[ARC5] My ARC-5 40 meter station

[Charles]

That undersized modulation transformer may have a lot to do with it... my 
homebrew screen mod would start to distort below 400 Hz and at 150 Hz it 
looked like crap. Once I got rid of the modulation transformer (actually a 
15VA, 115v 60Hz isolation transformer which was a reasonable match) and used 
the 6AQ5 driver as a triode-connected cathode follower to drive the screens 
directly, it generates a near-perfect waveform at any frequency from 30 Hz 
to dog-whistle :)

The schematic of the BC-456 I found does not have values for the parts - in 
particular, the coupling caps. How big are they?

Yeah, carbon mikes are not exactly known for high fidelity. I would think 
that the most important frequencies (containing most of the speech energy) 
are between 400 and 1500 Hz, and if there's rolloff above and below that it 
will be quite intelligible but not broadcast quality!

-Charles
WB3JOK/0

-----Original Message----- 
From: Chris Bowne
Sent: Sunday, February 22, 2015 1:48 PM
To: Charles
Cc: ARC-5
Subject: Re: [ARC5] My ARC-5 40 meter station

I can try to get you a trace image. I feed in outboard line level audio to 
the normal carbon mic input using a mil RM-12 remote control unit that was 
used to feed transmitters like the BC-191with audio from remote locations. 
With a high quality audio chain it's amazing how good a totally original 
screen modulated command transmitter can sound. I have swept mine with my 
HP-200 as a source and it's very clean from 150 Hz to way past 10 kHz. Below 
150 Hz there is some distortion evident. Not sure if it's in the transmitter 
or the audio feed.   When fed into a dummy load, test audio content sounds 
really nice on a high quality receiver.  But plug in a T-17 or RS-38 instead 
and it's back to the bandits at two o'clock carbonium microphonium sound!

Sent from my iPhone

> On Feb 22, 2015, at 9:35, "Charles" <charlesmorris800 at centurytel.net> 
> wrote:
>
> Thanks for the suggestion. I hadn't realized the BC-456 modulator is 
> indeed a screen modulator, whereas the MD-7 is a plate modulator with a 
> screen winding! I would be interested to see the output waveform of your 
> transmitter producing 15 watts carrier at 100% screen modulation (with a 
> stock BC-456 modulator), if it's even capable of 100%. Can you please post 
> a scope picture?
>
> I do have to wonder, though, has anyone read Dave Stinson's "ARC-5 Notes", 
> to which I posted links yesterday? There is discussion of the various 
> matching methods (including the series cap, which is what I'm using), AND 
> actual harmonic output measurements of each. Not much difference between 
> them for either power output or harmonics, so I went with the simplest, a 
> doorknob series cap. There is even room inside the transmitter to mount it 
> after removing the unneeded antenna relay behind the top of the front 
> panel.
>
> Rather than add an additional transformer and variable cap, I simply added 
> one turn to the 40m output tank, (two turns for the 80m), in series with 
> the output variometer link. I can now adjust the loading into 50 ohms from 
> "too little" to "too much" via the front panel knob. I'm a firm believer 
> in the KISS principle :)
>
> -Charles
> WB3JOK/0
>
>
> -----Original Message----- From: Chris Bowne
> Sent: Sunday, February 22, 2015 8:14 AM
> To: J Mcvey
> Cc: Charles ; ARC-5
> Subject: Re: [ARC5] My ARC-5 40 meter station
>
> Great work Charles. To get the best loading and max your power output, try 
> using the autotransformer matching network that Walt, KJ4KV published in 
> an ER in Uniform column many years ago. It works very well, I use it on my 
> command sets to get about 50 watts out on CW and 15 watts on AM, using the 
> original BC-456 screen modulator and DM-33 dyno.  Consists of a parallel 
> tuned circuit with the transmitter fed to a tap on the coil a few turns up 
> from the ground end through a 150 pf series cap and the 50 ohm antenna fed 
> off of a second coil tap a few turns above the transmitter input tap. I 
> threw one of these together using a pi network coil set from a junker Eico 
> 720 transmitter and a 200 pf transmitting variable.  As I recall the input 
> tap is about 5 turns up from the ground end and the antenna tap 3 turns 
> above that. Use a cap with relatively wide spacing as the voltage across 
> the plates can get quite high at resonance.  I had arcing problems with a 
> narrow spaced cap.  You tune for resonance with the autotransformer cap 
> and then peak output with the command  set transmitter variable roller 
> inductor. With this setup your output should peak with the transmitter 
> coupling control at about 2/3 of full travel,  I can't recall the ER issue 
> the circuit appeared in but it was around 1989-1990 as part of a series of 
> Walt's ERIU column on command sets.
>
> The circuit should also help with transmitter harmonic output suppression.
>
> Sent from my iPhone
>
>> On Feb 21, 2015, at 18:19, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
>>
>> You get alot more than 8 watts out. The output was designed for low 
>> impedance (5-12 ohms ) a few hundred ohm capacitive 5-J200 antenna.Specs 
>> say you should exceed 40 watts in CW  and have a 15 watt voice 
>> carrier.Perhaps try using a 9:1 "unun"
>