[HomeBrew] Need info/suggestions

[N2EY at aol.com]

In a message dated 7/13/05 7:48:14 AM Eastern Daylight Time, 
n6kyr at netzero.net writes:


> I'm in the process of building a 2 tube cw transmitter (6AG7 with an 
> OD3/VR-150 reg on the screen). 

I'd call that a 1 tube transmitter. Rectifiers and VRs aren't usually 
counted.


It's going to be s/s on the rectifier, using an 80 uF/450 VDC filter (no 
> choke,sorry). 

The single cap should provide adequate filtering for the job.


I have recently gotten a hold of a Stancor transformer rated at 440 VAC CT at 
50 
> mA with a 6.3VAC/.5 A filament and a 25.2 VAC 100 mA xtra winding (perhaps 
> a biasing winding). 


hmmmm - are you sure those are the ratings? The heater windings seem very 
light. I used to have a pair of Stancors rated similarly but with heavier heater 
windings. Nearest I could guess was that they were tailor-made to be an AC 
supply for an unmodified Command rx using a 6X4 or 6X5 rectifier.

What's the number on the transformer? If it's a PC-8417, the heater windings 
are rated 6.3 V @ 0.6 A and 25.2 V @ 0.5 A. 

> 
> First off, on the transformer: I think it is good enough to handle the 6AG7 
> current demands. Does this sound right?
> 

No question on the plate side, it's adequate. The 6AG7 heater needs 0.65 A, 
which is a bit above the rating but since the 25.2 volt winding won't be used I 
think it will be OK.

With a capacitor filter, fullwave center tap rectifier using SS diodes, you 
should get about 300 volts B+, which is just about right. 


> Also, I would like to place on the screen of the 6AG7 the VR-150 tube to 
> stabilize the voltage "behind" the 47K screen supply resistor. Good enough to 
> pipe down "the chirps?"

Maybe - chirp can come from a lot of sources. Won't hurt. I've used a lot of 
VR tubes in homebrew gear and had no problem *if* the circuit is properly 
designed. 

There are six VR tubes of various types in the power supplies of the 
Southgate Type 6 and Type 7. All were obtained used. No more problems than any other 
tube. 

If you use a VR, the screen dropping resistor has to be changed from the 
original 47K value. I'd use 15,000 ohms, 3 watts as a starting point. 


> 
> Finally, on the placement of the crystal socket, I may be placing the socket 
> near some AC sources (about an 1" or so away from the pilot lamp and power 
> switch). Now I shouldn't expect any hum pickup now, right? 

Probably not. 

Let me know as I haven't punched the 7" by 5" by 2" chassis yet. I'd like to 
get 
> any lingering tech issues out of the way before "Mr. Greenlee comes 
> punching." 

If this is your first GB project or first GB tx, I'd recommend a larger 
chassis. Not because you need the space for the rig you envision, but because it 
will be a lot easier to work on, both when you're building it and in the future 
if you decide to modify it or add features. 

One advantage the Ancient Ones had that we don't is that they could usually 
get exact replacement parts. We have to work with what we can get, and a 
replacement may be bigger or differently shaped than the original. 

73 de Jim, N2EY