The AM Forum

Had a question posed to me.

Let's say you have a freakishly large amplifier.  Capable of a LOT of headroom.

And lets say you have two seperate stations.  Two antennas, feedline and exciters.  You have bpf on each radio to protect it from the other upon TX, etc.  Separate  input circuits.  The tube is the common element.

Would it be possible to amplify say a 15 meter and 10 meter signal, using separate pi networks on the output for each?

I've simulcast a second am signal before by amplitude modulating a 60 kHz tone through an analog series modulator, so I know two close in signals can't be generated and amplified.

After seeing the 3cx3000A7 amp post, I'm sure you can deduce where the question is coming from. 

On the top, it seems doable.  It would require a large cabinet for two pi networks, but any ideas?

--Shane
KD6VXI

I don't know why you say "two close in signals can't be generated and amplified"?
Not sure what you're referring to.

But there would be significant issues with IMD to contend with.

Any time you put two signals into an amplifier there are mixing products produced.

Why would you want to transmit on two different bands at the same time?

            -confused

Let's take an extreme example for the sake of discussion:

If we had an 80M and 20M pi-network in parallel, wouldn't the C1 80M input capacitor shunt the 20M signal to ground, like a low impedance direct short?   I would think that the lower frequency network would dominate, effectively making the interaction intolerable.


T

I think what Shane has in mind is an amplifier that could be used by two contest operators simultaneously on two different amateur bands.  Seems the goal might be sharing the expensive investment of a QRO power supply and large dissipation tube, and associated circuitry.

I think it might be possible, but likely not practical.  

First, the issue of mixing products between the two signals....   Mixing occurs in a non-linear device, where sum and difference frequencies are generated.  If the amplifier is operated in a theoretical 100 percent linear fashion, then mixing should not occur.  It appears this would require the amplifier to be run in class A, and the signal must be kept within the straight portion of the characteristic curve, or linearity would suffer, and mixing would occur, generating undesirable spurious outputs.  This would preclude running SSB or CW.  AM or FM should be possible, but the envelope MUST be kept within the amplitude range of purely linear operation.  Possible, but not practical.

Recall the early AM/FM receivers that shared intermediate frequency (IF) amplifiers at 455 kcps and 10.7 mcps  (KHz and MHz).  They had no switching mechanism; rather, they had IF transformers for both ranges, connected in series on the input and output of each transformer.  The result is a high impedance to both IF frequencies simultaneously.  This worked well for both AM and FM, and with AGC, the tubes were kept in their linear operation range.  Necessary for AM, but not critical for FM.

The problem of connecting input and output signals for two bands on a QRO amplifier could conceivably be accomplished in a similar, but slightly modified manner.

Bias and plate voltage could be handled in the traditional manner with shunt feed RF chokes and bypass capacitors.  It would not matter whether the tube is operated in grounded grid or common cathode mode, and it also would not matter whether a triode or tetrode is used.  

Consider an input circuit using link coupling to a separate parallel tuned circuit for each band.  Then couple each of these parallel tuned circuits to the input of the amplifier stage using a series tuned circuit, tuned carefully to the desired band.  The parallel tuned circuit would provide a low impedance to the desired frequency, coupling the input signal of the desired band to the tube input.  It would provide a high impedance path to all other frequencies, effectively isolating the other band from the input parallel tuned circuit.

The same approach could be applied to the output of the amplifier;  two separate series tuned circuits coupling the two desired band frequencies to the input of two separate parallel tuned circuits, each link coupled to the desired load.

This is somewhat different from the IF amplifer described above, where the two frequencies appear in series across each individual IF transformer.  I am not certain what would occur at the mixing point of the two separate series tuned circuits, it is just an idea I threw up for discussion.  Perhaps Stu might care to shed some light on this, whether it could possibly function as described.

Again, I think it might be possible, but it would be highly impractical.  Perhaps a better choice for a remotely-operated QRO contest station might be a shared power supply using separate tubes for each band.  The power supply at hand could easily handle the load of several legal-limit amplifier tubes, and the complexity would be significantly reduced.  A common cooling system could service several amplifier tubes.  The idea of using oversized tubes, such as 3CX3000As, would allow linear operation in a very conservative fashion, and would not be susceptible to failure during remote tune-up operations.

Just my two cents, for sake of discussion.  I am interested in hearing what other people think, and other options to Shane's query.

Rick, you hit the usage nail on the head, and Tom killed the idea with the problem of one plate circuit shutting the other.

I believe a couple to a few amps off a brutal power supply would be best.

Interesting idea, though.

--Shane
KD6VXI

There is a good book showing an 1KW amplifier that can simultaneously amplify many signals over its wide band width. This book is hard to find, probably still copyrighted, not online yet. The chapter 10 covers design with an example. $180 at amazon.

Radio Transmitters by Victor Owen Stokes
The Marconi Company Limited
Van Nostrand Reinhold Company, London
copyright V.O. Stokes 1970
Library of Congress Catalog Card Nr. 75-109568
dewey 621.38413   S874r

If anyone read this far, push pull is not explicitly stated but it supposedly eliminates half the harmonics.

The example amplifier uses eighteen 4CX250B tubes in a push pull distributed amplifier covering 2-30MHz and having tapered lines. HV is 1100V to allow operation into 2:1 SWR. Plate iron is 4KVA. Class A operation is not stated but may be implied because it is stated that AB or B reduces the gain bandwidth product. IMD is lower than 40dB down from 1KW PEP level.

18*250 = 4500W of anode..

I have that entire book, I'll have to check that chapter out.

--Shane
KD6VXI