Guitar amplifier as a low-level microphone driver

[ka1tdq]

Hello

I'm building another AM transmitter and this design will be a single 4-65A being plate modulated by a pair of 6CA7's for 50 watts carrier.  I've got the RF side of things down but need a little help on the modulator section.

I'm going to use a push-pull audio output transformer on the grids of the 6CA7's.  My question is what to use to drive the 8-ohm load.  I was thinking about buying a cheap 7 watt solid state guitar amplifier and using that.  I know guitar amplifiers distort by design, but the particular one I'm looking at (Peavey Audition) has an "Overdrive" control which I'm guessing is a distortion control.  I could just cut that back to zero.

I don't need much power to drive the 6CA7's so that's why I was looking at a cheap guitar amplifier.  Most audio amps on the market today are 100+ watts.

Any and all input is much appreciated! 
Thanks!

Jon KA1TDQ

[WD8BIL]

Hi John,

That'll work great. I've got the sister amp to that Peavey, Rage8, and it's pretty clean when used without the overdrive.

[KC4VWU]

I was thinking around the same lines but just using a big arse guitar amp and another output tranny in reverse. I have two real deal 50w Marshall heads, a 65w Peavey combo, and a few smaller solid state amps. Only problem is I was figuring on a pair of 4-65A's in parallel. I think I'll need at least a 100w amp and that may be leaving no headroom.

Phil

[KX5JT]

KN4ME, Dr. John in Alabama uses a Peavey guitar amplifier to drive push pull/parallel 813's modulating a 4-1000.  He puts out a great sounding legal limit signal.

[ka1tdq]

Thank you for the input everyone!  Wow, a guitar amplifier doing a pair of 813's... I guess my setup will work also but it's just a scaled down version of his. 

[W3RSW]

Doesn't Radio Shack still have 25 to 35 watt PA amps with 3 to 5 EQ bands, 3 or 4 mike inputs, 70 v to 8 ohm outputs, etc.?

My model MPA-45 has 3 EQ's and is used to drive 811's, 572B's through the backwards AF output xformer trick.  Used to modulate single 813, twin 813's, etc.

Yeah, your looking at too much audio for your 4-65 application, but a 3db pad and careful gain riding from a small PA amp should drive your EL34's nicely.

And yes, you are borderline.  Just get a slightly larger SS PA amp and modulate the 4-65 directly through a larger 'AF output' xformer.  But I guess audio xfor's might not be able to handle the 4-65 plate voltage long term, so just put the 'mod' tranny on insulators.

[ka1tdq]

I've looked at Radio Shack and they only offer larger models now, from what I've seen anyway. 

I still want to stick with modulating the 4-65A with a pair of 6CA7's, just because I want to.  I already bought a good modulation transformer off of ebay and I'm still going to use a Heising circuit after that.  My last transmitter had an all solid-state audio section, but it was really boring.  I want to be able to get on the air and say, "Yeah, I'm running a 4-65A modulated by a pair of 6CA7's." 

Thanks for the tip on the attenuator pad... I'll do that. 

Jon KA1TDQ

[W1RKW]

Use an LM1875 IC audio amp. You won't have an mic preamp which you'll need to drive it at line level but LM1875 will drive the audio transformer and grids of the 6CA7's with plenty of headroom.  The LM1875 is rated for 20 plus watts depending on the rail supply. I'm pushing it to 30w.  I use it in my 813 modulator for the 813 rig. There's plenty of headroom in the audio. Very simple to build. Use National Semi's circuit. You'll be good to go if you feed it with a line level source. One caveat. It's an unbalanced input but I don't have any RF problems with it. I do convert the input from unbalanced to balanced right at the input.  Another plus is the foot print is about 6 sq. inches.

[KE6DF]

"I've looked at Radio Shack and they only offer larger models now, from what I've seen anyway. "

Here is one that might work:

http://www.radioshack.com/product/index.jsp?productId=2103829

It's 40 watts.

It has the usual speaker outputs, but also it has a 70 line volt output.

It might be possible to drive high level 500 ohm line to grid transformers like UTC 59AX by using the 70v output.

I don't know if that would work, but seems like it might.

[ka1tdq]

I already purchased the guitar amp last night and it sounds good through a speaker using a Heil microphone.  I'm going to get a hifi Hammond PP audio output transformer to drive the grids of the 6CA7's.

I've drawn up the final schematic for the audio section tonight and I'm going to use the 6CA7's in triode mode by connecting the screen to the plate with a 100 ohm resistor.  Follow up the modulation transformer with a Heising circuit and this should sound p-r-e-t-t-y gosh darn good.  I'll even throw on a negative peak limiter on the Heising choke so I can go over 100%. 

Thank you AM window!  :-)

Jon KA1TDQ

[W3RSW]

Try it without the neg. peak limiter first.  Look at your modulated output on a scope, get a feel for things.  See how you and others like the fidelity.  Then lash in the neg. peak limiter and check for distortion by monitoring yourself and by others.

Smaller AM rigs (well all of them really) benefit greatly from learning best settings.
You'll find if the power supply and AF amps have head room at lower frequencies and if you've set mike and following throughput phasing correctly, that you'lll easily get 125 to 130% or greater positive modulation with very little neg. mod. over 100% with a normal, assymetrical voice.

 All this without the grunge that neg. peak limiters may produce.  Your already more than halfway to excellent audio by using Heising coupling.

If you change rigs and mikes a lot and generally are in a hurry, then a neg. peak limiter might be the way to go.

[k4kyv]

I have noticed  that, at least with my voice, the vast majority of the envelope waveform modulates to about the 60% range.  Just a few, very thin peaks extend up to 100% or beyond, but even with the broadcast peak limiting amplifier I can't completely get  rid of them.  Those are the ones you can shear off by using limiting or clipping, without generating grunge.  Ideally, the peak limiter would be activated by real time audio, but the limiting action performed on the audio after a slight delay.  That would eliminate the inevitable overshoot that occurs with real time limiting, without generating the harmonic distortion caused by hard clipping.

Those short-duration high amplitude peaks that get past the peak limiter impose a cap on the average percent modulation of the waveforms that make up most of the voice envelope, if you want to avoid overmodulation.  But you want to avoid clipping or limiting the main body of the waveform envelope.

[ka1tdq]

I had used a Heising circuit on my last homebrew AM rig along with an overmodulation meter ( a mA meter that monitored negative peaks).  It did deflect when I pumped too much audio into the finals.  I installed a negative peak limiter using zeners and a diode and the meter never deflected after that.  I also got great audio reports with the limiter installed. 

For this rig, I'll try it without the limiter first (like you said).  I have to anyway to find out what my modulated B+ values is.  And I'll see what it sounds like. 

I will say though that for this rig (I've re-done the numbers for 70 watts carrier) it's going to be SUPER heavy with a metric butt-ton of iron... all for just 70 watts!  But what the heck!