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 Post subject: Johnson Valiant Static Modulator Current
PostPosted: Jul Tue 28, 2020 9:24 pm 
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Joined: Jun Fri 23, 2017 5:36 pm
Posts: 80
According to the manual the static modulator current should be 60 ma for AM operation. So being both an AM mode and a Valiant newbie I tuned up the Valiant in CW mode as directed and achieved maybe 175 watts out at 450 ma plate current and 8 ma grid current. But for AM the plate should be 330 ma so I decreased the loading (turned the knob to the left, I am still confused about what increased loading means, apparently its decreasing the capacitance) and resonated the plate circuit to get the dip to be more or lass 330 ma. Switching then to AM mode I find that the modulator current is way low, maybe 10 ma at best. So I review the bias settings notes I did some time ago and find that the drive should be 0 when the bias is set (which it was). Turns out reducing the drive increases the modulator static modulator current to where it is supposed to be. I am sure some of you know how this is supposed to work so please tell me. The now pretty much restored Valiant (long job) seems to be working well but there is now the small matter of how to operate 1956 style which I have never done. Still haven't found an antenna change over relay.

Thanks for any help.

Bill N5VR


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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Wed 29, 2020 2:58 am 
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Location: Costa Mesa, California
I believe the Drive is used to adjust the plate current where as, Plate tuning resonates the plate and is always adjusted for minimum. The loading (coupling) matches the antenna system and is adjusted for max output, but no more. Because it interacts with the plate tuning, they both are readjusted a little after either is moved. It has been a while since I had my Valiant off the shelf.

Norm


Attachments:
Valiant 1.jpg
Valiant 1.jpg [ 221.61 KiB | Viewed 463 times ]
Panel front.jpg
Panel front.jpg [ 201.87 KiB | Viewed 463 times ]

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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Wed 29, 2020 7:10 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 4598
Location: Lincoln City, OR 97367
Greetings to Bill and the Forum:

Here's my quick tutorial on Pi Network transmitter output circuits. Figure 1 is a conventional Pi network. I then morph it into figure 2 and last to show what it really is, I simplify it into figure three:

Attachment:
Pi Network Analysis.jpg
Pi Network Analysis.jpg [ 242.43 KiB | Viewed 450 times ]


I hope this helps with your understanding of the loading control. If I'm clear as mud, please feel free to ask additional questions and I will try to clarify.

Regards,

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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Wed 29, 2020 3:29 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Somers, CT
The loading, drive or tuning should NOT affect your quiescent modulator bias. You need to check at least two things. First is the grid bias voltage at the grids of the modulator tubes shilfting for some odd reason? Is the screen voltage on the modulator tubes dropping more than expected when the rig is being tuned?

In the AM mode, the modulator quiescent current should remain where it is set, unless audio is present.

Pete

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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Wed 29, 2020 8:02 pm 
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Joined: Jun Fri 23, 2017 5:36 pm
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Pete,

I was afraid you were going to say something like that. I will investigate and get back.

Norm,

Nice Valiant!

Jim,

Thanks for the explanation, I have tried others but yours is the best I have seen. Much appreciated.

Bill N5VR


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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Wed 29, 2020 9:00 pm 
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Joined: Jan Sat 12, 2008 12:25 am
Posts: 203
Location: Kingwood, Texas
I believe in early versions of the Valiant, the drive control will cause the modulator current to vary. If the transmitter in question doesn’t have bias adjustments on the side of the chassis, it’s the early version.


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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Wed 29, 2020 9:23 pm 
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Joined: Jun Fri 23, 2017 5:36 pm
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It is indeed the early version. I have considered the possibility of changing to the 'pot" method of setting the bias but I have never seen an article on how to do that so I haven't done it figuring that the old version would work since it was the original design. I was able to get the voltages set pretty close per the manual but at the time I did it I couldn't check the modulator current as the front panel was off so no meter.

Bill N5VR


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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Thu 30, 2020 11:59 pm 
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hbr nut wrote:
I believe in early versions of the Valiant, the drive control will cause the modulator current to vary. If the transmitter in question doesn’t have bias adjustments on the side of the chassis, it’s the early version.


I stand corrected.

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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Fri 31, 2020 9:59 pm 
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Joined: Jun Fri 23, 2017 5:36 pm
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Thanks to everyone who responded, it was helpful. I went back to the bias adjustment procedure and found that I had done a pretty good job and the voltages were pretty close to spec. I was within 2 V on the PA and 4 V on the Modulator grids. Then I noticed that the meter was reading 65 ma modulator current against a spec of 60 ma. I did not change a thing so the only difference is in the setting of the controls. So something about the way I had it set up before apparently was the cause.

I'll post some pictures of the completed restoration later. Now all I need to do is to get this into an operating position and find someone to talk to on AM. At least the former is not a trivial task given the size of this thing and the SX-101A I intend to mate with it (already restored and working well). Oh yes, and finding a changeover relay.

Bill N5VR


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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Jul Fri 31, 2020 10:26 pm 
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As posted previously, modulator idling current does vary with grid drive level. If the 6N2 transmitter is used with the Valiant to supply its power and modulation, the Valiant must be modified with the "two pot" mod that keeps the modulator idling current fixed regardless of final excitation.

Either the original or later two pot bias circuit works fine if not using a 6N2 with the Valiant but the later circuit is far more convenient and doesn't require sliding a contact on a fragile power resistor to set idling current.

With the early Valiant, the final amp plate current also reads 30 mils high because the plate meter also shows the VR tube regulator current. It isn't a big deal but Johnson allowed loading to 360 instead of 330 mils for AM, not that roughly 18 watts of additional input made any difference.

Rodger WQ9E


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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Aug Sat 01, 2020 3:15 am 
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Joined: Oct Sun 15, 2006 12:57 pm
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Location: Liberty, Missouri
Nice to see another Valiant going on the air. I've enjoyed mine for a few years now and use it occasionally. Mine has the 2 pot bias mod.

You might consider finding a Johnson electronic T/R switch. It simplifies the station wiring, and works with any transmitter and tube type receiver, just run the transmitter's RF through it on the way to the antenna. If you plan to use it with SS receivers, you'll need to add a couple back to back diodes on the receiver output jack.

Image

Image

Image

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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Aug Sat 01, 2020 2:48 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
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Location: Santa Rosa, CA
Bill,
The meter shunts are notorious for being out of spec. Often by quite a bit
leading to incorrect meter readings. This is especially true of the PA plate
and mod plate shunts which are simply small loops of wire.

Verifying them is easy. Use a variable current limit bench supply and
with power disconnected from the transmitter (!!!), apply a known
current across each meter shunt that corresponds to full scale for
that range.

Of the 8 Johnson transmitters here, all had problems with this.
Steve

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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Aug Sat 01, 2020 3:14 pm 
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Attached is a PDF file I put together for another ham restoring a Valiant that contains some of what I have learned from restoring a number of Valiant transmitters over the years. A Valiant was my novice rig back in 1975 and it was the first model I acquired when I got involved with restoring vintage gear in the mid 1990s.

Rodger WQ9E


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Valiant info.pdf [47.5 KiB]
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 Post subject: Re: Johnson Valiant Static Modulator Current
PostPosted: Aug Sat 01, 2020 11:59 pm 
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Location: Liberty, Missouri
Quote:
...The meter shunts are notorious for being out of spec...


I just replaced them with 1% 1/2 Watt metal film type.

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