A Series Modulated Tube Rig

[ka1tdq]

Back in the day, I made a CW rig using two 6L6's in the final.  No tube plates were exposed, so I built it so that all the tubes were on top of the chassis.  I really like how the transmitter looked and I'm thinking about building something similar.  This time, it'll be a series modulated rig with a 5763 driving a 7984 compactron. 

I was looking at this design on the AMwindow for the overall concept:

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

Could I use a 6L6 in place of the 6AQ5 in the modulator? 

The power transformer I have on hand is 476vct (for the high and low voltages).  It also has 12.6vct for the filaments (the 7984 filament would use a different transformer), and it has a 5 volt winding for the DDS VFO to drive the 5763.

I'm just thinking about building this since I already have most of the parts.  It'd be PW, but sound really good.  Did I mention it would look really cool?   

Jon

[Opcom]

Looks great!

[w4bfs]

a series modulated transmitter will give the series pass regulator a real workout since it is linear class A and the rf out tube is class C.

this is an application where it is to your advantage to do a careful paper design around worst case conditions to make sure you get what you want.

what carrier power level for the 7984 ?  how much b+ ?

a solid state series pass will give you most of the swing available.  a tube series pass will drop 60 to 100 volts internally and therefore will need that much more supply B+

you are starting with 476V ct  ... is that either side of ct or total secondary ?

there was a thread here for a solid state series pass modulator done by a W1??? ... looked like it could be useful

[ka1tdq]

476 volts is the total secondary side voltage with 238 as the center tap.

I just did a quick calculation and at best I could achieve 9.5 watts carrier at 100% modulation (less for more modulation) at 2500 ohms load impedance.

476 vdc / 2 = 238 vdc

Iplate @ 2500 ohms and 238 vdc = 57mA (class C)

57mA x 238 vdc x .7 = 9.5 watts

It's a lot of effort to get good sounding AM at QRP.  A single FET blows that out of the water.  I'm just toying with project ideas.  The coolness looking factor is high with this transmitter, but in practicality it'll probably just wind up as a show piece on a shelf and not get actual use.  That's what happened to my 100 watt, 75 meter class E transmitter.  It sits on a shelf behind me.  Why use 100 watts when you can use 400?

I've seen the series solid state modulator thread, and it would work much better than a 6L6 tube modulator.  I just wouldn't have the glass lighting up the chassis.  

...or this could be a 10 meter AM transmitter to be used a few times every 11 years, since high power on 10 isn't needed when the band is open.

Jon

[w4bfs]

ok Jon .... consider a voltage doubler but honoring keeping the xfmr ct at ground potential .... it takes four diodes and three capacitors to do this but even so this is still only about 500V .... you can get to 1kV by doing a quadrupler ... that way will need research if going to keep ct winding at ground potential ..

NEW NOTE a tentative possibility for a quadrupler requires 6 diodes and 5 capacitors and will need to be breadboarded to prove the concept..

 time to start looking for new xfmr

[w4bfs]

ain't Google just wonderful ... voltage multiplier circuit shows what is needed .. to do this voltage quadrupler full wave from the grounded center tapped secondary indicates that 10 diodes and nine capacitors will be needed ... just to be on the safe side