Using a toroidal transformer as a mod transformer Part II

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Several months ago I posted my results (which became a very long thread) on using a toroidal filament transformer as a modulation transformer with my Johnson Ranger. This is a follow-up post on my results in scaling that experiment up to full legal limit.

First to recap what I did with the Ranger in the earlier experiments:

Externally modulated
40 watts output at carrier (500 volts plate voltage x 125 mA plate current => 62.5 watts input power)
Modified Heising configuration: 20H choke, 1uF capacitor (essentially no unbalanced current in the toroidal modulation transformer)
Toroid = 25VA Antek filament transformer wired for 12 volts in and 240 volts out (20:1 step up)
Audio source was my regular audio chain as input to a Samson "Servo 120a" audio amplifier (using one of the two channels)

Results: excellent (on-air tests revealed that listeners could not tell the difference between the Antek modulation transformer and a Hammond 1629SEA output transformer).
Frequency response essentially flat between 50 Hz and 7000 Hz (audio input => demodulated off-air monitor output)

Scaling up to legal limit

I'm gathering the components needed to scale the earlier experiments up to legal limit. I plan to plate modulate my homebrew amplifier (currently configured as a linear amplifier), which uses a pair of Russian GS-35b triodes.


Class C operation
Plate voltage (at carrier): 1600 volts
Plate current (at carrier): 300 mA
Power input: 480 Watts
Power output: ~ 375 watts (78% efficiency)
Modulation impedance: 5333 ohms
RF load impedance: 2667 ohms

Heising reactor (50 H 300 mA Peter Dahl NOS)
Capacitor: 1 uF (must be rated at > 4500 volts)
Required audio power ~ 240 watts (for 100% positive peaks)
Required audio voltage at transformer output: 1600 volts peak

Desired step up ratio to present a load to the audio amplifier of between 4 and 8 ohms: roughly the square root of (5333 ohms / 6 ohms) = ~ 30:1. [For 8 ohms: ~26:1]

Modulation transformer:

I decided to use off-the-shelf Antek transformers, available from his Ebay store. I am using two (2) step up transformers in tandem (which means that any artifacts from the transformers will accumulate). A single, smaller transformer (300VA rating) would be cheaper, but he doesn't have one, off-the-shelf, that also has the output voltage rating I need. [If the output voltage rating is not high enough, then the core will saturate at low modulation frequencies (v ~ dB/dt).]

Transformer 1 (as specified):  63VAC input => 115 VAC output (rated at 800 VA)
Transformer 2 (as specified):  115 VAC input = 1600 VAC output (rated at 800 VA)

Tandem combination: 63 VAC input => 1600 VAC output (rated at 800 VA), implying a step up ratio of 1600/63 ~ 25.4:1*

 *As measured with audio applied at 400 Hz: 2.19 volts in : 55.3 volts out => measured step up is 25.3:1

Initial results:

I decided to try the new transformer with my Ranger... i.e., substituting the new transformer for the 25VA transformer I used previously. Otherwise, everything is the same as my previous experiments.

The results are excellent. The demodulated output waveform from the off-air monitor overlays perfectly (after scaling and offset) on the audio waveform from my audio chain... for frequencies between 50 Hz and 7500 Hz (the Ranger's plate and screen bypass capacitors are the limiting factors at higher modulation frequencies). I.e. the modulation is linear, and has a flat frequency response from 50 Hz (lowest frequency measured) to beyond 7500 Hz.

Obviously, with these transformers, there was no problem reaching 100% modulation.

While I have yet to try this at legal limit power, the fact that it works so well with these transformers in combination with my Ranger implies that there is no problem with the parasitic component values in the larger transformers (leakage inductance, capacitance). This is what I expected from some back-of-the-envelope calculations... but its nice to see it in practice.

Next step (probably next month): plate modulation of my GS-35b amplfier at full legal limit

Best regards


Interesting info - will be watching for your test results.

I notice that Antek now lists a couple audio transformers that will be available starting September. Assuming your tests work out as well as expected, it might be interesting to see if they would consider entering the mod transformer (and perhaps even reactor) market. The toroidal design would make it relatively easy to offer a 'multimatch' setup.

Bought several big power supply transformers from them for LF and MF transmitters. They are excellent quality, delivered quickly and work exactly as advertised. E mail correspondence indicates that they will do custom work although I haven't gone that route.



Hello Stu,

I find your mod transformer testing very interesting.  Since you are also using a mod reactor, isolating the dc current from the transformer secondary, I was wondering if a simple intermediate test would be valid.  Instead of strapping to the GS-35B amplifier, suppose you load the mod transformer with a 5000 ohm resistance, and tap a segment of the load with the scope and distortion analyzer, then sweep the audio chain.  Would this method verify the modulation transformer performance, or are there other issues to consider?  I was considering using this method to test my modulation equipment before fully integrating all the components.  Obviously, this method would not verify the audio chain is immune to RF feedback, but for preliminary testing it would limit the number of unknown variables. 

73, Rick

Hi Stu ... just excellent ! ... that is one of the things I really like about this group ... excellent innovation and sharing skills ... reminds me of working at GE in the good ole days ... one question... what are hi-pot ratings for these torroids, particularly winding to winding ?


Thanks for the feedback and the words of encouragement!

Rick: Right now, I am limited by the output power of my audio amplifier. I'm feeling a little "buyer's remorse" about the cost of the transformers and the choke. When that passes, I'll probably purchase an amplifier that can put out around 600 watts in bridged mode. Then I can try what you suggested.

John: Antek's specification says that their transformers are hi-pot tested at 3500 volts:

"Dielectric Test:
TEST CONDITION: Apply dielectric meter between primary coils and secondary coils; and increase voltage up to 3500VAC. No initiate any spark."

Nevertheless, I intend to be very careful in how I perform the full power test.



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