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Re: [TenTec] century 22 setup driver and pa

To: tentec@contesting.com
Subject: Re: [TenTec] century 22 setup driver and pa
From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
Reply-to: geraldj@weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Sun, 30 Jan 2011 23:44:53 -0600
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
Yup you are right, I didn't look at all the bias circuit connections.
So my technique would be: on receive, turn the driver and PA bias pots 
to zero. Turn down the RX audio (to minimize the supply current) and 
measure the total radio current. Turn up the driver bias for a 30 ma 
rise in total current. Go to transmit after turning the front panel 
drive control all the say down, by closing the key or the lock switch. 
Turn up the pa bias for 130 ma rise in total current. No wiring breaks 
required. These will be off a little because the bias current will rise 
and it comes off the +12 all the time, but good RF transistors have a 
current gain of at least 100, so that change in current won't be 
detectable on the half amp supply meter. Besides the bias to the driver 
is shunt regulated by the diode junction drop of Q4 (which I presume 
samples the driver heat sink temperature for temperature compensation of 
the bias) so that circuit is not going to change supply current itself. 
There will be more change at the PA bias circuit because of that 100 ohm 
resistor to ground. But not MUCH. .6 volts across 100 ohms would mean a 
current change outside the PA by 6 milliamps. More than the PA 
transistors would draw bias current with a beta of 100, like 1.3 ma for 
raising the collector current 130 ma.

Then I'd check the output keying envelope rise and fall times for key 
clicks with my handy scope and the drive turned up for normal operation. 
You might have someone fairly close by listen for key clicks but that 
can be subjective modified by a receiver with too fast attack audio 
derived AGC.

In a zero bias tube, like a 3-500Z, 572B, T40Z, or 811A, it does take 
negative bias to be in class C, zero bias gives class B, a bit of 
positive bias would give class A, with AB somewhere in between A and B.

A better definition of Class C is that the output terminal current 
(plate, drain, or collector) flows for less than half a cycle. A class C 
stage is most efficient if that output current can be made into a 
saturation current with a flat top for significantly less than half a 
cycle of drive. The 304TL was especially good at that, and some fixed 
frequency transmitters had distorted the drive to add some third 
harmonic to square up the drive with some third harmonic impedance in 
the output side to accept that harmonic current. That's called third 
harmonic peaking and can make a plain class C stage do 85% efficiency 
from DC to RF where without it the efficiency might be more like 70 to 
75% (except for the 304TL, a pulse tube that can do 85% without third 
harmonic peaking).

A better definition of Class B is that the output terminal current flows 
for one half cycle.

The best definition of Class A is that the output terminal current flows 
throughout the cycle. It changes but never cuts off.

In class AB that output terminal current flows for more than half a 
cycle but not the whole cycle. The device cuts off for part of the 
cycle. Sub groups of that for tubes only are AB1 and AB2. In AB1, the 
tube is never driven to grid current. Collins 32S transmitters control 
the ALC by detecting grid current. In AB2, some grid current is allowed 
and RF drive peaks. The 4CX1000A is a class AB1 tube, no grid current is 
allowed, its grid would overheat if there was any.

Its not such a good definition to define these classes on the input 
bias, because different devices act differently. Zero bias tubes usually 
take zero bias for class B and if run AB would need positive bias, but 
triodes that have that characteristic usually aren't run in AB modes. 
Some tubes, like 807 and 6146 need negative bias for all the classes, 
just they get more negative for the shorter conduction angles. And the 
drive power is greater driving the grid into conduction on the positive 
going peaks but that's not a requirement for class C, just typical of 
tubes in common use. Then bipolar transistors can run with zero bias for 
class C, but need a little forward bias for class B and more for AB and 
A. Some FETs need bias just like a tube, but some enhancement mode 
(think mostly power MOSFETs like the IRF510 that works at RF) needs a 
little positive bias for class C, more to reach the threshold of class 
B, and more yet for AB and for A, yet if it draws gate current (besides 
that for charging the internal capacitances which can be significant) 
its blown on that cycle because to much voltage has blown the gate 
insulation layer good for only 20 volts for a standard level FET and 10 
volts for a logic level FET that's turned full on at less than 5 volts 
on the gate. And to add to the confusion there are N type and P type 
FETs where the normal terminal voltages of the P type are inverted from 
the N type.

73, Jerry, K0CQ

On 1/30/2011 9:18 PM, Bwana Bob wrote:
> Yes, the note from G3VTT was typed on the inside front cover.
> In the Century 22, with key up, the finals do not receive bias voltage.
> The final bias is only applied when the "T" voltage is present, but the
> driver bias is applied all the time.  After I replaced the finals but
> before touching the bias adjustments, I measured the current into the
> driver/final board as 93 mA in receive and 159 mA in transmit with no
> drive. More investigation is needed.  Maybe somone on the list can make
> some measurements on his Century 22.
> Correct me if I'm wrong, but I now recall that class C uses negative
> bias; class B uses zero bias. Class A uses positive bias to put the
> device into the middle of its linear range. I'm not sure about class "AB".
>                   73,
>                   Bob WB2VUF
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