Toroid Modified Heising for DX-60

[K1HH]

DX-60B and Modified Heising Modulation

I have a Heath DX-60B and would like to apply modified Heising modulation by using the Antrak toroid mentioned by Stu, AB2EZ, in his threads on this reflector.

I have the toroid, two 1mfd/2000dc/800ac oil filled capacitors which I would parallel to be 2mfd, and two 10 Henry/200ma chokes.

I assume that one end of the toroid secondary is grounded with the other end going to one end of the two caps in parallel. The opposite end of the caps tie to the top end of the two chokes in series, which goes to the plate of the 6146, through any RF chokes, etc. The bottom end of the two series chokes goes to the B plus supply.

In looking through some old Ham Handbooks, they show breaking both the B plus line to the plate and screen grid lines and applying the high level modulation. Is this the best method? Or do you just break the plate supply line and insert the modulation?

Do these leads need to be shielded?

I’m sure someone has done this to a DX-60 and would appreciate any advice.

Thanks in advance,

Rodger
K1HH

[Bacon, WA3WDR]

Tetrodes and pentodes are good for modulated stages, but they need some screen modulation along with the plate modulation to force some variation in plate current.  Without any modulation on the screen, plate current will not vary much with plate voltage until the instantaneous plate voltage swings pretty low on the modulated waveform.  This results in badly distorted modulation.

Most simple designs for low and medium power tetrode and pentode modulated stages just connect a screen dropping resistor from modulated B+ to the screen(s).  But this overmodulates the screens, and that causes distortion as well.  It is better to connect this resistor to unmodulated B+, although there is an even better way to do it (see below).

The screens want about 50% to 60% of the modulation compared to the plates.  The varying plate voltage will cause variations in screen current, and this can be used to self-modulate the screens if a resistance, or inductance of high impedance, is in series with the screen.  This self-modulation is a little bit light, so some additional modulation should be applied, as shown in Deano's article (link below).  The simplest way to add decent screen modulation is shown in my own article (link below), which works, but is not quite as good as Deano's solution.

This is the best way to do it:

Class C Optimization for Ultra Low Distortion
Dean, WA1KNX
http://www.amfone.net/AMPX/71.htm

And this is a simpler way - not quite as good, but you'll definitely get the idea:

THE SELF-MODULATED SCREEN GRID
Bacon, WA3WDR
http://www.amfone.net/AMPX/98.htm

  - Bacon, WA3WDR

[WB2RJR]

I would suggest the following.

Read the manual on how Heathkit tells you to load up the DX-60 for AM.

Now get an old ARRl Handbook, 50's early 60's, and read how they tell you to tune the PA when you are screen modulating.

You will see the difference!

From page 293 of my 1963 ARRL handbook:

"In general, the amplifier should be heavily loaded.Under proper operating conditions the plate-current dip WILL BE LITTLE MORE
than JUST DISCERNIBLE. Operate with the grid current as low as possible......etc etc etc."

Try tuning up your DX-60 like you should.

You will not need any mods!

Tune it up right and feed it into an SB-220, you can get 225 watts constant carrier out put. I used this as the stand-by for my Globe King 500B for many years. People could just barely tell the difference
in the audio. You know, I had to tell tell them what transmitter I was on so they could "hear" the difference.

Good Luck and have fun, the DX-60 is a great transmitter!

73

Marty

[AB2EZ]

Rodger

Hi!

You will get a lot of different perspectives on how to modulate your DX-60, because there are a lot of ways to do it that all work pretty well. As Bacon points out... you must modulate the screen (which really controls the plate current) as well as the plate. As Bacon points out, there are a number of ways to modulate the screen (I've tried all of them... and they all work) including (but not limited to) the ways that Bacon mentioned and referenced.

As Marty points out, if you do screen modulation properly, and if you are willing to run the transmitter at about 50% of the peak power output that you could achieve (with the same tube) with plate modulation... then people who hear you will not be able to tell whether you are using plate modulation or screen modulation. The key is to load the transmitter properly for screen modulation... and I won't repeat what has already been said, except to say that, in my experience, loading the transmitter properly for high quality (audio) screen modulation is best done using a scope to observe the modulated envelope of your output signal with an off-air monitor.

If you want to plate modulate your DX-60 in Heising mode, my experience (with my Johnson Ranger) is that you can obtain excellent audio by using the simple method of connecting a 30k ohm resistor between the modulated B+ and the screen. There are, as said, many other ways ... but from the perspective of how people who hear your signal on the air will rate the quality of your audio, using the simple "screen dropping resistor" method works fine on my Ranger, and it should work fine on your DX-60. Make sure that the resistor has a high enough wattage rating (20 watts or larger), and make sure that the rf bypass capacitor from the screen to ground is .001 uF   (i.e., not a larger value). The resting (no audio applied) screen voltage should be around 200 volts.

Keep in mind that the voltages that will appear on the plate side of the Heising  choke will exceed 2x the B+,  and the voltage that will appear across the series capacitor (a.k.a. B+ blocking capacitor) will exceed 2x the B+. [Perhaps this is not obvious, but it is due to the series resonance between the capacitor and the equivalent magnetizing inductance of secondary of the toroidal (or whatever type you use) modulation transformer... that occurs at some low audio frequency that depends upon the various component values].

Using a single 1uF capacitor as the series capacitor (a.k.a. B+ blocking capacitor) is sufficient.

Best regards
Stu

[K1HH]

Thank you to all who replied. I am actually going to try both methods of modulation over a period of time. I'm rebuilding the DX-60 for better frequency response with screen modulation and actually have have a Heath SB-1000 linear that I'm rebuilding too. But I have the other parts and a couple of audio PAs and some audio processing gear and I thought I'd try the hi-level path as well. I had already bookmarked the article on setting up the screens but was facinated with using the low cost toroid as a modulation transformer.

Rodger