Dear Geert,
The HF amplifier just finishing up here uses two 3CPX1500A7
triodes that consume 22 amperes at 5.00 volts on the paralleled
filaments.
As the filament design uses a fully floating supply with bifilar-wound
filament chokes bypassed on their cold sides, providing a D.C. filament
supply that also floated was possible with the outboard HV supply
removing some components from the RF head chassis.
Surplus Sales fixed me up with four fifty ampere, stud-mounted
rectifiers and individual heatsinks capable of radiating the 21 watts
dissipation. These were mounted on "Rhyolite," or 1/4-inch red board
with the heatsinks not touching anything else metallic and suspended by
threaded rods so they remained floating and in the air-flow.
Step-starting is crucial, as the tubes, when cold, will consume greater
amperage if not restricted. This was done with a DPDT relay and a
Crouzet timer on the 240 volt primary. By using 25 ohms at roughly
200 watts capacity for 5 seconds I see the tube filaments start out
at 3 volts and then climb to 5.
Reid Brandon at EIMAC supports the concept of floating filament supplies
and also supports DC filaments for amplifier design where all effort is
made
to reduce hum. However, as Mr. Brandon points out, filaments voltages
can safely be rectified without deleterious effect to the tubes if the
voltage
is under ten volts. Over ten volts and the filaments display an uneven
heating
pattern where one side of the heater wire is hotter than the other. Some
DC filament
users periodically reverse tube filament polarities to overcome this
effect when
operating over ten volts.
Sincerely,
Hal Mandel
W4HBM
> Reading through the various reference works, I see that most tubes
> are heated with AC. However just went through AG6K's docs on
> amplifiers as well as a recent update of a GS35 design from a French
> amateur, and I see DC is used because of ease of regulating
> (correct) voltage . I am building my very first amp (with a GI7B,
> will later build one with a GS 31), so can't seem to find in the
> literature any negatives on using DC iso AC except for the
> additional circuitry and am inclined to go for DC. Any comments ??
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