[HomeBrew] 6AG7/6146 xmtr progress

[N2EY@aol.com]

In a message dated 7/24/02 5:11:29 PM Eastern Daylight Time, 
catman351@yahoo.com writes:

> Been putting some time into the project while I read your comments. Good 
> stuff here! So far, I took note on the notes that my xfmr choice may be 
> marginal. I also had a 800 VCT, 465 mA xfmr laying around that was about 
the 
> same girth as the 600 VCT, 250 mA xfmr. I'm going to use that instead. 

Oh mama, that's some nice iron!

> Additionally, I'm not going to bridge rectify the transformer. Instead, 
I'll 
> do the traditional method of full wave rectification by taking a diode of 
> each "high" leg of the transformer and series them together at the cathode 
> side of the diodes to the L/C  filter.

You mean parallel them, I think.

> I may do a variation of the Johnson 
> Courier method of putting he 12 Hy choke on the center tap of the xfmr and 
> ground. In that way, I'll reduce the h.v. tension on the choke. 

Not really a concern at the voltages you are working with.
>  
>  I just built the 6AG7 osc. chassis.  I'm using the osc coil from a Viking 
1 
> parts rig combined with  a 100 or 140 pf transmitting cap. Feed through 
"caps"
>  will be homebrew (I'm using #10 compressed fiber washers, 2 soldering 
lugs, 
> and a #10 bolt to pipe through the DC, filament, and RF output (this is 
o.k. 
> to do, right?)

Sure. But these are just feedthroughs, not capacitors. You need some disc 
ceramic bypasses on the inside of the 6AG7 subchassis. Bypass the non-signal 
leads.
>  
>  I'm going to have to mount four more things to the chassis to make it 
> complete: a key jack to key the 6AG7, an xtal holder, and a switch to 
select 
> between the xtal and an outboard VFO. 

Yup. Be sure that the switch shorts out the cathode RFC in the 6AG7 circuit 
(you're using the "grid-plate" circuit there, right?)
>  
>  Tank circuit...well, someone commented that I was using a broadcast cap. 
> That is true for the loading circuit. Traditionally, they have been the 
> source for loading caps in the past. I don't think I'm using a broadcast 
cap 
> for the plate circuit (unless the Cardwell cap I'm using was used as such 
and 
> it doesn't appear to be a multi section receiver cap with all these healthy 
> looking insulation on it and the large thick plates). Additionally, I 
decided 
> to limit my  operations from 160-20. I'm not going to go 15 or 10 meters (I'
> ll let my Ranger do the talking there). Hopefully, with the reduced amount 
of 
> voltage I'm going to run on the power supply and with my selection of bands 
> not including 10-15,  the tank circuit problems discussed will be reduced. 
I 
> took a look at some of the other designs and also took a look at mine 
(again).
>  I can get the farthest point of the coil to be within 2" of the 
bandswitch. 
> The coil is 90 degress out of line with the bandswitch. Also, I forgot that 
a 
> neutralization cap could be two wires twisted together. 

OK, but I'd try something other that two wires for the neut cap.
>  
>  Back to the power supply: I suspect with the reduced voltage (at 400 x 
1.414 
> rms) or now 560 VDC (maybe less with the choke), I'll be running on the 
order 
> of around 50 watts or less.

Hold on a sec - there's some confusion. I apologize if you already know the 
following.

You will only get 560 volts out of that transformer if you use a 
capacitor-input filter. With a choke-input filter, you'll get about 360 
volts. The Courier uses a choke input filter. 

>Additionally, I can now run the 6L6 plate 
> modulation circuit with this supply. I'll have to figure a power supply 
> voltage divider so I can get useful voltage for the 6AG7 driver and 6146 
> screen circuits. 

Consider building the power supply as a completely separate unit. It will 
save space and weight, and can be used with other projects.

Here's another idea:

With choke input, your supply will deliver about 350-360 volts at well over 
400 mA. If you use two tubes in the final, you can get 70 to 100 watts input 
at that low plate voltage, and the 6AG7 can operate at the same plate 
voltage. Suitable tubes for such a rig (listed in order of my own preference) 
include the 6DQ6, 6BG6, 6L6, 1625/807 and many others.  A pair of 6DQ6s or 
6BG6s can be loaded to 240 mA at 350 volts, for an input of 84 watts and 
output about 55-60 watts. At such low plate voltage you can use broadcast 
variables for all uses in the pinet. 
>  
Main problem is the need for a suitable power supply choke and bleeder 
resistor(s)

Such a setup will leave over 200 mA (70 watts) for the modulator, which 
should be enough to generate the necessary audio from a pair of the same 
tubes listed above.

73 de Jim, N2EY