Durward Tucker Transmitter Historical Restoration Project

(1) The large variacs (main 1 and 2) on the front panel have 'soft' spots at about 70% of 120V. Why?

(1 Answer:) In the days when the transmitter was used, this would have been 70% of 110V, equating to 70% of 4800VAC on the secondaries of the transformers or about 3360VAC.
With the old filter and rectifier in place at this setting (3360-0-3360, full wave CT, dual section LC filter: 4H/2uF, 5H/2uF) and considering the combined nature of the PA and modulator supplies, the combined HV would have been 2960VDC with a 600mA load (carrier conditions, 1.1KW carrier) and 2600V with a 1 amp load under 100% sinewave modulation (equivalent PEP of 3510 watts (and virtual 878 watts carrier) as derived from regulation characteristics of the power supply including the variacs).
Ok, so certainly Colonel Tucker would have rightfully used this setting for the weekly radio nets in the Texas State Guard (no power limits).

(2) The high voltage can be raised in excess of 4000 volts with the variacs. Since the HV transformers are designed for 110V to 4800V at 1 amp, this is not surprising. Unfortunately this presents a safety hazard to the rectifiers, filter elements, and modulation transformer.

(3) For the purpose of the remaining notes, it is assumed that the line voltage is 120VAC as it is today, not 110VAC as it was in 1957.

(4) The HV transformers appear to be built with two separate secondary windings, each 2500VAC. These seem to be connected in series by strapping and one end is currently grounded. The other end at 5000VAC feeds the plates of the HV rectifiers. When the two 120V wall plugs are correctly inserted into a pair of outlets which are served by 240VAC, each entire transformer secondary becomes on half of the (virtual) secondary of a full-wave bridge rectifier at voltages from 0 to 5000-0-5000. This puts forth the requirement that both variacs output voltages be adjusted to exactly the same voltage in order to eliminate a 60Hz ripple caused by unequal voltages from one half of the secondary to the other. The operator must carefully adjust the variacs. This requirement for 'fiddling' may have led to the soft spots mentioned in note (1).

(5) It is our intent to retain the use of the variacs and to have the PA and modulator HV both separately adjustable to reasonable levels between 2000 and 3000 volts.

(6) If further investigation reveals that the transformers have split secondaries, and are indeed each independently configurable for 2500-0-2500V, then the power supplies may be rewired as full-wave center-tap supplies. There will be two choices for filtering. Capacitor Input is acceptable because of the large peak currents permitted by the 575A or 673 mercury vapor rectifiers. Choke Input is always a better choice for regulation. In notes 7 and 8, the scenarios assume 100% settings on the variacs and a 70% efficiency in the PA. Also considered is the additional DC curent load imposed on the power supply by the modulation. Only the PA power supply operation is considered. (The modulator power supply is for discussion another time and that discussion may revolve around regulation over the period of an audio cycle.)

(7) Choke input configuration, LC is 16H and 40uF. Tramsformer is 2500-0-2500VAC with an impedance of 163 ohms on each side of the CT. Duncan Amps' PSUD simulates the power supply to give these DC outputs, and the RF outputs and audio inputs are calculated on top of those.

(7a) B+: 2199VDC, Bleeder load: 145mA (318WDC)

(7b) B+: 2120VDC, Carrier load: 145mA+279mA=424mA (318WDC+591WDC=909WDC supply load and 414 watts or so carrier @70% PA efficiency)

(7c)Condition: B+ 2038VDC, 100% modulation.
Peak load at crest of modulation:
Peak Current: 145mA+558mA=703mA,
Peak Voltage= 2038VDC+2038Vpeak_audio=4074V at peak of modulating waveform
4074V*558mA=2274Wpeak PA input (@peak of audiowaveform where B+ E and I are doubled)
(318WDC+2274Wpeak=2592Wpeak supply load
output is 1607 watts or so PEP
-BUT- since the average additional current loading due to 100% modulation is significantly less than the peak value and the filter capacitor also acts as storage for power consumed during the positive half-cycle of the modulation waveform, the power supply average or RMS loading figures are closer to those shown in (7d).

(7d) Condition: B+ 2065VDC, 100% modulation.
Average load at crest of modulation:
Average Current: 145mA+471mA=626mA
Average Voltage: 2065VDC+1460VRMS=3525Vaverage (RMS of the positive B+ half-cycle at the 100% modulation condition)
3525V*471mA=1678Waverage input to PA at 100% modulation condition
1281VRMS*336mARMS=491WRMSaudio
2065V*(276mA+(276mA*.707))=
2065V*(276mA+195mA)=
2065V*471mA=983W from power supply to PA
983W+491Waudio=1474Waverage input to PA
1042 watts or so average RFout
1473 watts or so PEP RFout

(7e) Case is made for legal-limit operation with choke-input filter in (7c). Reservations are held towards operating the 4-1000A at such a low plate voltage as 2000 volts, as it may be inefficient.