[HomeBrew] Need help with AM BCST hb tube radio

[Steven Weber]

Carl,

>-The way I'd like to do this is to inject the DDS output into the first RF
amp/mixer. Can I do this at the cathode if I use a tranformer to do this?
The approximate Z of the DDS VFO is 2.5 kohms. Also, how much signal should
be used (i.e. what is the ratio of the input signal to the DDS VFO output)? 
>

You'd probably be better off feeding the VFO into the first grid instead of
the cathode. Typically, the first grid and cathode are used as the VFO in a
 VFO/Mixer tube such as the 6BE6, so that grid would be available to inject
the DDS VFO into. As for level, you'll need at least 5 volts p-p. Maybe
more. Since the input impedance of the tube is very high, (set primary by
the grid resistor you use), you can make a transformer on a ferrite core to
step up what ever level the DDS VFO gives you and add isolation. From the
Handbook, voltage ratio of a transformer is Es = Ep (Ns/Np) You can do the
algerbra from there :-) You may want to add a simple emitter follower
buffer amp to the output of the DDS to drive the transfromer though. 

I've used a step up transfomer with my old AD7008 DDS VFO to get enough
voltage to drive a tube transmitter in the past with good success.  

>-Where can I get a source for tube style IF cans (at 455 kc, of course). 
>
Try Antique Radio supply in Az. Or find an old all american 5 kitchen radio
to salvage parts out of. Be aware though, that often the capacitors in the
IF cans go bad. I've fixed a number of old tube sets where I had to rip out
the silver plated mica caps (which are not encased in anyway) and replace
them with something else. 

>-Would it be possible to use this same circuit (if I changed the DDS VFO
freq output) to cover 160 meters? I'd like to use the radio for both. 
>

Sure, just have to retune the input. Depending on the range of your DDS,
you could tune the whole HF band. Just a matter of some band switching and
tweaking the tuning cap. 
72,
Steve, KD1JV
"Melt Solder"
White Mountains of New Hampshire
http://www.qsl.net/kd1jv/