The Beginnings of HB Project?

[KC4VWU]

Here is the start on the RF deck...

[N2DTS]

All the parts look too small to me.
What are you going to do with the rig?

The RF parts look good for 200 watts, and you can get 700 watts carrier out of a pair of 813's....

Brett

[KC4VWU]

I don't plan to exceed 2KV Ep and that is what I figured it for, but your right. After I actually checked the C/tune air gap, I found out it was too small for that; it's only 3/16" and needs to be at least 1/4" for that voltage. I could reduce the plate voltage to 1.5KV, but then I don't have enough C. The tank coil looks like #14 and I could bump that up to #12-#10, but it would have to be hand wound; that's not a problem though. The C load will probably be too small of an air gap also. Trying to juggle all the figures and finding the necessary parts is a lot harder than I realized.
I'm not looking to build a powerhouse on my first time out. I just want to be sure I'm doing all the figuring correctly and have good construction practices.
I'm basing all my calculations on info from the Bill Orr handbook. So, to figure plate load resistance, I got my starting figures from the RCA handbook:

For 813

ICAS Ep=2000v
Ip= 200mA   x2 tubes=400mA
For RF plate load resistance
ohms=Ep/2Ip

=2000v/.800mA
=2500 ohms

Am I on track here, or have I goofed up?

Phil

[N2DTS]

Your calculations are ok, but with small parts you get arc over and melted coils.
I run my 813's at 2000 volts and 400 ma, they do that all day long, but with mod peaks you can get 5/6 KV across parts with the extreme audio peaks.
For loading caps, you cant have enough, 2000 pf is a place to start, and I always disliked fixed caps switched in.

For the plate tuning cap, I think Bill Orr lists 4x the plate voltage, which is a place to start, but more is better.

I suppose you could run 1500 volts and 300ma, or any combination, but must resist the urge to crank it up..

When I build, I always overbuild, things last that way.

Its easy to wind a coil with thick copper wire, or tubing, solder on some terminals and mount it on insulators, or install bannana jacks and plugs and switch coils for bands. The grid is easy to make multi band, but the pie net handles a lot of power and voltage, and its nice to not have big switches and shorted coil turns to cause problems and lower Q.

Yes, parts are getting hard to find, and sometimes you have to go with what you can get, or make better parts yourself. You can make all kinds of coils, wind enameled wire on PVC, I did that with my 813 rig, seperate coils and links 160 to 10, you can make big coils also, big caps would be tough.....

What is the modulator going to be?

Brett

 

[N2DTS]

Also, I would not have made a hole in the front panel, with 813's there should be nothing to see, no plates glowing, and a lot of RF will come out and infect your microphone.....

Brett

[KB2WIG]

The tubes look a bit close fer my tastes....

klc

[KC4VWU]

O.K., nothing is written in stone at this point. I still have more parts stashed that I'll have to go through. I could use the front panel for the mod deck, use a different tube or combination of tubes for RF, or totally bag both and start over. Let me give a run down on the tubes I have on hand, that's probably a good starting point to decide from.


4-400 QTY-3
4-125 QTY-6
4-65   QTY-2
813    QTY-10
304TH QTY-1
572B   QTY-2
811A   QTY-10

I'm going to include the triodes. Since this will never be more than a 75/40M Tx, I wouldn't imagine that neutralization would be much of an issue.
The largest plate iron I have is the Stancor from a BC-610. Remember, this Tx doesn't have to make max smoke; I'm sure I'll build another pretty soon. I'm not going to really worry about the modulator for now: I'm just going to concentrate on the RF deck and the power supply.
This is basically just a learning experience, and a chance to hone my layout and construction skills. so that when it is time to build "The Ultimate", I'll be 100% sure I'm doing it right.
Phil

[N2DTS]

Phil,
This is the fun part!
Makes you think, and its fun.

I might use 813's modulated by 813's, 813's are very rugged and problem free.
A pair of 811a's at 1500 volts would be a bit light for a pair of 813's but would be simple, run the entire rig at 1500 volts, you would be around 12.4k ohms and 340 watts out into 3750 ohms for 600 watts input.

813's as modulators at 2000 volts would be 16k and 335 watts out, but in AB1 (no driving power).

Part of the design must reflect what parts you have or can make.

If you had killowatt plug in coils for push pull, 811a's (or 812a's)  modulated by 811a's works real well and is very simple for 300 watts out. If using 811a's, you need no bias supply for the modulator or rf deck, no screen supplies, just a grid leak resistor....

Mod transformers you can get might also restrict your choices.

I have a stanley mod trans 8000 ohm to 2800 ohm 300ma trans (with screen winding),
a thordarson multi match 300 watt (T11M77), a freed 20,300 ohm to 9100 ohm 250 watt job, and an old bc610 mod trans 16k to 8325...
I could trade  any of those for something...

Brett




   

[KM1H]

Sink those tube sockets so the internal grid lead is below deck otherwise it most likely will need neutralizing. Its also a good idea to use sockets with the bayonet base and then ground the shell. The sockets you have are OK for GG or audio.

A pair of 572B's or 4-125A's will modulate it well.

Id stick with that Tune cap for now, see how it performs. Ive had no arcing in a TX with a single 2 KV 813 at the same spacing. A lot depends on how well you do the pi-net impedance transformation. The ART-13's dont arc either when the voltage is cranked up.

Carl
KM1H

[N2DTS]

I dont think you need to do anything with the sockets at 80 and 40 meters....

Here is another neat window idea....dont make holes in the front panel, but mount the tubes (modulator) on a small seperate chassis, mount the small chassis with the tube sockets on it on the top of the modulator deck front panel, then install a piece of glass in the rack/cabinet in front of the tubes.

That way you dont have to cut holes in panels, a piece of glass is about $4.00, and it looks sharp.
I use screw terminal strips to connect the tubes to the mod deck.

For the RF deck, I would rather try and shield it, or at least not have big holes to let the RF out, although in many cases it does not seem to cause problems in the shack.
It MIGHT allow harmonics out (pre pie net) and into your TV's and so on....

Brett

[KM1H]

I dont think you need to do anything with the sockets at 80 and 40 meters

It never hurts to do a quality build instead of a schlock one. Now it will be ready for other bands if this builder or another wants. Besides 40M can get squirrely also with a nice high gain pair of tubes.

Carl
KM1H