using electric guitar amps as a modulator

[KK4RF]

Has anybody here ever tried to use an electric guitar amplifier as a modulator for an old cw transmitter? I have 5 sons who've collected a bunch of guitar amps, tubes as well as solid state, in the 30-50 watt output range and wondered about using a tube audio output transformer backwards with some filter chokes in a Heising type modulator circuit. I wondered if anybody here had tried that.
     I'm new to this forum. Any opinions on this would be appreciated. Thanks.
                                          ---Marty, KK4RF---

[W3SLK]

Marty said:

Has anybody here ever tried to use an electric guitar amplifier as a modulator for an old cw transmitter? I have 5 sons who've collected a bunch of guitar amps, tubes as well as solid state, in the 30-50 watt output range and wondered about using a tube audio output transformer backwards with some filter chokes in a Heising type modulator circuit. I wondered if anybody here had tried that.

No they won't work! You need to send them all to me immediately! 

Seriously, with some slight modifications, you can make them work as a dandy speech amp. I use a Bogen PA amp to drive my 805's on my HB modulator. Find a UTC multi match xformer and you will have something along the lines as an Eico 730.

[kd4afp]

Hi Marty;

Here's a link to a tutorial by Stephen Ickes, WB3HUZ that shows how to do what you describe:

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

73,

Rick KD4AFP

[w1vtp]

Marty

Here's what I did.  Look for the PDF file and take a look at it.  Yes, you can use an audio xfmr backwards.  Just be careful about the amplifier you use.  I had a so called 50 watt amp and the iron in it was pathetic.  Remember, garbage in, garbage out.

http://amfone.net/Amforum/index.php?topic=16771.msg115834#msg115834

73 es GL Al W1VTP

[KK4RF]

Gentlemen,
     Thanks for all the replies to my question on guitar amps as modulators. We've got 3 tube amps here rated at 30 watts out and 1 solid state unit rated at 50 watts. For the most part, they sit idle here and I've been wondering they could be pressed into service as a modulator. I'd seen the nice article about using Hi-Fi amps as modulators which is what got me to thinking about all these guitar amps around the house here.
     I've been getting on 3885 kHz with a couple different rice-boxes and have been amazed that people actually can hear me on AM with just 25 watts out. I do have an old WRL Globe Scout which I'm buying some crystals for and hope to have up on 3885 in the next couple weeks. I suspect my output from that old tube . rig would be close to my rice-box units. It would be fun to get on AM with an old tube rig! Thanks again to all for answering .   73s, Marty, KK4RF

[N4LTA]

One thing to keep in mind when using a Push Pull output transformer in reverse. A push pull output transformer is not designed to pass DC current and may quickly saturate when used as a modulation transformer. In a push pull output transfomer - the currents  in the core cancel each other. That will not be the case when using it in reverse as a modulation transformer.

I have used one of the heavy Hammond SE series of transformer as a modulation transformer with a solid state amp driving it. The large ones are rated for over 100 MA current.

[WBear2GCR]

actually the thing you need to be most cautious about is the setting of the gain and tone controls, since most guitar amps are designed to distort quite a bit. I'd put it on a scope and look for the best settings - use a square wave at 1khz. and adjust for the best looking flat top you can get, then use a triangle wave at the same tone settings and look at the input level vs. output level for the best gain control settings...

                _-_-bear