meter with a known value. Doesn't seem to add up.
600mA*3100 V =1860W input*60% efficiency=1116W out?