Gerald,
I pulled out my copy of the RCA tube manual and read the section on AB1
amplifiers. I didn?t find anything that explicitly referred to a K factor
for AB1, but then maybe my edition is an older one that didn?t have that
information.
I redid my calculations using the Eimac tube performance computer and did
find a previous error of mine in reading the chart. The new values are more
in line with what I had expected. At 3000 V on the anode, 500 V on the
screen, and 750 mA anode current, I get about 1250 watts output. Idle plate
current is 250 mA and the grid voltage swings from ?90 V at idle to ?14 V at
full input. Peak RF current is 1.06 A.. This calculates to a load
impedance of 2200 ohms.
73
David
-----Original Message-----
From: TexasRF@aol.com [mailto:TexasRF@aol.com]
Sent: Tuesday, September 07, 2004 10:29 AM
To: dhallam@rapidsys.com; amps@contesting.com
Subject: Re: [Amps] Plate Load Impedance
The RCA Power Tube book shows plate load impedance= (plt voltage/plt
current)/K factor. Note that plate voltage is the d.c. value from the power
supply and plate current is as measured by the d.c. plate current meter; not
the peak values seen on a graph or load line.
K factor is shown as 1.8 for class AB. K factor is determined by the
conduction angle of the plate current. A K factor of 1.8 implies an angle of
(1/K) X 360 or 200 degrees. If the class of operation were A then the
conduction angle is 360 degrees and K=1. If the class is C then the
conduction angle is less than 180 degrees and K will be greater than 2 and
possibly as much as 3.
K factor is altered by the value of zero drive plate current. As the zero
drive current is made lower by additional grid bias, the conduction angle is
made less, which raises the K factor. Conversely, raising the zero drive
plate current will lower the K factor. This is intuitive since we would be
moving toward class A operation in this case.
The RCA book discusses the proper level of zero bias plate current for class
AB as being related to 1/3 the tube plate dissipation rating. For an 8877
running at 4kv this comes out as .125 amps which is pretty close to what we
typically use.
73 de Gerald/K5GW
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