The AM Forum

Hello,

        I ended up playing around some (3) ART-13's the
last couple of weeks. I wanted to display a Trapezoid
pattern. A little schematic tracing brought fourth this
easy to implement approach.

        C128 is at the Modulated B+. A small "L"
bracket was fashioned to hold a HV 500pF blocking
cap. I used 5 300K +/- ohm 2 W resistors in series
off that with a 20K 2 W to ground. Pick off
Modulated B+ at the top of the 20K resistor gives
me about 20 V P/P audio for the Horizontal feed
to my Scope.  My Vertical scope connection goes
to my regular inline sampler.

         I removed the "RECEIVER" binding post
and installed a BNC jack there. It's so easy and
cheap I made up 3 of em', I'm just going to leave them
in there.

         I haven't used the Trapezoid pattern much
only because it's such a PIA to implement. Once I
come up with a way to Bias the "Z" axis input on
my scope with my T/R system, I may stick with it.

         Just using my junk box for the pieces parts.
Nothing is critical. My series Resistance is about
2 Meg.

73

/Dan

Nicely done, scope pattern does not look great though...

    Yeah,

       Not sure if it's just a Tetrode thing, or
that Tertiary winding on the mod xfmr. All 3
of em look about the same. Of course these
things all have HB supplies of various voltages
and tubes in various states of decay. One even
has holes in the 811 plates. One has a pretty
soft 813.  Amazingly they all look Ok with a
swept wave display and sound OK on the air.

       Hence, my desire for a trapezoid display.
I think I will put a Variac on the LV supply.
Maybe I can find a "Sweet Spot".

73

/Dan

  Dan,

   You will be happy once you got that Trap ability all dialed in. The neat thing is using a scope on X;Y mode you get Trapezoid, and then turn the sweep to 1 ms / division, and you get the modulated envelope. You can trigger on the audio too to get better scope trigger to your voice. This gives you one hell of a lot of information with minimum fuss.

   The voltage divider you have isn't compensated, and therefore there is phase shift. Think of how a 10:1 or 100:1 scope probe is designed. What I would do is reduce the 20K to 2K and see if that looks better. The junction of that voltage divider must drive the cable capacitance in parallel with the scope input capacity. A 10' rg-58 cable and typical scope could add up to 350 pf load or more. Going down to 2K would lower the source impedance 10X, so that should help. The 500 pf capacitor isn't really needed. As you have it now, bypassing that capacitor will put about 22 volts DC into the scope when the ART13 B+ is 1 KV. No big deal, just AC couple the scope input. Better yet going to 2K from 20K will lower that DC voltage to only 2.2 volts. Use a 1K and only ~ 1 volt DC..

   The RF sampling needs to be clean and without power line components mixed in with it. At the 100 watt level, I like to sample the RF in the rig's 50 ohm output with a 5K 2W non inductive resistor. Then hook that up to a BNC connector. Then add a shunt 50 ohm resistor across the connector. So at 100 watts we have 100v peak into 50 ohms, and at the connector there is 1 volt peak..ideal to feed your scope. The resistor will dump 1 watt.

   I attach an image from one of my rigs that sample the RF this way.

Other options of course are possible.

My two cents :-)

Jim
Wd5JKO

   Hey Jim,

       Thanks OM, I did as you suggested and eliminated
the blocking cap and fashioned up an RF tap per your
advise. it cleaned up a lot..

Thanks again,

/Dan

 Dan,

   Good deal on how those changes you made worked out. I wonder what you dropped that 20K resistor to? Was it 1K? Higher ratio dividers for higher power rigs would be necessary.

   The fact that the walls of the trapezoid are straight is quite telling. I believe the audio and RF taps are presenting valid data now. This validates the ART-13 design as quite good. The 813 plate modulation with a tertiary screen winding is behaving quite well. I wonder how your trapezoid looks at audio frequencies of 3 Khz and higher?

   Many plate modulated rigs using the 6146 will show non linearity on the trapezoid. This is not the fault of the 6146, but more due to the simple screen dropping resistor arrangement. The result is an excessively modulated screen grid, and RC phase shift between the screen dropping resistor and the screen bypass capacitor.

   What you have done with two simple taps (1 audio, 1 RF) could easily be added to many rigs.

Jim
Wd5JKO

   

   Hey Jim,

       I ended up adding a 1 meg. 2W "R" in place
of the blocking cap so I didn't have to move the board
with the rest of the resistors on it.

That brought my series resistance to a bit more than
3.5 meg ohm. I then just strapped a 4.7K 1 W across the
20 K to bring that down to about 3500 ohms.

       For the RF tap I used 4.7K 2 W carbon to a
47 ohm 2 W carbon.

       This ART-13 only makes about 70 Watts out, My
other one will do a 100 W. The third one with the soft
813 makes about 60 watts but will not make ANY positive
modulation either with the mic. or in MCW. I'm
expecting another 813 this afternoon.

       The vertical input was still a little distorted,
a little bit elliptical, not a lot but noticeable. A 220
pF disc across the 47 ohm did the trick!

Thanks again Jim,


/Dan