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Author Topic: Cathode modulation info for WB4BFS  (Read 11780 times)
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W3RSW
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Rick & "Roosevelt"


« on: December 13, 2007, 10:16:11 AM »

Called CQ on 7295 y'day at noon and WB4BFS, John, Athens, TN came back; interesting converstation about cathode mod; a few minutes later WA9YXG, Cliff, Chicago and WB0VMC, Robert, ST. Cloud, MN joined in. 

Basically, John has a Viking II with bad mod iron and wants to put in a low mu triode or parallel set of triodes such as a 6Y6 into the cathode of the 6146's and feed audio to the 6Y6.  Has anyone done this with this combo or of similar lower voltage tubes without transformer matching?

Various circuits, one in the west coast manual about 450Th (I think) with 4 ea. 6L6's in its cathode circuit. All agreed that the 6L6's were very hot on ratings in that circuit and might better be 6550's, Kt88's.  Big discussion ensued about cathode mod. really being a combo of grid and plate modulation.  Controversies and misunderstandings about cath. mod.-  Also some of the old circuits in my Ryders showed transformer coupling with a capacitor tap for audio back into the bias side of the class C grid's rf circuit.  Best eff. was 58% for this combo.

John wanted to know if anyone's tried a low mu triode in the cathode of 6146's or actually experimented,
built up such.  Most of us thought that he ought to try to find a transformer for best impedance match and less loss of voltage across two sets of tubes, etc.  One of us, can't remember who, mentioned he knew of a ham who used a 115v to 80v at 6 amps. pwr. xfor for a cathode match and it worked well.

 Well, to sum up we all told John to 'go for it.'  Feel free to look him up and offer any advice.  He's aware of most of the formulae and graphs showing Po vs. Po fully plate modulated, calculating cathode impedances for various combo's of grid and plate modulation by the cathode, he's not going to get full strap of the orig. Viking, etc.
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RICK  *W3RSW*
kf6pqt
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« Reply #1 on: December 13, 2007, 11:06:17 AM »

http://www.montagar.com/~patj/cathmod01.htm

-Jason kf6pqt
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W6IEE, formerly KF6PQT
The Slab Bacon
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« Reply #2 on: December 13, 2007, 12:22:19 PM »

for good series cathode modulation you need to use a good low Mu triode and a very low plate resistance to have linear modulation.

IMHO the absolute best commonly available tube for this would be a
6080 (or 6AS7G) with both sections tied in parallel. It is very low Mu
and has an extremely low plate resistance, and a pretty high current capability. One would prolly be enough for a V-2, a pair could possibly do a 300 watter easily.

The only disadvantage in this would be the drop in carrier output due to the low efficiency of cathode modded class C finals. instead of the normal 100w outpoot of a V-2 you would prolly only get 30 or 40w. But, it would still be fun to do.

                                                  The Slab Bacon
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Ed/KB1HYS
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« Reply #3 on: December 13, 2007, 02:01:11 PM »

What if you built a nice Cathode modulated rig, like the one on the AM WINDOW tech PW pages, that puts out about 5 or so watts and shot that into a llinear amp??

All the audio and RF work is done at relatively low levels, and the Amp gives you the PUNCH.  Is it that easy, or is there something I missed??
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73 de Ed/KB1HYS
Happiness is Hot Tubes, Cold 807's, and warm room filling AM Sound.
 "I've spent three quarters of my life trying to figure out how to do a $50 job for $.50, the rest I spent trying to come up with the $0.50" - D. Gingery
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« Reply #4 on: December 13, 2007, 02:42:29 PM »

What if you built a nice Cathode modulated rig, like the one on the AM WINDOW tech PW pages, that puts out about 5 or so watts and shot that into a llinear amp??

All the audio and RF work is done at relatively low levels, and the Amp gives you the PUNCH.  Is it that easy, or is there something I missed??



This is kind of the theory behind HF plastic rigs now. Smiley

ALTHOUGH!!!  LInears are HORRIBLY inefficient at carrier levels.

--Shane
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K4QE
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« Reply #5 on: December 13, 2007, 02:43:34 PM »

Ed,

What's missing is the efficiency.  Yes, what you describe will work, and, in fact, many do just this.  The driver may be a low power series modulated rig or a ricebox running AM, but, either way, you'll never get more than ~35% efficiency out of that leenyar.

If you are willing to compromise and pay the extra bux in your electric bill (especially in the summer for your A/C in the ham shack), then this is a viable solution.

If not, Class-E w/PWM all the way!
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73, Tony K4QE
Steve - WB3HUZ
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« Reply #6 on: December 13, 2007, 02:47:56 PM »

Nothing new with that approach. It was common practice in amateur radio in the 20's and early 30's, until push-pull based plate modulation caught on.

I wouldn't worry about efficiency. Unless you are on the air A LOT, the difference in your electric bill will be minimal. I've never noticed a difference in my bill when running plate modulation vice linear amplification.
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k4kyv
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Don
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« Reply #7 on: December 13, 2007, 06:09:24 PM »

The total efficiency, that is, rf carrier power delivered to the antenna vs a.c. power drawn from the mains, is pretty close to the same with plate modulation compared to linear amplification or low level grid modulation.  With the plate modulated rig you have the modulator tube filaments, plate loss in the modulator tubes, loss in the modulation transformer, power consumption of the audio driver plates and filaments, in addition to the final amplifier plates and filaments to account for.  The peak efficiency of the leenyar should be on the order of 60%; the resting unmodulated carrier power with AM runs at approximately 30%.

Continental Electronics used to point that out in their broadcast transmitter promotional literature.  They never produced any plate modulated tube type transmitters, but relied on various schemes of low level modulation until the advent of pulse width modulators.  Of course that all changed with high efficiency solid state transmitters, which is the reason why we are finding so many tube type broadcast transmitters free for the hauling away.

Linear amplifiers are just as inefficient with SSB as with AM.  Class B amplifier efficiency is directly proportional to the amplitude of the rf power output.  As with AM, the maximum efficiency on SSB occurs during voice peaks that use up the available headroom.  But the human voice has a very high peak-to-average power ratio, so with SSB most of the time, just as with AM, the amplifier is putting out far less than its maximum peak power and thus running far below peak efficiency. Therefore, its average efficiency with SSB is about the same as it is with AM.  The noticeable difference lies with the steady 100% duty cycle of the AM carrier, which is predominates with AM.  With SSB, power output and therefore the 30% or so average efficiency occurs over a much smaller duty cycle compared to AM, but the high peak efficiency occurs over about the same duty cycle with AM as with SSB.

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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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K1MVP
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« Reply #8 on: December 13, 2007, 11:02:09 PM »

Just a note,--on cathode modulation,--I have cathode modulated a single 6146 homebrew rig
and also a DX-40 which also uses a single 6146 in the final, with good results running "PW"
out at about 15 watts of carrier(with both rigs).

My homebrew "junkbox modulator" uses a single 6DQ6 fed by a 12AX7 fed by a D-104.
I also have used the 6146--cathode modulator combo to drive a linear( SB-200) which will get me about 120 watts of carrier out, and have had  good signal and audio reports.

Not the most "efficient" set-up, but it does work,--I have also thought about cathode
modulating a pair of 6146`s, but am sure I would need another 6DQ6 (in the modulator)
to do the job.
                                                 73, K1MVP

P.S,--The modulator is basically just in series with the 6146,(just a form of series modulation)
                                                                                                 
       
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N3DRB The Derb
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« Reply #9 on: December 14, 2007, 05:36:35 AM »

my 1939 stancor TX used cathode mod. PP 6v6's modding a single 812. I used to use it on 40 meters and brought it out along with a SX-16 on field day one year, made several AM contacts on 40. i was yellowy until I bypassed the series resistorwih a .5 which was in all the circuits in the cathode modulation handbook by Jones.

the best thing about it is no huge plate mod iron. a 100 watt audio transformer will fully mod a 500-700 watt carrier with room to spare.
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W7XXX
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« Reply #10 on: December 14, 2007, 09:10:56 AM »

I have had excellent results replacing a burnt mod xfmr in a Viking 2 with an ART-13 mod xfmr. However if one is set on cathode modulation it is of consideration that cathode modulating tetrodes may meet with disappointment unless one can hold the screen voltage at the potential of the cathode as far as audio is concerned.

If one is into serious mods then changing the 6146's to a 8005, doubling the PS voltage and series cathode modulating the 8005 with 4 6AQ5's or 6V6's might work, but would be still bound by a 50% efficiency at best. An output of 110 watts is possible with this combo if 200 ma can be drawn from the PS with the voltage doubling. Since the plate mod PS requirements are now omitted this might make the difference. Another consideration is whether the plate tuning cap would stand the increased voltage on mod peaks.

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Ed/KB1HYS
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« Reply #11 on: December 14, 2007, 11:14:28 AM »

Since the  original issue was the demise of the fellows Modulation transformer and getting back on,  perhaps he should look into replacing it with an old TV power transformer. Probably not easy to find, but I would bet they are less expensive than equivalent mod trannys.   TimTron had a mod for that on 100 watt transmitters, on the AM Window tech page.

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73 de Ed/KB1HYS
Happiness is Hot Tubes, Cold 807's, and warm room filling AM Sound.
 "I've spent three quarters of my life trying to figure out how to do a $50 job for $.50, the rest I spent trying to come up with the $0.50" - D. Gingery
WA1GFZ
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« Reply #12 on: December 14, 2007, 11:36:43 AM »

My 4CX3000A would drive right out of the shack in a 1/2 hour in the summer time. It would go from basement cool to hot real quickly. Class e rig I can run all day and maybe the heat sink might hit 115 degrees F after an old buzzard.
Now in the winter it is tubes warm glow
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Steve - WB3HUZ
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« Reply #13 on: December 15, 2007, 09:00:56 PM »

If you have access to old QSTs, look at November 1939 for an article by F.C. Jones (more or less the godfather of cathode modulation) and December 1939 for a follow-on article by W2DIY (no kidding).

F.C. Jones also published a book entitled "Cathode Modulation" a year or so before. If you can find one, it's good reading. West Coast hand books of that era also had pretty good coverage of cathode modulation.

UTC made some transformers specifically for this type of cathode modulation. They were designated by the CM-xx model numbers.

You can calculate the modulating impedance by the following formula.

Z = (2P)/(I^2)

where Z is the modulating cathode impedance,
P is the audio power in watts required for 100% modulation (1),
I = DC cathode current of the RF final amplifer

(1): The audio power varies with the amount of desired efficiency desired from the RF final amplifier - basically it's the ratio of plate to grid modulation - most designers aimed for audio power requirements of 10% to 25%  of the carrier input power of the RF amp.
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N3DRB The Derb
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« Reply #14 on: December 15, 2007, 09:11:32 PM »

yeah, thats the one.
Quote
F.C. Jones also published a book entitled "Cathode Modulation" a year or so before. If you can find one, it's good reading.
I think I still got it.

it also had 50+ circuits using every tube around back then, including crazy stuff like H&K Gammatron's, which is still IMO the coolest name for a tube evar.

http://www.geocities.com/rxtxtubes/pa01074.htm

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WBear2GCR
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« Reply #15 on: December 21, 2007, 05:13:38 PM »

Low plate Z tubes?

The 6AS7, the 6080 sure...
but the 6336 is mo' biggah and better...

Then the Ruskie 6C33C has 'em all beat, and it is super rugged and inexpensive too.
Quite a few places import them here.

         _-_-bear
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_-_- bear WB2GCR                   http://www.bearlabs.com
w4bfs
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« Reply #16 on: May 03, 2008, 03:43:19 PM »

Hi  finally got around to getting registered   just wanted to say thanks for your comments and i am still considering just what to do to the JVII   73
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Beefus

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W3RSW
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Rick & "Roosevelt"


« Reply #17 on: May 04, 2008, 11:07:57 AM »

- Glad to see ya' on the board John.
Best of luck with whatever you decide to do. We'll all wait with baited breath to see what lash up works.  Grin
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RICK  *W3RSW*
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