Monobloc amplifier use as a modulator

[k7mdo]

I almost hate to exppose my ignorance but I "need" to know something that has puzzled me for some time.

How?, does one employ a simple mono amplifier like a Heathkit WM series as a modulator on some hombrew transmitter?

What is needed in addition..? you can't just use the 4-8-16 ohm output for a plate modulator without having another transformer to step up the impedance back to 6-10,000 ohms .... can you? Or, can you somehow utilize the existing transformer?

Tom 

[N2DTS]

You can do it two ways that I know of, use the 8 ohm to drive an 8 ohm to 5000 ohm transformer to drive modulator grids, or you can change the output transformer to something that allows the rf final B+ to flow through it.
That would be something like a 5000 ohm push pull to 5000 ohm single ended, designed to have DC through it, or not with modified hi zing (choke and cap).

The 8 ohm to grid setup is very common, works well for class B or AB2.

All my modulators at the moment are set up that way, its easy and clean, one mic, one processor, dual channel power amp feeds 2 different rigs with the exact same audio.

WB2CAU used to use some sort of filiment transformer to directly drive the B+ on a rig with a big Harmon Kardon audio amp, and he sounded very good.

[WU2D]

There is a third way that can work if the impedances match up roughly. Basically you leave the output winding open and connect a HV capacitor of a few uF from one of the plates of the audio amplifier P-P primary ( choose the side that puts you in phase with upward modulation) to a Heising Choke feeding the final. Try using the same plate voltage that the amplifier uses or higher. In this way, a 35 Watt class amplifier could likely modulate a single 6146.

[w1vtp]

Here's what I did.  Worked well.  Just a 60 watt Hammond with a 20 Hy, 100 mil reactor and a 2 MFd ( 2 x 4 MFd) as a blocking cap.  The classic, incorrectly named, "modified Heising."

http://amfone.net/Amforum/index.php?topic=16771.0

I'll try to dig up the schematic if you're interested

Al

[W2VW]

Aren't the WM Heath amps Williamson designs with the screen taps? If so you would need those taps on any single mod xfmr scheme.

[flintstone mop]

IF Tom is willing to do major surgery to a Heathkit amp; you could find a mod transformer that will match the Power amp tubes in the Heathkit. Feed the screens with proper voltage dropping resistor. And the secondary will have to match the PA. tube in the transmitter. The existing transformer will not cut it.
I may be missing something with the Williamson design and the screens being tapped from the output transformer, as Dave mentioned.
What are you going to modulate? A single 6146 or a pair? There will be issues for impedance match and the amount of audio power to plate modulate the RF power amplifier. As mentioned earlier a single 6146 would require less audio power.
Daisy chaining two transformers back-to-back to be a modulation transformer would be a waste of time. A lot of WATTS wasted.
NOW to drive a modulator stage already existing in a transmitter with nice audio from a mic preamp and EQ and the OP's Heathkit amp would require a back-to-back arrangement. The second transformer PRIMARY will be towards the modulator grids and the center-tap in the primary will allow the bias voltage to be applied to the tubes.
Get an old Bogen PA amplifier to be the guinea pig for this, unless the Heathkit is going to be the sacrificial experiment.
Interesting thread
Fred

[WA1LGQ]

Tom, my Viking I is modulated by a solid state rack mounted PA amp that feeds the low impedance winding of a rather large audio output transformer that I was lucky to make a trade for years ago. It was made by Olsen. The primary winding is shunt fed in "modified Heising" fashion and it works absolutely great. There probably is some loss in the system but so what?
Take a look at these from the Amwindow.org site:
http://www.amwindow.org/tech/htm/modheising.htm

http://www.amwindow.org/tech/htm/shuntfedmod.htm


.....Larry

[k7mdo]

From the responses it is clearer to me that any direct use of an audio amplifier is just going to need some more iron.  I l just didn't know for sure if I was missing an application just through ignorance of some method unknown to me.

No problem for me as the mono block Heath that I have is currently tagged onto the diode load of my 390a where it works well.

I am building a small HF transmitter and will eventually build a modulator for it but will probably copy the Eico 730 style with some modifications. I have scrounged a 730 modulation transformer from a donor unit and was just trying to reduce my workload for building the rest from scratch.

Thanks for the insights.  Tom