Note
The HP-20 replaced the UT-1 and was nearly identical to the UT-1. Indeed, the two units are virtually indistinguishable in outward appearance. The only real difference between the two is the addition of a –130 volt 30 mA bias supply on the HP-20. This gave the unit a lot more versatility for use with newer Heath rigs as well as non-Heath rigs.
The basic specs are the same, although the HV current ratings have been sneaked down a little bit. The unit also added a line filter to aid in minimizing TVI.
The filament supply can be wired to provide 6.3 volts at 8 amps, or 12.6 volts at 4 amps.
Note: Heath stated that the loads on the 600 volt and 300 volt terminals should be adjusted so that the total load does not exceed 120 watts. For example, a typical load might be 600 volts at 150 mA (90 watts), and 300 volts at 100 mA (30 watts).
There is conflicting information for values stated in the specifications and those given in the text of the assembly manual. The company said that If the 300 volt supply is not used, then the load on the 600 volt supply could be as high as 200 mA, and that If both the 600 and 300 volts supplies are used the maximum rating are 150 and 100 mA respectively. This would be a total of 250 mA, which conflicts with the rating of 120 total watts. Better to error on the side of caution.
There is also conflicting information about the filament current. The specifications page and the step-by-step instructions state 6.3 volts at 11 amps and 12.6 volts at 5.5 amps. However, the schematic states 8 and 4 amps respectively. The power transformer part number (54-109) is listed as having two 6.3 volt 4 amp secondary windings.
Examples of the HP-20 have been found with the 3D Heath script logo and the Heath/Daystrom screened logo. Units with the screened logo would be from later production runs. The front panel has an on/off switch and a pilot light. The rear panel has an octal socket for power out and the AC power cord. Housed in a louvered steel cabinet in two-tone Heath green.
References:
Improvements. 73 Amateur Radio. Mar 1966, p. 74.
Filament voltages: 12.6 volts at 8 amps OR 6.3 volts at 4 amps
High voltage:
600 volts at 200 mA
OR
600 volts @ 150 mA AND 300 volts @ 100 mA
Regulation (600 volts, from 50% to 100% load): 10%
Bias: –130 volts @ 30 mA
Photos, general information and specifications from "Heathkit: A Guide to the Amateur Radio Products," by Chuck Penson, WA7ZZE. Used with permission.
