[HomeBrew] Transmitter design concerns/help

[N2EY@aol.com]

In a message dated 7/22/02 11:34:24 AM Eastern Daylight Time, 
catman351@yahoo.com writes:

> All:
>  
>  I'm currently building a CW (AM capable with modulator) 2 tube transmitter 
> that I'd like to have generate 50-60 watts between 160-10 meters. Just 
wanted 
> to know if the following is possible or should I know about any particular 
> problems:
>  
>  Design is based partly on WA6VVL's dual 1625 xmtr with a 6AG7 osc. Instead 
> of the 1625's, I'm opting for a single 6146. 

So far so good. 807/1625 can get squirrely above 15 MHz or so, and the drive 
requirement of a 6146 is lower. But a 6146 is only rated 90 W input on CW and 
67 W input on plate-modulated AM. 

> Most of the tank circuit parts 
> came from a Bendix Marine transmitter that originally used a 6146 variant 
and 
> produced 90 watts out. Tank coil is 1.75" in diamter, 2.75" long at 15 tpi. 
> Plate cap is a variable Cardwell at about 350 pf and the load cap will be a 
> triple 500 pf broadcast type unit. Bandswitch will be a 6 position BC-375 
> unit acquired from Fair Radio a few years back.

Before making an ironclad commitment to that, figure out the needed pi-net L 
and C for each band. Recall that the 350 pF Cardwell may have significant 
minimum C.

> The driver circuit will be an 
> xtal/VFO switchable 6AG7 unit. Here are my questions. 
>  
>  -I have a 600 VCT, 250 mA xfmr I also bought a few years back from Antique 
> ES. Can I full wave bridge the secondary to produce 600 VDC while I bring 
out 
> the center tap to a cap/inductor circuit to provide 300 VDC for the lower 
> voltage circuits? I suspect that between the amp and the osc/driver, I 
> shouldn't be "railing" the power supply's 250 mA capability. Sounds ok? 

If the transformer delivers 600 volts AC to the rectifiers, you won't get 
600/300 volts DC. Using silicon rectifiers and a capacitor input filter, the 
voltages will be on the order of 840/420 volts DC no load, and somewhat less 
under full load (depends on how big C is and the winding resistances). With 
silicon rectifiers and a choke input filter, the voltages will be on the 
order of 540/270 volts. Tube rectifiers will deliver a bit less, depending on 
the rectifier type, but they add other problems. 

The 250 mA current rating is fine for CW, since a single 6146 will only need 
about 170-180 mils and the load is intermittent  and low duty cycle. AM is 
100% duty cycle, so it may be inadequate. What matters most is the total VA 
required.
>  
>  -Position of the coil. I'm trying real hard to keep the bandswitch as 
close 
> to the tank coil as possible. I think I can get it within 3" at the 
farthers 
> point with the nearest point having the coil and the switch almost touching 
> each other. Any problems here? 

Consider using two bandswitches, positioned for shortest leads. At the very 
least, shield the coils from each other and arrange for the switch shaft to 
be grounded where it passes through the shield.
>  
>  -6AG7 circuits should be shielded/enclosed from causing any interaction 
with 
> the PA circuits. Am I correct in this assumption? 

Absolutely. Also, be sure to neutralize the 6146. 
>  
>  -The 1962 ARRL handbook shows the 6146 with a 10 pf feedback cap for 
> neutralization. How necessary is this circuit (as I haven't seen EFJ 
Rangers 
> or some other transmitters with a neuralization circuit)? Let me know on 
this!

Some rigs will work without neutralization because:

1) they load the grid circuit with resistance (wastes drive)
2) they reduce drive by detuning (wastes drive and reduces rejection of 
unwanted spurs)
3) they used a tube and layout that happened to have low feedback (they got 
lucky)
4) the final doubles on some bands
5) they used a low-impedance driver circuit so that the grid was effectively 
loaded
6) some combination of the above.

Allow for neutralization from the start. Much easier than trying to add it.

Note that many unneutralized rigs will exhibit reaction between the grid and 
plate circuits that complicates tuneup (you adjust for X mils grid current, 
but when you dip the plate the grid current goes all over the place, so you 
readjust the grid tuning, but that upsets the loading...you get the idea.)
> 
>  -Will I have enough "juice" from this transmitter to add on an additional 
> outboard p-p 6L6 driven by a 6SN7/6SL7 plate modulation set up? 

I wouldn't try to use the same power supply for both the rig and the 
modulator.
>  
>  Let me know and thanks. Any other questions not addressed, please let me 
> know. Cal, N6KYR.

Good luck!

73 de Jim, N2EY