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Author Topic: Jphnson Valiant modulator question  (Read 1817 times)
WA2DTW
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Posts: 91




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« on: July 29, 2004, 09:37:14 PM »

Hi
Trying to get the valiant working.  Getting normal grid drive and RF output; the bias is set properly (-70 volts on final grids and -35 volts on modulator grids) but the resting modulator current, which should be about 60 ma is >200 ma.  Any ideas?
Thanks
73
Steve WA2DTW
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W8JI
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« Reply #1 on: July 30, 2004, 06:12:50 PM »

Steve,

Are you sure you really are setting the grids at the proper bias?

The OC3 regulator tubes, how are they? Do you have proper screen voltage? The VR tubes regulate the modulator screens.

If not either of those, you may have a bad meter shunt. A high-resistance shunt can cause the meter to read high.

You'd have to test the shunt with an external current source. I can tell you how to do that if the other things are OK.

73 Tom
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WA2DTW
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« Reply #2 on: July 30, 2004, 07:29:35 PM »

Hi Tom
I adjusted the modulator bias resistor to get 60 ma resting modulator current.  The valiant is now on the air (for the first time, for me) and I'm getting an OK audio report.  (don't have a scope set up).  Did not go back in and measure the voltage on the modulator grids, but would assume that it's higher than -35 volts.  
Next will have to decide what to do about the "clipper", which seems to be quite controversial.
Many thanks and 73
Steve WA2DTW
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W8JI
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« Reply #3 on: July 31, 2004, 03:45:47 PM »

Steve,

I measured clipper distortion and other things(I have a full modern RF test lab), and concluded most of the mods to the modulator section do nothing, and some are actually harmful.

I added negative feedback with very minimal results, I measured frequency response with a sweep generator and a detector on the RF output, and found that changing the RF bypass capacitors did nothing except make splatter beyond 6kHz up and down worse.

Adding a simple grid leak resistance (not in any of the mods) improved modulation linearity, and the choke in the screen grid mod made things worse...not better.

The one thing that is necessary and does help is increasing the size of coupling capacitors. I went from standard to .05uF for all interstage coupling. This cured a severe low frequency rolloff. Also, I've found the cathode bypass capacitor in the driver tube (12AU7 I think)bad. I replaced it with a 22uF 50V.  

I plan on eventually publishing, with display readouts from test gear, this info on my web site.

In the meantime, I suggest making minimal changes other than the coupling caps or bad parts. The Valiant was already pretty good, and the modulation transformer won't take much extra power anyway. Some of the mods, like a diode clipper in the modulator output and reducing shunt C in the RF secion bypasses, actually greatly increase splatter without increasing anything helpful!

I hope you enjoy your Valiant. I am now restoring a Globe Scout 65A and a Ranger II.

We just finished a new silkscreen for the Globe 65A, and it looks EXACTLY like the old panel except factory new. We stripped and re-shot the panel in new paint, and screened over it in a process duplicating what Globe did. The panel is amazing. We even left the errors in, where Globe had things slightly off-center.

I'm hoping to do the same to the Ranger II, and eventually to my Valiant.

The Globe was my very first (used) commercial transmitter, and I bought a Valiant (used) in 1970. Hope to hear your radio someday soon.

73 Tom    
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WA2DTW
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Posts: 91




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« Reply #4 on: July 31, 2004, 09:33:36 PM »

Again, many thanks, Tom.
It is good to hear an opinion in favor of keeping the valiant as it is!  I have no intention of altering it unless I get really bad audio reports.   The clipper probably helps when there is a lot of QRN to get through.
I hope to catch you on 75 and 160.
73
Steve WA2DTW
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WA2DTW
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Posts: 91




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« Reply #5 on: August 02, 2004, 11:39:26 AM »

Tom
Got the Valiant working, attached the Dow Key Relay,
and paired it with the old HQ170.  Was making QSO's,
getting good reports, everything was great!
Then, the next thing I knew, the modulator stopped working.  I can measure base current in on the meter in the MOD position, but no movement of the meter with speech.  RF output is normal.

I realize that this can be almost anything wrong with
the audio chain.  But does anything obvious come to
mind?
73
Steve WA2DTW
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W8JI
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« Reply #6 on: August 02, 2004, 05:55:51 PM »

You'll just have to trace through it. Look to be sure a tube filament isn't out, and you might try wiggling the tubes. Careful of that HV. It is everywhere!! Even with the TX switch off.
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N6AYJ
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« Reply #7 on: August 10, 2004, 02:08:17 PM »

Thanks for the tips, Tom!  I have a Valiant too, which I am trying to get on the air, and I am sure I will find your advice very helpful.  I bought it only a couple of months ago.  My elmer, Lee Grabowski, K6UJR (SK) had a Valiant back in the late '50's and it is one of my earliest recollections about ham radio.  I have always wanted a Valiant, and now I finally have one.
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WA2DTW
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Posts: 91




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« Reply #8 on: August 14, 2004, 09:11:14 PM »

And my thanks also to Tom.
Was able to isolate the problem to an open resistor R35 in the audio circuit.  (a 47K resistor between the clipper and the third audio amplifier).  Strangely, there were no signs of smoke or wear on the resistor.  Happened to have a replacement resistor with twice the wattage.  The rig worked after changing the resistor.
Tom- I hope to work you soon on AM.  
Also- best of luck to N6AYJ.  Perhaps we'll get our Valiants together sometime on 14.286.
73
Steve WA2DTW
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N6AYJ
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« Reply #9 on: August 15, 2004, 09:58:25 PM »

I would enjoy a boatanchor QSO with you a lot, Steve.  I still don't have my Valiant on the air, but did recently install a new Peter Dahl power transformer in my HT-32, so I could talk to you on that in the meantime.  My boatanchor RX is a 51J-4.  73 from California, the land of fruits and nuts.
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W4PTO
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« Reply #10 on: August 16, 2004, 05:34:14 PM »

Hi:
How are you measuring modulator current? Putting a meter in series with the supply, I suspect.

I have to second on not modding the Val's power supply. I had one and despite complaints from others that my audio sounded "tight", I felt that to battle the band conditions on 75 meters, the extra audio "punch" was worth it. Additionally, I felt it would be appropriate to have others hear how an original Valiant from the 1950's sounded on the air. Cal, N6KYR.
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WA2DTW
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Posts: 91




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« Reply #11 on: August 16, 2004, 07:30:03 PM »

Hi, Cal
Nice to hear from you.
Appreciate your opinion on the Valiant modulator.  So many folks insist that the whole modulator section should be drastically modified and the clipper section eliminated.  (I still need to figure out what to do with the clipper).
For "modulator current" I was just using the meter mounted on the valiant, with the switch in the MOD position.  According to the manual, it measures modulator cathode current.
73
Steve WA2DTW
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K5CEY
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Posts: 217




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« Reply #12 on: August 17, 2004, 02:32:15 AM »

Well,
   I've had a Valiant since 1971. It was built from a kit in '61 and I bought it for $65 from a local Ham from a newspaper add.

For it's time and day, the Valiant was the bee's knees. Medium power, and all band capability. For you guys that were around then, AM was the normal mode, and the terrific banshee heterodynes on 75 meters at night were unbelievable.

That hot,clipped, piercing audio from the Valiant did the trick.

In today's world, when these old dinosaurs are restored and put on the air, everyone thinks they should sound like broadcast stations. It will never happen.

The modulator chain is just not capable of producing 100 watts of clean, smoothe audio. Under ideal bench conditions, a pair of 6146's in AB1 with 750 volts on the plates and properly matched output transformer will deliver about 120 watts of audio power. With that chumpy mod transformer,and +HV about 650 volts, it just won't make it.

There are more audio mods for the Valiant than I have fingers and toes.

Mine sounds OK, if I do say so (if I don't push it). Raised mod HV to 800 volts with an external supply, eliminating the driver xfmr and driving the 6146's directly coupled with a dual cathode follower. Using a Heising choke and coupling condenser and a peak limiting circuit from a 1945 handbook, and a respectable amount of inverse feedback from the secondary of the mod xfmr to the 2nd speech amp stage.

I've made other mods in the RF and power supply sections, but that's a subject for another time.

It's a nice looking rig and easy to tune and put on the air and looks like a '57 Pontiac.

         John

 
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W8JI
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« Reply #13 on: August 18, 2004, 08:21:11 PM »

If you look at the modulator waveforms in the valiant, the grids of the 6146's go into current (AB2) before you can reach 100% modulation when you run more than 250mA or so of plate current.

I spent extensive time looking at all of the Internet mods, and most were nonsense. For example I ran an audio sweep generator into the input and looked at detected AM output with sweep, and the HF rolloff is compensated by a peak in the choke and low-pass following the clipper tube. This compensates any rolloff caused by bypass capacitors in the HV supply to the PA.

The screen grids are ALREADY nearly optimum modulated by the feedthrough in the dropping resistor, which is fed modulated audio. As I added or removed more audio voltage on the screens, I only made the modulation distortion increase. Viking already had that system down pat.

If I added a diode negative limiter in the modulation secondary, it simply made bandwidth wider without increasing "in-band" performance. The idea of clipping is to reduce splatter, not increase it.

I do agree that raising HV on the modulator would be better, but the same end result will occur (100% symmetrical modulation) if you just run less power and you won't be hammering the small iron used in the modulation transformer.

To increase modulation linearity, I added a grid leak in series with the CT of the bias pot for the PA. This increase gid bias, and also pulls the clamp tube harder out of conduction. It makes grid current setting much less critical.

I measured distortion with and without negative feedback, and found virtually no change. All of my distortion was when I allowed the 6146 grids to swing positive. That squares off the audio waveform driving the tubes.

In order to minimize that problem I used a larger dual triode with higher quescient current and lower plate output impedance. I also loaded the secondary of the modulation transformer with a resistance, so the load on the driver tube did not swing from near infinite (no grid current) to a lower value when the grids swing positive.

I think the increased modulator voltage would be a good change, but the transformer scares me.

The idea of cathode drive and direct coupling is a good one, but I wonder why not just add a second cathode follower push-pull tube driven by the existing transformer?

This would solve the grid loading problem because the cathode follower would be very low source Z and less affected by the 6146's falling in and out of grid current.

Very interesting approach, changing the driver to a cathode follower. I only tried to "doctor" the driver by making it a lower source Z through a lower plate impedance tube and resistor swamping. Someday I'll have to try that.

My overall conclusion was most of the mods made by people were self-feeding mods. They changed one thing and the effects of that required them to change more. Before you know it, the entire system was changed.

After I tried all the mods I could find and saw what was happening, I decided to pull them back out and start over. My goal was to regroup and fix the minimum amount possible. If I wanted to gut the rig, I wouldn't have gone the path of making it wider without any benefit to on-frequency quality.  

73 Tom
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K5CEY
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Posts: 217




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« Reply #14 on: August 19, 2004, 05:56:41 PM »

Tom,
  All of your comments on the Valiant are viable. You mentioned about the mod grids pulling current and going into AB2. This is the reason I decided on using cathode followers and direct coupling to the 6146's. (From an old CQ article from the Fifties, using a 6AS7 dual low mu triode cathode follower to drive a pair of 810's in class B.) Prior to this I tried many things. For a while I just connected the driver xfmr primary from the 12AU7 cathodes to ground and strapped the plates to +300. Untold hours of mindless tinkering. I wonder how many times I said "hello" into the dummy load?

I was not aware of the peaked response in the clipper filter which negates reducing RF bypass values to help high freq AF response. Of course, most folks completely remove the clipper circuit 1st thing. I would never reduce the PA bypass values, especially considering 160 and 80 meters. As a matter of fact, I fattened up the PA output coupling cap (which effectively shunts the modulated HV) from 500 pf to .01 uf. That impressive looking 500 pf HV doorknob cap has about 180 ohms of series reactance at 1.8 mc and caused max PO to occur off the "dip" on 160. 180 ohms is about 10% of the total plate resistance of the three 6146's. I now have max output on the dip and picked up 10 or 15 watts PO on 160 and a slight increase on 80.

I said I was using peak limiting. A better description would be AMC or peak compression. I use a string of diodes connected to the modulated +HV and they are biased so they begin to conduct just before the negative half cycle goes to zero. This is felt as a DC level which is filtered and is fed back to the control grid of a variable mu pentode in the 2nd speech amp stage.

The inverse feedback loop really cleans up the audio for me. ( Audibly that is. I've never felt ready to sweep the audio and look at it on a 'scope ).

The reason I added the modulation reactor and condenser was to relieve that tiny mod xfmr of having to carry over 300 mils of steady PA plate current.

I've always wanted to try a pair of 6DQ5's in the modulator. Specs for AB1 linear service look pretty good at 600 volts or so and should put out more than the 6146's. Been 7 or 8 years since I've had that featherweight chassis turned up on end.

 John
 
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