B&W 850A

[W7SOE]

I recently purchased a B&W 850A.

It is missing the 10m coil and appears to have been "reconfigured".

It makes no sense the way it is now.  Does anyone have some documentation showing the original configuration? 

Thanks

Rich

[WA1GFZ]

The Coil was 5 turns of strap that went between the 10 meter switch tap to a screw on the other side of the ceramic back plate. The strap was about 1/2 inch wide and maybe .06 thick. The turns were spaced between the two mounting points. Note there is no switch tap in the 80 meter position because you use the whole coil.

[The Slab Bacon]

IIRC there are 2 versions of that inductor. The 850 and 850A (or maybe the 852??)
One of them is for the glass type higher impedance tupes, and the other one is for the ceramic jet engines. The higher z one is definately more inductance. It is wound with a mix of heavy wire, tubing and the 10m portion is a piece of strap, like Frank said. The other one is wound with all tubing and strap for the 10m coil. They are somewhat application specific and not necessarily interchangable, depending on what final you are using.

[KM1H]

The original 850 version was designed for Class C Eimac glass tetrodes and doesnt work that well at AB1, I believe the 850A was the replacement that supposedly is OK for all modes. There is also a 850/160. Both are claimed to match 2500-5000 Ohm loads.

The 852 is for 1500-3000 Ohms which includes some ceramics as well as a pair of 3-500Z's and 8877's.

The 851 is the QRP version rated at 250W AM and 2500-5000 Ohms.

Carl

[WA1GFZ]

I hear the 852 works with multiple 8877s if you make the 80 meter coil tune 160, but the switch contacts die young.

[W7SOE]

Well this one is the 850A, a coil and tubing.  I am going to use it for the 813x813 project.  Still looking for an instruction sheet or something.  Otherwise I will just have to rework it as I think it should be.


Rich

[WA1GFZ]

My guess you will need to reduce the 10 meter coil to 4 turns or connect the plate tune at turn 1 or 2 to get a good Q on 10M. The rest of the coil should be fine.

[w8khk]

Hopefully a few pictures will help you restore it......

[WA1GFZ]

Looks like someone modified it to run at a lower Z. There is a turn or maybe 2 off the 10 meter section and some turns removed from the 80 meter section.
It may work out as it sits now for a pair of 813s. Just bend the 40 meter tap away from the coil and go for it. you want the plate tune cap as close to the coil as possible and also close to the tubes. The 813 has a high plate C so you want to avoid stray C so it works on 10.

[W7SOE]

Thanks Rick, that helps a lot!

Rich

[KM1H]

Tapping the 10M coil in the center will require 4X the C to tune, its a good way around tubes with high output C. You can tap it anyware with a correspondenr step up C value. I had to do that to get the 4x 813's on 10M and has been done for many decades by SW broadcasters.

[Steve - WB3HUZ]

According to B&W the 850, 850A and 851 are all designed for amps with plate loads in the 2-4 k Ohm range.

The 850 and 850A are the same except the switch control shaft is at DC potential on the 850. The 850 and 850A are rated for 1 kW AM, 2 kW PEP SSB, 4kV plate voltage.

The switch control shaft on both the 850A and 851 are at ground potential.

The 851 is rated for 250 W on AM and 500 W PEP SSB, 2 kV plate voltage.

[W7SOE]

According to B&W the 850, 850A and 851 are all designed for amps with plate loads in the 2-4 k Ohm range.

The 850 and 850A are the same except the switch control shaft is at DC potential on the 850. The 850 and 850A are rated for 1 kW AM, 2 kW PEP SSB, 4kV plate voltage.

The switch control shaft on both the 850A and 851 are at ground potential.

The 851 is rated for 250 W on AM and 500 W PEP SSB, 2 kV plate voltage.

Thank you Steve for this and the docs.

And thanks to the rest for the help.  I have this thing all sorted out.

Rich

[w8khk]

Looks like someone modified it to run at a lower Z. There is a turn or maybe 2 off the 10 meter section and some turns removed from the 80 meter section.

Frank, it appears the 10 meter coil has been compromised, thanks for the tip.  I attached a pic of my brother's (W2WM) 850A, also purchased at the Dayton Hamvention 2010.  His has four turns of flat strap for 10 meters, while mine has three.

We compared the wire and tube sections, they seem to be identical, so it seems unlikely that these sections were modified.  The wire section is 12-1/2 turns, and the tube is 3-3/4 turns.  My main interest with my amplifier is 75 and 40, so I doubt the missing turn on the 10 meter section will have any impact on operation.

[WA1GFZ]

You should be fine. 1 less turn will not effect anything on the low bands and strays will get you there on 10. Just keep your connections as short as possible between the coil and plate tune. Then again between plate of the tube and plate tune. The plate blocker should go to the cap then the cap to the coil. Many guys run the plate blocker to the coil then the cap to the coil.
This adds an inductance in series with the cap .

[w8khk]

Just keep your connections as short as possible between the coil and plate tune. Then again between plate of the tube and plate tune. The plate blocker should go to the cap then the cap to the coil.

Sorta like this....

The amp is finally completed.  Ready for the smoke test.

[WA1GFZ]

Exactly.
My rig I split the plate tune cap into 2 sections and added a wafer to the band switch so I only had 3 stator plates 20 through 10 meters. Presently i'm converting to vacuum variable caps.

[KM1H]

Nice layout but are you going to shield those meters?

Carl

[WA1GFZ]

Just found my B&W book
850A L= 13,6, 6.5, 1.75,1.0,.8uh
plate c= 150,80,70,55,50pf
looks fairly high z but spec says 2500 to 5000 ohms
the 852 had 5 turns on the 10 meter coil looking at the picture.
the picture shows silver buttons on the contacts. I have never seen one with silver contacts. I would easily rate the silver contacts at 1KW am 2KW ssb.
The new pressed contacts no.

[w8khk]

Nice layout but are you going to shield those meters?

Carl

No I did not plan to shield the meters.  I left room so that I can add a meter shield, or shield each meter individually, if necessary.  I could even add a screen cage over the entire final deck, under the meters, but I do not expect it to be necessary.  Time will tell.

If this was a class C modulated final, I would have planned for shielding.  But I have never needed it on the 4-400 and 4-1000 linears I have built, so probably will do OK here as well.

Frank, thanks for posting the specs on the B&W.