B&W 850A and working Q

[KC9LKE]

Well… either I’m close to right or you all will start the weekend with a good laugh.

I to am starting to build the 813x813 transmitter and picked up an 850A for use with it.

Looking into various pi net tables and calculations I see a disconnection of the 850’s L values for a given plate load. 80, 40 are together, high plate load, and 20,15,&10 are grouped at a lower plate load.

 I also understand that the 850A is a “do all” design and I’m free to move the taps if I wish.

My main concern is with the 10M coil, no taps. IIRC Its stated to be 0.8 uH and I also measured it at 0.8. If I add an additional 0.8uH to the 10M coil then all the taps tend to group closer for a given plate load and a Q of 12. This is on paper.

I don’t have every thing in front of me so I’ll make some general statements….

Using a pair of 813’s, in class C they’ll have a plate load around 3300ohms for legal limit, at 80M lets say, and the L values for 80 and 40 are OK for a Q of 12.

The 813’s max rated frequency is stated as 30 Mhz, and I’m aware of the 813’s high plate capacitance.

What I expect will happen:

At higher frequencies the 813’s efficiency will begin to suffer so to maintain the same power output they will have to be reloaded at a higher plate current, effectively reducing the needed plate load. Then the 850’s L values for the higher bands will be closer to the correct value while maintaining a Q of 12.

Correct???
OR
“just go buy a rice box kid”  

Thanks
Ted / KC9LKE  

[WA1GFZ]

850A= 13.6,6.5, 1.75, 1.0, .8 uh
When you lay out the tank circuit set it up so you can move the plate tuning cap connection up the 10 meter coil. Come up with a clamp so you don't have to trash the strap inductor with solder. A bit of inductance between the tube and pi network will help you resonate on 10 meters and still have a good Q. It won't hurt the other bands if you use nice short direct connections. You want to avoid a long lead to the plate tuning cap. You also need a plate tuning cap with low minimum C. I suspect the lower inductances on 20, 15, and 10 allow for strays adding L to the circuit.

[KC9LKE]

Frank:

Glad You chimed in on this.

I plan to post the layout for any advice before I drill any holes.

I understand about adding some L to reduce the plate C.
I remember You mentioning it before.
If we are on the same page it goes before the blocking cap.
I did the calculations sometime back, but will post at a later date.

Thanks
Ted / KC9LKE

[WA1GFZ]

No not before the blocking cap. This will make the 10 meter inductor too large. Just tap the plate tuning cap 2 or 3 turns from the plate blocker connections to the inductor. IF you have short leads the other bands will be fine. You could put it before the blocker if you make the 10 meter coil smaller.
Might be better to change out the inductor and wind something with 1/4 inch copper tubing so you can dial it in. Then once you know what you need  reinstall the 10 meter coil with required change so it doesn't get bunged up.

[KM1H]

Tapping the 10M coil at mid point with the Tune cap wil give a step up ratio of 4X the required C value.

Its an old 40's trick used by commercial SW transmitters using high output C tubes. Adding an additional coil just complicates things.

You will also see many 850A's with the 10M coil missing 2 turns.

Carl
KM1H

[KC9LKE]

OK Frank & Carl.

Actually I have two 850’s, one is buggered up pretty bad and missing the 10M coil.
I’ll give it a try.

Also for others reading this….I searched the board this morning and found your replies to W7SOE’s 850A thread.

Thanks for the info.

Ted / KC9LKE