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Single 811A AM Rig




 
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Author Topic: Single 811A AM Rig  (Read 1986 times)
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w8khk
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« Reply #25 on: March 06, 2019, 07:58:13 PM »

Tom, thank you so much for your very kind words.  I know that would make my dad very happy too!

I don't want to hi-jack Michael and Don's 811A thread, but perhaps some of this might be of interest to them as well.

Regarding your question about using the GFZ solid-state driver:  I plan to reserve that for modulating the push-pull pair of 304TLs that my dad built in 1949, when I was only 2.  I was thinking about using another pair of 304-TLs for the modulator, but Frank says there might not be enough grid swing, so maybe a pair of 4-250 or 4-400, or even a pair of venerable 813s?   It had a 300 watt UTC mod transformer sporting four Taylor TZ-40s in push-pull parallel;  we still have the tubes, but the modulator deck did not make the trip from NJ to FL when dad retired.

I have been thinking about building a series-modulated AM rig for some time, but the dissipation is the killer.  I am not as concerned about the heat and inefficiency as the ability to find tubes that can take the abuse.  I got lucky and came into a few 3CX3000F7 tubes, and the light went on and said use one for the series modulator, and the other for the Class-C final.  Thinking they will be used initially for 75 and 40, the layout should not be too difficult.  The greatest challenge will be designing the air flow plenum with enough insulation such that it will be quiet in proximity of the operating position.  It will NOT be in a remote location.

I am awaiting delivery of some 4D32 and 810 tubes, to prototype the RF final and series modulator, respectively.  If I place the modulator between the positive supply and the RF amplifier, it could run as a cathode follower, and would need a large grid swing to fully modulate.  It would never run any grid current.  I am looking at using a 4-250 or 4-400 with a resistive plate load to drive the 3CX3000 grid, not for the dissipation rating, but simply to handle the extremely large plate voltage swing.  However, with the modulator elevated to such a high positive potential, audio coupling will be challenging.    Another alternative is to place the modulator below the RF deck and ground.  If the cathode of the RF deck is the common return for the grid circuit, this would still be high-level plate modulation, NOT cathode modulation.  In this case, audio drive would be much simpler, and link coupling of RF to and from the final would be a trivial method to isolate input and output from the high voltage.  The jury is still out on the method, but testing with several 810s in parallel for the mod and 4D32s for the RF will help narrow down the issues and circuit design.  In any case, I do not plan to go with grounded grid; instead I expect the topology will be a conventional neutralized RF amplifier.  I thought of using a 4CX3000A, but I would really rather keep it simple and use a triode.  This series mod configuration might make it quite challenging to properly modulate the screen grid of a tetrode!

Your numbers are just what I was thinking..... I have used 4KV for many amplifiers and transmitters, so 5KV is in the comfort zone, 8KV is a bit extreme.  Having a 5KV 1A CCS power supply from a Collins 5KW broadcast FM transmitter fills that requirement.  While I can crank the voltage down for the prototype testing, with the 3CX rig I would run the RF deck at about 2KV, with 3KV across the modulator tube to allow for positive peaks, although they are not critical for my purposes.  I plan to have audio processing carefully control peaks and modulation level, that project is almost completed, and soon to be revealed.......

The "proto" configuration reminds me of your sweep tube PDM and quad of 4D32s you recently documented.

One of the triggers that made me excited about the series modulation approach is the performance of the HP 606A RF generator, which provides AM via a 6CL6 series modulator.  The audio is clean and can pass square and triangle waves with no visible distortion or phase shift, and I was very impressed. I would think that during the global cooling seasons in New England, you should consider that 3CX5000 series modulator for your 4-1000A.  If the shack is cool and drafty, you would just need to get into a competition with Tron/HLR at how much longer each of you could keep the old buzzard transmission going!  Mix it up a bit and have some fun!



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Rick / W8KHK  ex WB2HKX, WB4GNR
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« Reply #26 on: March 07, 2019, 02:18:11 PM »

Lots of interesting points here especially amplification factor vs watts of drive. I looked up the 812A and they are scarce & expensive. For now I'll go with a 4-125A or 813 as a next project though I have a couple 4-65A's that I could play with.
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« Reply #27 on: March 07, 2019, 02:34:03 PM »


One of the triggers that made me excited about the series modulation approach is the performance of the HP 606A RF generator, which provides AM via a 6CL6 series modulator.  The audio is clean and can pass square and triangle waves with no visible distortion or phase shift, and I was very impressed. I would think that during the global cooling seasons in New England, you should consider that 3CX5000 series modulator for your 4-1000A.  If the shack is cool and drafty, you would just need to get into a competition with Tron/HLR at how much longer each of you could keep the old buzzard transmission going!  Mix it up a bit and have some fun!


Hi Don,

You can't go wrong with the 813s. Cheap and possibly the most rugged (>100 watts) transmitting tube ever made.

Good luck with the project!


Rick,

Yep, running square waves thru an AM rig and seeing them reproduce is amazing.
Of course most AM rigs can't do this but your series modulated rig certainly can.

There is something appealing about a direct series modulator - the simplicity of design and minimum of critical parts, unlike a tube PDM rig or p-p mod xfmr situation.

The air flow and maintaining a quiet room is difficult for sure.

Keep us updated on your progress and prototype testing, OM!

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
w8khk
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« Reply #28 on: March 08, 2019, 04:32:00 AM »



There is something appealing about a direct series modulator - the simplicity of design and minimum of critical parts, unlike a tube PDM rig or p-p mod xfmr situation.

The air flow and maintaining a quiet room is difficult for sure.


Yes, the cooling air is the challenge.  With proper acoustic absorbent materials, it should be cool and quiet right there in the shack.  In the meantime, the prototype low-power (PW) implementation will probably be three or four 810 series modulators, and three 4D32 Class-C finals.  Since they do not need a fan, I will probably do a Frankenstein Pine-board implementation, the likes of which would make Ross Hull shudder.  Pine boards have been used for transmitters long before the Heil rig, and will probably be used long after as well.

In order to test the scrote 3CX3000F7 modulator, instead of an RF deck, I have a string of 60 watt incandescent bulbs in porcelain sockets on a wood board, all in seriesl  They will take about 3000 volts, and let me fine-tune  the modulator with no RF deck or dummy load.  Again, this is a winter operation that heats the shack nice and toasty, but cannot be attempted in the summer!
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« Reply #29 on: March 08, 2019, 11:19:36 PM »

I was hoping someone would challenge a specific idea I  mentioned above. No takers, so I will challenge it myself because I am wondering the answer.... Wink

T

As you said. You answered the question as far as I understand it which is incompletely.
 
In audio, we probably will not hear even 1% distortion (except Bear?) due to the path from tube to speaker, whether the speaker is at a receiver elsewhere, or in the same room. Speakers could have some 3-5% distortion, and more for cheap ones in radios.
1% is -20dB. If by negative feedback, this is reduced to 0.1%, OK fine -30dB. The spectrum for audio sound is only 3 decades wide, 30-30,000Hz, and help via NFB is pretty straightforward.  Then of course we mess that up with n-diode limiters and other stuff.


In RF the negative FB and other measures are not so simple, and now some people can resort to predistortion, but not at most stations.
The spectrum for an HF linear amp is 5-6 decades wide (30Hz to 3 or 30MHz) because the real spectrum includes both audio components and RF components.

So, most tubes can make very good sounding audio amps and modulators but not as many are good enough for RF amps with a -40dB specification.  Is that agreeable?
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« Reply #30 on: March 10, 2019, 01:23:04 PM »

Hi Pat -

Thanks for the info.

So it looks like we are going to end up with the premise that tubes operate cleaner in audio service than in RF service for various reasons.   Our conclusions seem logical, though I am not 100% convinced we are correct.  

The next time I talk to Chuck/ K1KW I will see what he thinks.  He was an engineer at Varian (Eimac) back in the 70's/80's  and usually answers my tube and amplifier questions with little effort.  Many times my intuitive assumptions are wrong and he gets me squared away... :-)

I'll let ya know.

T
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Frank / WA1GFZ says when he's working near high voltage, as a warning he sings this song by Jay and the Americans: "Come a little bit closer, you're my kind of man, so big and so strong, come a little bit closer, I'm all alone and the night is so long."
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« Reply #31 on: March 12, 2019, 06:25:34 PM »

Well, that was interesting! I'll stick with 813's!
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« Reply #32 on: March 13, 2019, 10:21:25 PM »

T, eager to learn his opinion!
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