"PW" transmitter

[k7mdo]

K1JJ's little "PW" transmitter has come to life in Canby Oregon!  I got the itch to build a small AM transmitter for very local communication with another ham down the street from me and I selected Tom's circuit as a good starting point.

I did change a few things, some notable.  Mostly it worked from the first switch on!

I found a couple of hitches but they were easily overcome.  One thing was that the power supply as depicted seemed to have too much hum so once that was solved the next challenge was too much gain in the audio amplifier stages.  I reduced the gain by 5 fold and that solved that....  but mostly it was probably my fault for changing the modulator to incorporate "parallel" 6AQ5's instead of the original designed single tube. 

I cooked a good tube before I had those hurdles behind me!

Then I ran out of diodes for the cathode bias!  Tom had 30 or so and I only had 25 or so and figured I would need at least 15-20 more to stay within the limits of the tubes.... so, I subbed in a 400 ohm resistor in the series... seems to work fine with that.

I get about 4.5 watts max and very even modulation +/- on the REA monitor.... though you can push it a bit by lowering the power output down to 2 or so watts.

My first contact (1/2 mile away) found me at 20 over S9 with very clear speech!

As time permits I will try some of the more distant guys (out to 50 miles?)

Anyway, great project, with good results.

73, Tom

[W7TFO]

That is a cute lil' devil...

73DG

[K1JJ]

Yep, Tom did a nice job on his version of the rig.  

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

As he mentioned, the  HV power supply needs extra filtering than depicted. Make it a C-L-C filter and the hum will disappear.

Also, using two paralleled 6AQ5 modulators (class A) makes for longer life and no plate color /  strain.  The single final 6AQ5 is class C, so can remain as is. The RF section uses just one tube - as simple as it gets. (xtal controlled power oscillator)

That little rig will do 200% modulation with very clean audio peaks - and hi-fi flat. The VXO circuit will swing a xtal up or down 3Khz or more.

Tom, you might post some of the pics you sent to me of the underside and other shots. Your's is a very nicely constructed rig.  I still have mine after 20 years - ready to go.

T

[k7mdo]

I will post a couple of "progress" pictures taken during construction.  Almost every part came from my "extensive" parts treasures chest.... better known by the wife as a "junk box"!

Thanks again for the great little rig idea.

73, Tom

[Steve - K4HX]

Nice PW rig! And that mod tranny is strapping.

I wonder if a 6BQ5 could replace the two 6AQ5s?