[Hallicrafters] Hallicrafters HT-33A

[Glen Zook]

I joined the amps group about an hour ago and have already sent the person advertising the 8295A an E-Mail.  Am waiting for his reply.  If I can obtain the 8295A then that will make things MUCH easier!

One thing that I have been thinking about is the actual power that the HT-33A is capable of without any problems.  Of course the unit was manufactured when the maximum INPUT was 1000 watts and the tuning instructions in the manual are for that value on CW.  The instructions are to use the amplifier with a cathode current of 390 mA which supposedly equates to 370 mA of plate current.  At 2700 volts this makes a plate input of right at 1 kW.  They also say for SSB operation to tune up to 520 mA of cathode current which equals 500 mA of plate current.  That equals 1350 watts input.

Now "typical operation" of the PL-172 is shown at plate voltages of 2000, 2500, and 3000.  At a plate voltage of 2500 volts and a plate current of 960 mA this shows the PL-172 giving 1870 watts output.  This is for CW and represents an efficiency of almost 78%.  In Class AB-1 (which the HT-33A is capable of running) at 2500 volts at an average input current of 400 mA (but a "peak" input current of 1.6 amps) the "typical operation" of the PL-172 shows to be 2310 watts output.

In the "real world", with the circuit losses in the final tank, etc., the efficiency is probably going to be more in the 65% range.  Maybe a little better, maybe a little less.  But, assuming that 65% is the efficiency then to get 1500 watts output would require right at 2300 watts input.  At 2700 volts this would require a plate current of pretty close to 850 mA and a cathode current of 870 mA.  At 2300 watts input and 1500 watts output the plate dissipation is going to be 800 watts which is well within the ratings of the tube.

Another thing to take into consideration is the higher line voltage that is common today.  My line voltage generally runs pretty close to 127 volts at my breaker box and the breaker box is located about 3 feet from my main operating console.  Therefore, for all practical purposes, the voltage drop due to the conductors can basically be ignored.  Assuming that 2700 volts is the output from the 3B28 rectifiers then the additional input voltage of 127 volts (which is a little over 8.5%) should produce a voltage of around 2930 volts.  Then, I am planning on replacing the gas rectifier tubes with solid-state devices.  Now the voltage drop across the 3B28 tubes when compared to solid-state should result in an increase of maybe 25 volts to 50 volts, probably closer to 25 volts than 50 volts.  The main reason for replacing the tube rectifiers is to get the B+ off of the filament winding on the power transformer with a slight increase in voltage being a potential
 "bonus".  This should result in a plate voltage of around 2960 volts which is within the ratings of the tube.

The maximum scale of the meter in the cathode current position is 800 mA.

Assuming a plate voltage of 2950 volts and a plate current of 750 mA this would result in an input power of a little over 2200 watts.  Then assuming an efficiency of 65% this means an output power of just under 1450 watts.  At this point the plate dissipation of the PL-172 would be right at 775 watts.

After all this reasoning actually comes my question.  How "hefty" is the power supply in the HT-33A?  The physical size of the high voltage transformer is considerably larger than in my Tempo 2001 amplifier that can put out 1400 watts.  In fact, there is only a single transformer in the Tempo linear which provides heater and control voltages as well as the high voltage.  Now the Tempo has solid-state rectifiers from the "get go".  The high voltage transformer definitely weighs considerably more than the entire Tempo amplifier including cabinet.

My concern is not with the capabilities of the r.f. portion of the amplifier but in what is a safe level to run the power supply.  Since the instruction manual states that 520 mA of cathode should be run to comply with the old 1000 watt "average" input regulations of the past and that is a very conservative rating.  Therefore, I am looking for ideas from those who have run the HT-33A or HT-33B as to what to expect.

Glen, K9STH

Website:  http://k9sth.com