>Some people are very persistently claiming tetrodes being
>superior to triodes in IMD sense. Some tube data sheets
>do support this claim. I do no think that this is
>always true, however. For example 3CX800A7 and
>4CX250B are an example of one triode being
>much more linear than one tetrode.
// Agreed, however, my 2, 4CX250 2m-SSB amplifier turned out be as clean
as the Kenwood TS-700. 10w transceiver that drove it.
>As W8JI pointed out some time ago some amateur HF
>amplifiers using (Russian) tetrodes seem to have worse
>IMD than other amplifiers that are using
>3CX800A7 or 3CX1500A7 triodes.
// Svetlana gives a clue on how to improve IMD with RF negative-feedback
resistance. A diagram of this circuit is at Figure 10 on my Web site.
>My interest on the subject arises from the facts that I
>own Acom 2000A (2 x GU74b / 4CX800) and
>that I have started a 144 MHz amplifier
>project with GU84b (close to 4CX1600 ?).
>To the utmost disgust of some QRP-fanatic
// hi hi A laugh a day helps to keep the doctor away.
> PSK-31 operators I have run 1 kW (Finnish legal limit) when
>comparing MFSK-16 and PSK-31 on OH-W paths during
>slight auroras. (MFSK is superior to PSK, BTW).
>Before committing that "crime" I did make sure
>that my signal is clean. Acom 2000A clearly is
>much cleaner than my IC-746. With a professional
>spectrum analyzer I could not see any difference
>on the relative IMD levels of barefoot 30 or 40 W and
>Acom driven with 30 or 40 W.
>According to the QST May 2000 test of Acom the
>3rd and 5ft order IMD products are -37 dB and
>-60 dB from PEP at 1.5 kW output, respectively.
>Some modern 2 x 3CX800A7 HF amplifiers seem
>to have slightly lower 3rd order but higher 5th order
>IM product levels.
>According to QST September 2001 test of Ten-Tec
>Titan II (4CX1600) worst case 3rd and 5th order
>IMD levels are -29 dB and -31 dB from PEP at 1.5 kW.
>Even the 11th order IMD was only about -50 dB rel PEP!
>According to articles on
>at least 4CX250B amplifiers must have good screen regulation
>in order to achieve even acceptable IMD performance
// True. However it is my opinion that this is "hyped" by some of those
who sell such regulators.
>On the FCC database you can find quite a lot of material
>about the amplifiers that are type-accepted in USA.
>Both Acom 2000A and Ten-Tec Titan II schematic
>diagrams can be found there.
>It seems to me that Acom has a well-regulated screen
>supply that has a low output (DC) impedance for both
>normal and reverse screen currents (not a rare phenomenon
// Amen, Illka. Even my tetrode with handles amplifier, "The Plywood
Box", exhibited reverse screen-current from the 8170/8171 on occasion.
25mA of bleeder current took care of the problem. Without the bleeder,
it would quite likely have self-destructed.
> As far as I understood the operation
>of the Titan II screen supply it has a pretty high
>output impedance that is even different for normal and
>reverse screen currents.
// On smallish tetrodes, a zener-string of 10v - 20v, 5w zeners
provides: V adjustment, screen overcurrent protection, loss of anode
supply protection, and a drain for reverse screen-current - all with
maximum simplicity.. [note: Eimac first proposed this brilliant scheem
in 1956 using shunt gas V-regulator tubes.] However, for tetrodes that
need more than 700 or so screen volts, the series-pass regulator and
bleeder is clearly the best choice. - see Figures 5 and 8 on my Web
>... ... ...
- R. L. Measures, 805.386.3734, AG6K, www.vcnet.com/measures.
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