[AMRadio] problems

[Brett Gazdzinski]

Ronnie,
 
> 
> Parallel, straight out of the 64 handbook, the One Band Kilowatt 
> Amplifier, but I put in a B&W 850A tank circuit and some good 
> heavy duty variables, don't really have anything in there for tube 
> balance, what do you suggest?

That design is the same thing that got me started on my 813 rig!
I took some design ideas from it.
Its a great setup.


>  I'm using a separate screen supply, and it will be delivered through 
> the modulation transformer which has a separate screen winding. 
> I'm not using a clamping tube or anything, think I should?

That is a good way to modulate the screens!

I would use a resistor before the mod transformer, so if the 
screen current goes up, more voltage is dropped in the resistor.
Figure dropping something like 50 to 100 volts, and make the power supply
100 volts extra.
That is what Collins did in my 30k-1.

A little variac is great here, as well as a 2.5k 25 watt pot,
but you can use fixed components if you run the deck
at a somewhat fixed point all the time.

For balance, try a small resistance in the individual screen circuits,
and maybe a little in the cathodes or plates.

I had a lot of tubes to choose from, and found a matched set, but
plan on adding some resistance to balance the load.
After all, if one tube has slightly different characteristics,
it may get most of the load.

Plugging in about 15 individual tubes one at a time, I had
plate and screen currents all over the place.
One may give 200 ma, another 100 ma.
If I used both of those tubes together and ran the thing at 400 ma,
One tube could run 250 ma, the other 150 ma...or worse...

I would need to calculate the resistance, voltage drops, power
of the resistor, etc in the plate or cathode, the screen
circuit would be a bit easier.

This is bad news in SSB, but great in class c am operation.
Makes things nice and smooth, more current, more voltage drop
in the resistors, sort of self regulating.

I included a current overload circuit in my screens.
Its a relay with a pot across the coil, above a set current, the relay
closes and connects the screen voltage to a resistor that
holds the current high and the relay closed
(and the voltage off the screens), and also lights up the reset button.
The reset button interrupts the resistor to ground circuit.
Almost any relay will work...

I was also careful to not apply the screen voltage till
AFTER the plate voltage was up to full on the step start.
The step start relay activates the screen voltage relay.

> as stated above, using B&W850A with good air variable,
> will have around 2250 volts and the tranny will deliver an amp
> so I'll have all the current I need. I'm using a separate HV 
> supply for 
> the 805's.

Sounds great!


> What value dropping resistor and where did you place it in your 
> supply?

I forget the value, I think its something like 2500
ohms, a big 50 watt pot from fair radio.
You want it to drop about 50 to 100 volts at the correct screen
current...about 75ma for a pair of 813,s.
1500 ohms would drop 112 volts at 8.5 watts.
A 1500 ohm 25 watt resistor sounds nice.
Or 3 500 ohm 20 watt resistors...

I suppose you could use individual resistors at each screen, say
750 ohms and that would help balance them, but not sure
what it would do to the modulation.
Maybe experiment with a combo of both, say 100 ohms at each
screen, 1200 ohms in series with the power supply...

Isn't this stuff FUN?
A whole bunch of simple circuits adding up to
a complex rig that works well!
I tend to make things so I cant have a brain burn out
and fire up with no antenna and burn something up.
807,s, altzimers, just plain stupid, I am ready for it!

  
Brett
N2DTS

> 73's
> 
> Ronnie
> 
>