Using a toroidal transformer as a mod transformer Part II

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AB2EZ:
Update:

I have everything looked together... and with the audio amplifier I have on hand (120 watts peak into 8 ohms in bridged mode) I was able to comfortably modulate about 30-40% with 400 watts of input power (1600 volts and 250 mA) and 350 watts of output carrier power. See the attached photo

Note: 1600 volts / 0.25A = 6400 ohms. My step up transformer ratio is ~ 25.3 :1... so the impedance that the audio amplifier is driving into is  6400 / (25.3 x 25.3) ohms ~ 10 ohms. As a result, the amplifier (which acts roughly like a voltage source) probably can only put out 120 watts x 8/10 = 96 watts into the load it is looking at.

It looks good on the scope (The output of my off air envelope monitor is tracking the audio input to the audio amplifier)... and it sounds like my transmitters normally sound when I listen to it.

I gave a lot of thought to the issue of dumping the energy stored in the Heising choke when switching from transmit to standby. I finally decided that the energy would, in fact, be dissipated in a safe way (no big voltage spikes on the choke or the step-up transformer) in the audio amplifier. The DC choke current is diverted into the secondary of the step up transformer... which, in turn, produces an opposite current in the primary*... which flows through the output resistance presented by the audio amplifier. Since the toroidal step up transformer has very low leakage inductance... the transition of the current in the Heising choke from flowing into the plates of the rf tubes to flowing into the secondary of the modulation (step-up) transformer should be very fast. There is a brief period (probably less less than a few microseconds or so) when the current flows into the .0044uF bypass capacitor (to ground) at the modulated B+ side of the plate rf choke. The current can also flow briefly through the output rf coupling capacitor (.0066 uF) to ground via the tank coil and the safety choke. When that is occurring, the resistance of the rf choke also provides some absorption of the stored energy in the Heising choke. I tried to measure the voltage across the secondary of step up transformer when I switched the transmitter from transmit (~300 mA through the Heising choke) to standby. I didn't see anything larger than about 1300 volts... and the associated transient waveform was rather complicated... including ringing at around the resonant frequency of the tank circuit . The way I measured this was to measure the transient waveform across one of the 6.3 volt windings on the secondary of the step up transformer, rather than measuring across the 1600 volt winding of the secondary. I scaled the waveform I observed on the 6.3 volt winding by 1600/6.3

[Come to think of it, the combination of the 0.0044 uF r.f. bypass capacitor and the 0.0066uF rf coupling capacitor are providing a total of a .011 uF path to ground in parallel with the tube's 5400 ohm modulation resistance. These are diverting significant modulation current for frequencies above 2680 Hz... so I need to reduce both of them to .001 uF.]


*The primary current is larger than the secondary current by a factor equal to the turns ratio of the transformer.

In the picture (don't do this at home with high voltage circuits):

A. the larger toroid is the Antec 800VA, 115VAC input : 1600 VAC output transformer

B. the smaller toroid is the Antec 800VA, 63VAC input : 115 VAC output transformer

C. the cardboard file box contains the plate transformer I use with this linear amplifier (now running as a Class C plate modulated transmitter... with about 120 VAC across the primary of the plate transformer, instead of 240 VAC)

D. the 50 Henry, 300 m.a. Heising choke is sitting on top the the cardboard box (covering up the "warning, high voltage" sign)

E. to the left of the Heising choke, on the wooden base, is a row of ten (10) 18 uF 450V electrolytic capacitors, in series; each with a 100k ohm 5 watt balancing resistor across it. The resulting Heising capacitance, 1.8uF, is optimal in conjunction with the 50 Henry choke... given my target of 1600 volts and 300 mA at carrier. I.e. 5400 ohm modulation resistance. This capacitance resonates with the Heising choke at 16.8 Hz. At 16.8 Hz, both the capacitor and the choke have reactances of ~ 5270 ohms.

w4bfs:
hmmmm   hmmmmm  doughnuts

w1vtp:
Stu

I am personally excited about this approach.  The biggest frustration of getting "iron" for an AM station is the lack thereof and now (especially in Ebay) Iron has somehow been touched by the Loadstone and have been turned into gold or some such precious metal.

The building  of an AM modulator based on currently "off the shelf" components open up new possibilities for the AMer.  Good luck on the venture and I'll be following your progress carefully.

Another great approach is the use of commercial reasonably inexpensive AF amplifiers.  The line of AMP you are using offers power all the way up to over 300 watts.  So more exotic amplifier design, should one want to avoid that exercise, could be avoided.

73, Al

WA1GFZ:
Seems to me Steve QIX built a couple solid state modulators using FETs and transformers. I would think it easier to do than the class H.
All you need is a phase splitter FET driving two banks of output devices.
A bias supply and your in business. This way you can put the stereo amp back in the living room. gfz

AB2EZ:
Frank

I wonder if I opened up the Samson 2-channel amp...

Would I find:

"a phase splitter FET driving two banks of output devices"  :o
 
Stu

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