To Ground or not ground

[W8ACR]

Well, the 254W rig is finished and running well. There are a few minor control issues to yet be resolved, but all in all I'm quite satisfied. Made contacts with Steve KL7OF and a few others last Saturday.

My next project is to get the rig on 75 meters as well. This time, I'm going to build an old fashioned push pull triode RF deck for 75 using a large B&W butterfly capacitor and an HDVL coil. Since the capacitor has a plate spacing of 0.5 inches, I can simply put the rotor at DC ground if I wish. However, after reading several old handbooks, I have found that although the texts talk about the feasability of putting the rotor at DC ground (Fig. 1), all the actual circuit schematics put the RF B+ on the rotor which is then bypassed to ground with a capacitor (Fig. 2). The only explanation offered for this is that this allows the rotor voltage to follow the swings of the modulation voltage, and allows for the use of a closer spaced capacitor. The implied message is that putting the rotor at DC ground is OK for CW service, but putting the B+ on the rotor is preferable for plate modulated service.

I was hoping to put the rotor at DC ground for simplicity and safety, but now I'm not so sure. Does anybody have any pracitcal experience with these circuits?

Thanks, Ron W8ACR

[WA1GFZ]

The frame at DC would allow higher breakdown voltages so I would not do it unless the plate spacing is too close for the voltage plus modulation peak you are running.

[KL7OF]

!/2 inch plate spacing on a B and W cap is OK in push pull for up to 3kv in my experience...Frame ground or rotor ground.....Gud luck...Steve

[KA2DZT]

Full 1/4 inch spacing is good for 9KV peak.  So, you have a cap that is 1/2 inch spacing, but is that between each rotor plate and the next stator plate or what??

Just checked my B+W catalog, they did make 1/2 spacing caps.  If you have the model number I can give you the capacitance if you don't know.

Fred

[W8ACR]

Hi Fred,

Yes, there is 1/2 inch spacing between each rotor and stator plate. Five plates of each. I believe this is either a CX-58 or CX-62. The model number is worn off, so I don't know for sure. My cheapo capacitance meter measures about 60pf per side.

Ron

[Ralph W3GL]

   It's  CX-62!  Will work FB with the B&W HDVL series coils...

[KA2DZT]

Hi Fred,

Yes, there is 1/2 inch spacing between each rotor and stator plate. Five plates of each. I believe this is either a CX-58 or CX-62. The model number is worn off, so I don't know for sure. My cheapo capacitance meter measures about 60pf per side.

Ron

My catalog shows, CX-68A and CX-59A as 1/2" spacing caps.  CX-62C is a 1/4" spacing cap.  The capacitance for each is the part number and it's for each sections.

To better tell exactly which cap you have, give me the spacing between the mounting holes, the catalog shows that for each cap.

There were three types A, B, C.  A 1/2"   B 3/8"   C 1/4"  These models date back to the 1940's

Later B+W catalogs only show smaller caps with only 1/8" spacing.

the real Fred

[W8ACR]

Fred,

Mounting holes at the base of the frame are 3.5 inches apart in width and 9 inches apart in length. As I said earlier, there are 5 stator plates on each side and 5 rotor plates, 1/2 inch from rotor plate to stator plate. It is also equipped with N8 neutralizing caps.

Thanks, Ron

[KA2DZT]

Fred,

Mounting holes at the base of the frame are 3.5 inches apart in width and 9 inches apart in length. As I said earlier, there are 5 stator plates on each side and 5 rotor plates, 1/2 inch from rotor plate to stator plate. It is also equipped with N8 neutralizing caps.

Thanks, Ron

Ron,

Seems that the mounting-hole spacing widthwise is the same for all models.  The 9" lengthwise spacing seems to be the CX-49A which the catalog shows as 9-1/16".  The rest in the "A" series are much different than 9".

All the caps in the "C" series are different from 9" and only the CX-70B is 8-7/8" but that would be only 3/8" plate spacing.

So, my best guess is that you have the CX-49A.  The catalog does not give the number of plates for the caps like other makers do.

Fred

[W8ACR]

OK Fred,

Thanks for the info. I'll need about 35-36 pf capacitance to resonate the HDVL coil at 3.880. I'm going to use a 25pf fixed vacuum cap padder. Tank Q will be 7-8.

Any chance you could scan that B&W document and e-mail it to me?

Ron

[Ralph W3GL]

   My bad...  Sorry.  

   It sounds like the assembly in my retired KW rig out in the garage
   that hasn't been operational since 1947.

   It's too cold out there to check the number but it has the whole
   assembly, neut. caps, padding clips and coil mount with swinging link
   as one unit.  The spacing is .5 inch rotor to stator...

   Accepts the HDVL coils and I'm using the 80 meter coil in my clip
   lead series tuned antenna matcher till I build something permanent for
   the station... (The operative word is "till"; the clip leads will probably be
   there for some time...)

   Edit:  I did use the 25 mmf vac. cap with the 80 meter HDVL coil...
            It is still on the assembly.

[KA2DZT]

OK Fred,

Thanks for the info. I'll need about 35-36 pf capacitance to resonate the HDVL coil at 3.880. I'm going to use a 25pf fixed vacuum cap padder. Tank Q will be 7-8.

Any chance you could scan that B&W document and e-mail it to me?

Ron

Ron,

My computer skills are piss-poor, so I'm not really sure.  I still can't figure out how to even post a picture on a computer.

My daughter might be able to help me do that but that would have to be at some later date.

This catalog I have is in mint condition and it shows all the coil sets that B+W made during WW2.  I also have a price sheet, and FYI the CX-49A cost 22.08 (amateur net) in July, 1946.

One last thing, I see that the catalog only shows neutralizing caps  N1 thru N4, yours are N8's.  Each type has which tubes they will neutralize, nothing for N8's.

Fred

[k4kyv]

Connecting the frame and rotor to +HV will allow twice the rf voltage across the tank circuit before the capacitor arcs over, than grounding the frame directly.  Directly grounding it means full modulated rf voltage PLUS full modulated DC between the plates.  Running the frame at +HV allows the capacitor to handle the modulated rf voltage only.

Another problem with grounding the frame directly is that in case an arc-over does occur, you will be pulling the arc from the HV DC (effectively shorting the plate supply to ground) as well as from RF.  Likely to cause more damage to the capacitor and/or plate supply than a mere rf arc would.

[W8ACR]

OK Fred,
Don't worry too much about scanning or copying that document. If'n it happens, great. If'n not, no biggie. At least now I know where to go to get B&W info. 

At 1800 VDC B+, I'm not too concerned about the capacitor arcing. Even with plate modulation, I think there is a good safety factor. The capacitor itself is in great condition - no dirt or carbon scoring etc. My main question was whether putting the rotor at DC ground is inherently undesirable for plate modulatede service or not.

Thanks again for all the input.

Ron

[KA2DZT]

Ron,

As a last bid of info,  B+W rated those 1/2" spaced caps at a full KW input.  At 1800 volts you should have no problem even with plate modulation.

But, just in case it's not enough, you'll be the first to know

Fred

[W2PFY]

At 1800 VDC B+, I'm not too concerned about the capacitor arcing. Even with plate modulation, I think there is a good safety factor. The capacitor itself is in great condition - no dirt or carbon scoring etc. My main question was whether putting the rotor at DC ground is inherently undesirable for plate modulated service or not.

Until the unknown factor shows it's ugly head but what the hell, that's what makes life interesting