PA Amp Modulator

[w5gnb]

I have a question for the group.  I have a Bogen M60A mono pa amplifier (60 Watts) and would like to use it to plate modulate a Eico 720 transmitter.  The output transformer in the Bogen is set up for speakers of 4,8, 16 ohms and also balanced outputs for 10 and 82 ohms.
Can I use the existing output transformer on the Bogen or should I replace it with something else ?? I have a modulation transformer from an old Heathkit DX-100 that possibly could be fitted.....

Gary - W5GNB

[KA2DZT]

You're always better off with the mod xfmr.  The output xfmr in the Bogen PA amp is design for constant voltage output 25 volt and 70 volt speaker lines.

The step down ratio would be too much to get any real modulation.  Also that secondary probably will not handle any real high voltage.  Go with the mod xfmr.

My comments are for a tube PA amp.  I have the Bogen 60 watt tube amp but don't use it for anything.

Fred

[Opcom]

The Bogen MO series are known for their good fidelity.

[Pete, WA2CWA]

The Bogen M60A has a number of feedback connections coming off the output transformer that would have to be dealt with if you're going to replace it with a modulation transformer.

[w5gnb]

Thanks for the Info guys.  I had a hunch the existing transformer might not be quite suitable for AM Modulating and the feedback winding is also a concern since a the replacement transformer does not have that feature.
How about this..... Taking a modulation transformer and adding it externally at the output of the Bogen transformer, and then using the external transformer to modulate the transmitter....  Would that work ?

[KA2DZT]

It would work but you need to match the load impedances between the highest impedance (70 volt terminals) on the Bogen amp to the mod xfmr.  Usually, you would do better using another output xfmr (if you have one).  Not every set-up is perfect,  you can try using whatever you have and see how it works.

Fred

[Donnie SWL]

I have a bogen amp not sure the numbers but it used 2 6550 tubes in it i took the transformer out of it and  put it in a apache transmitter backwards so to speak i use a solid state 150 watt to drive the the 8 ohm tap....i had to use a couple capacitors between the ss amp and the tranny to make the ss amp happy....it has a great sound an has been running like that for 3 years now.......but i'm sure their are people on here that know better than me...

[Opcom]

If you do not mind outboarding, you could leave the existing transformer with its feedback windings there, and attach a separate modulation transformer from plate to plate of the Bogen amp. The Bogen would not have to be hacked up, plus you could use one of the original output windings for monitoring or for tests because its waveform should be substantially like that at the modulation transformer's secondary.

[flintstone mop]

OR you could try to get the modulation transformer from an Eico 730 modulator and that would be a fair-to-good match to the Bogen finals and would definitely match the Eico 720 RF PA. The feeback circuits could be gotten from the phase splitter circuit in the Bogen OR get a little audio from the PA windings of the mod circuits back to the preamp of the Bogen.

Look at this Folks::::::Might be some nuggets in the following link
YUP definitely a little tap of audio for negative feedback


http://www.amwindow.org/misc/gif/eico730.gif

Scheezo of the Eico modulator ckt.

Fred

[N2DTS]

Trade/sell your amp and get a 730 modulator.
They can sound fantastic.

[WD8BIL]

Find a 40W output transformer from another amp (or buy one from Antique Electronins for $40) and "run it backwards".

Insert the primary into the B+line of the transmitter and drive the 8 ohm winding with the Bogen.

Been done by many on this board.

[KB2WIG]

G,

 Go commando.

Tim has used a power transformer for the mod transformer.

Do it for the children.

klc

[WD8BIL]

How to Make a Mod Transformer

Here's the Tron's article.

[flintstone mop]

How to Make a Mod Transformer

Here's the Tron's article.

YUP I did it for an Elmac AF67?? worked like a charm. Amazing match. You will have to pay attention to the detail about phasing the windings, but it works....GREAT!!
Fred

[N2DTS]

A LOOOOOONG time ago, Eric, wb2cau used a filament transformer to modulate a bigger rig, I forget what it was, but he used a tube HIFI amp. He worked for Harmon kardon I think.
This was about 30 years ago?
He ALWAYS sounded really good, even back then.