Well I have about finished up the KW rig. Seeing the HV meter rise to 3700V and having nothing explode was very rewarding. The HV is adjustable by two variacs and I will run much less, probably 2500V.
(http://208.190.133.201/tuckerkw/tucker_transmitter.html) I have a strange issue however. I hope the wisdom of the group will help me here. 1. I note that the modulator plate meter is in series with the B+ lead to the mod xfmr, and has the high voltage wiring to prove it. I can crank the HV up to 3000V and play with the modulator bias and see the meter work. This point is made to contrast with the next point. 2. The final current plate meter is more mysterious in its operation. It has only the regular 600V wire, and the leads disappear into a part of the wiring harness I have not investigated as of yet. The meter does not move, and to be honest I have not yet a safe way to observe the plate current of the final. 3. I did reduce the bias enough to see some screen current, (bias=100V G2=500V Ig2=50mA Ep=2500V), in an attempt to see if I could get a bit of color on the plate by burning up maybe 300W or so there. No such luck with the color, I do not dare to fiddle too much with this without a working plate current meter. I know that I have HV on the plate because I have used the final to slightly load the B+ and demonstrate that HV current is being drawn. The final's filament CT is grounded at all times. the unit is keyed by the final screen and the HV. Setting the screen volts and bias to zero results in a faster bleed-down of the HV when unkeying the rig than when bias is kept on the final, so I know it is drawing some current. (HV filter=40uF) 4. Ok, so now we would expect that the plate meter must be wired into the negative side of the B+ supply, and this would account for the low voltage wiring etc. Unfortunately this would not be the case, as the final's filament center tap is grounded, as is the B+ supply's negative side. The design does not seem to follow any of the usual schemes presented in various volumes of the ARRL handbook or the Radio Handbook. 5. Both sides of the plate current meter have a very low resistance to ground (about 1 ohm, not too easy to measure). 6. so.. any ideas how this might be wired up? It's very strange and does not behave as expected. My only other avenue is to further investigate (un-lace) the wiring harness and see where the leads go. 7. Another topic: Do mercury vapor rectifiers require a certain minimum current to remain in the conducting state? I only see them glow during the power surge when I key the unit and the filters charge. During the test described in "point 3" above, the screen current would suddenly jump from 0 to 40-50mA accompanied by a drop in high voltage from 3000 to 2500, and I adjusted the final bias. Adjusting it back to reduce the indication of current resulted in it sudenly shutting off to 0mA. Thus is strange behavior. It is doubtful that it is self oscillation as the plate and grid circuits are not tuned alike, and also i moved the frequencies well away from each other and repeat this reliably. Any comments and wild speculations will be welcomed! Anyway I got tired and quit for the night. I must remain alert when working on this dangerous equipment. Patrick
