Date: Thu, 31 Oct 2024 06:38:26 -0400
From: "Mike McCarthy, W1NR" <lists@w1nr.net>
To: amps@contesting.com
Subject: Re: [Amps] Two questions about tetrode amplifiers.
>
>> so I tried removing it from the socket and replacing
>> it with a 2500ohm non inductive resistor then I tried to probe the output
>> port. I think this didn't show me the correct values.
>
<Wrong method. You replace the tube with a mica capacitor simulating the
<output capacitance of the tube. Then you can adjust the tank tap
<positions using a dip meter.
<I think you really need to read up on power amplifier design. One of the
<best books is Radio Handbook by William Orr, W6SAI (SK). It is out of
<print but usually available on eB... I think the 23'rd edition was the
<last and latest.
<73 de Mike, W1NR
## You folks are making a mountain out of a molehill. No, you don't
remove the tube, you leave it in the socket. Temp wire a small resistor
(same as the calculated plate load Z, using the GM3SEK PI
spreadsheet)....and wire it between the anode and the chassis.
## The software will spit out the calculated C1 - L - C2 values....for a
given loaded Q.
## You map out the C1 tune cap ( position on the 0-100 skirt scale) vs
pf.......or skirt on the turns counter of the vac variable vs pf. Ditto
with the C2 load cap.
## You know how much uh is required for each band. Using a digital LCR
meter, each tap point is found and marked......and then the tap from that
point is run over to the bandswitch.
## If a roller is used, it too is mapped out for turns counter vs uh.
## nobody uses a grid dip meter. It's not 1962 anymore. A good lcr meter
will easily read down to just .1 pf...and also down to just .1 uh.
## I would take ORR's radio handbook with a grain of salt. I found 38 x
errors in his very last edition. It's overall a good primer though.
## The tube can't source anything, it can only sink current. Think SB-220
with it's 5 vdc zener bias. Tube conducts for 210 degs..and shut off for
the remaining 150 degs....in class AB2.
## On CW, drooping / sagging B+ is not too much of an issue. On SSB it is
an issue...since the load line is screwed up. Saving grace is.... on SSB,
average plate current is typ only 1/2 that of a cxr.....so average B+ sag
is not that much.
## Linear amplifiers are also known as variable efficiency amplifiers.
You only get max eff, when tuned for max PO. Tune the amp for 1.5 kw pep
out.... but then only drive it to 750 watts pep out......and the eff will
suffer. Linear AM is the worst. 375 watts CXR..and 1500 watts pep out.
The 375 watt cxr is only 25% eff..and anode diss is now a whopping 375 x 3
= 1125 watts.
## Tune up using reduced B+ will work, provided reduced plate current and
reduced drive is also done. That way the plate load Z remains the same.
Notice on all the old 2kw input / 1 kw input amps, the SSB voltage is
always 41% higher....and ditto with the plate current.
## You can do similar calcs for different voltage ratios. IE: normal max
B+....vs 1/2 B+. With 1/2 normal plate current, the plate load Z remains
the same...yet Power input + output is only 1/4...using the 1/2 voltage (
and 1/2 current).
## Many of the AM broadcast stations would use the same method to reduce
PO after dark. ( tube Pa's)..without changing tank values.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|