What''s your favorite brand of 'breadslicer' transmitting caps?

[N3DRB The Derb]

I dig Hammarlund myself.....better mechanical design. The TCD series is so stable you cant move or twist the plates from frame forces. I guess all are good in some ways, but the Hammurlund's are usually bargains at the shows.

I dont know how I could have forgotten the brands Don mentioned, but I added them and reset the poll.

[k4kyv]

There are also Nationals, Buds and Millens. 

I'm not sure if any are consistently better quality than others.  Cardwells are heavier and I think better constructed mechanically, but they are somewhat clunky, and tend to have high minimum capacitance.  Johnson and Bud are very similar in construction. The Millens are practically identical to the Nationals, since Millen Co. was an offshoot of National.

What I like about Johnson, Buds, Nationals and Millens is that, unlike Cardwells and Hammarlunds, the larger ones are easily disassembled, so you can reassemble the parts to custom make your own.  The final tank capacitor in my HF-300 rig was custom built by me. 

I took a BC-610 plate tank capacitor, which is a Johnson split stator 150/150 pf @ .175" spacing (7 KV rating), a 500 pf .175" single section and a 50 pf 0.5" spaced Johnson, and disassembled all three.  I used the longer shaft and connecting rods from the BC-610 cap, used the larger end plates, rotor and stator plates from the 500pf one and salvaged some additional plates from the 50 pf one.  The plate spacers came from the BC-610 capacitor.  I used the ceramic insulator bars from the 500 pf and the 50 pf ones, and fabricated the wiper assembly that goes to the middle of the rotor from one of the leftover larger size end plates, and reassembled these components to form a 240/240 pf, 7 KV split stator tank capacitor, which allows for plate modulation @ 2000 volts DC on the final with enough capacitance for the optimum tank circuit Q for 160m.  It is the big one seen at the left of the attached photo.

When combining parts from Johnson capacitors, one note of caution.  Older versions used ceramic stator insulator bars of uniform width, while later versions used a slightly thicker, tapered bar.  These are not interchangeable, because the mounting holes for the stator assembly do not have the same vertical alignment, and intermixing old with new will result in a capacitor that is impossible to align mechanically.

One problem that is common to all bread slicers is that the plates tend to warp or become slightly bent, so that the rotor and stator plates do not mesh exactly symmetrically, leaving the gap spacing at one side of some or all the rotor plates smaller than the gap at the other side of the same plate.  This degrades the effective spacing and thus voltage rating of the capacitor.  Most capacitors can be adjusted so that the rotor plates mesh equidistantly from each adjacent stator plate, but in nearly every case I have found it necessary to loosen the stator mounting screws and shift the stator assembly,  while bending and twisting some stator and rotor plates, to  get the adjustment as perfect as possible.  This alignment can be very tedious and laborious, but once accomplished, the adjustment should be permanent if the capacitor is handled carefully.

Vacuum caps are less suitable for balanced tank circuits, and one inherent disadvantage is that like with tubes, air molecules will eventually leak in to spoil the vacuum, although this might take decades.  But it is unlikely you could count on a transmitter with vacuum caps to still work 100 years from now.

There is no limit to the power level you can run with air variables; it is just a matter of making them big enough.

[W3SLK]

I liked the EF Johnson's myself but didn't B&W make the butterfly caps for P-P applications? I always thought they were pretty neat.

[k4kyv]

I liked the EF Johnson's myself but didn't B&W make the butterfly caps for P-P applications? I always thought they were pretty neat.

Yes, that's right.  One thing I never liked about the butterfly jobs is that the entire capacitance range, from minimum to maximum, occurs over only 90° of shaft rotation, instead of 180°.  This can make the tuning kind of touchy if you don't use a vernier or gear reduction dial drive.

But they are very compact and symmetrical, and make excellent capacitors for higher frequency, 10m for example, pushpull finals, and the  neutralisation capacitors are built in.  This construction makes for low stray capacitances in the final amplifier output circuit.

[WA1GFZ]

The power rating of the bread slicer is the current rating of the wiper to rotor connection. I took my 4CX3000A rig apart becaues the 2000 pf cardwell can't handle the current. I have a bigger cardwell to put in its place but only 1000 pf. so will add a 1000pf padder on 75.
Johnson caps are the easist to modify. I used the large size for a plate tuning cap. I shift the split for 2 stators on 20 through 10 and 40 and 80 get the whole 300 pf. This way the Q is right on the higher bands.

[WB3JOK]

The power rating of the bread slicer is the current rating of the wiper to rotor connection.

This is another useful property of butterfly caps - they eliminate the wiper-rotor contact (if you use them stator-to-stator at half the capacitance). In very high current applications, such as high-Q small magnetic loop antennas, it is mandatory since the radiation resistance is in milli-ohms, and a wiper resistance of the same order of magnitude eats up lots of efficiency.

[Tom WA3KLR]

Chef’s Choice 662.

[WD8BIL]

I'm partial to Bud

[k4kyv]

The power rating of the bread slicer is the current rating of the wiper to rotor connection.

True for the single section cap.  But with the split stator, just as in the case of the butterfly, there isn't a lot of current through the wiper cantact if it is wired in the balanced configuration.

[N3DRB The Derb]

I like Bud - my HB rig used one for the plate tuning cap.

[W7XXX]

National! I use stacked dual 50mmfd 6 KV Nationals in the triple 813. The bottom is the tuner and the top a trimmer to set tuner to mid range. Very smooth tuning.

[Steve - WB3HUZ]

Yea, I have a few here. They are a thing of beauty.

I liked the EF Johnson's myself but didn't B&W make the butterfly caps for P-P applications? I always thought they were pretty neat.

[WA1GFZ]

IF you study an EF Johnson you can see it can be converted to a butterfly like in the KW match box. Those big ones are quite nice though.

[steve_qix]

For contemporary parts, I like Palstar.  Large cap, and a very good mounting system.

If we're talking old parts, I like National, although there are many good ones!

Regards,

Steve