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[AMPS] About triodes, tetrodes and IMD

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Subject: [AMPS] About triodes, tetrodes and IMD
From: (2)
Date: Fri, 14 Sep 2001 09:14:14 -0700
>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 ?).
//  yes

>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
>on tetrodes!). 

//  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 
>... ... ...
cheers, Ilkka 

-  R. L. Measures, 805.386.3734, AG6K,  

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