[AMRadio] Re: Triode connecting 803's for a Modulator

Mike Dorworth, K4XM k4xm at arczip.com
Mon Apr 10 19:13:55 EDT 2006

The question was: how to do it with 803's The source listed is the Bill Orr
Handbook. He calls all of the circuits Zero Bias Tetrode Modulators. Even
the 807 and 813 in TRIODE connection is shown with the beam forming plates
grounded. The circuit is about zero bias, and no screen supply. The 803's
are shown since they were one very plentiful and cheap. My understanding of
the suppressor bias is that it allows the plate voltage to swing down to it
and increases both headroom ( work space) and increases efficiency. 510
watts output with clean audio is not bad for zero bias and no screen supply,
The suppressor voltage is actually the voltage used on the speech amp plates
which is says can be 6B4's or most any 8 watt speech amp. He does say these
tube require more drive than 813's but since the then price for a new one
was four dollars a tad more drive make little difference. In W6SAI's 14th
edition Radio Handbook page 595, chapter 27-3. Seybold's patent no.
2,494,3176 explains the use of  beam tetrodes in TRIODE connection to use no
bias and no screen supply. The drive is applied to the screens and a
resistor to the control grid. The 15th Edition  says the grid resistors are
not needed for the 803 or 813 tubes.. Hope this helps..


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