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[VE7RF]

The small value capacitors have greater 'resistance' (impedance) to RF current. So more of the current is diverted through the parallel high value ones (which present a lower 'impediment' to RF current flow). This is ok but emphasises the need for all components to be able to withstand whatever current/voltage they are individually required to handle.

He installed 5 x identical 100 pf  HT-50 caps in parallel.   Each of those caps is N750.  Each cap is rated for 3.5 amps @ 2.0  mhz.   So  5 x 3.5 = 17.5 amps CCS....at 2.0 mhz.

Per Glen's calcs, he only has 4.7 amps CCS (with a 1.5 kw cxr)..flowing through the  TOTAL padding. 4.7 amps / 5 x caps = .94 amp per cap...which is only .94 / 3.5 =  26.8% of each caps CCS rating.

5 x 100 pf caps will work just fine.  They won't heat....and they won't drift.

On the other hand, IF a single 500pf HT-50 cap was used, those are only rated at 7.3 amps (any freq > 1.69 mhz).  They have a worse TC  (N5250).  4.7 amps / 7.3 amps =  64.5%  of it's CCS rating.  The single 500 pf may well drift a bit....and it's C will REDUCE in value as it heats up.... due to it's negative TC.

On any of these doorknob setups, machine screw one end to the chassis.  Run one continuous piece of copper strap  across the tops of the caps.  With 5 x caps, they could be arranged in a row, or a row of 2, and a row of 3.... (rows side by side).  Then you have also heatsinked both end of all the caps.  That will dump any possible heat, and  less heat means less drift.

With 5 x caps, another method is to mount 4 in a square patter, and the 5th cap in the middle.   Then on wide piece of copper strap to cover the tops of all 5 of em.  Then a 1" wide piece of strap  from that mess...off to pad the air variable.

Our local metal shop has 3' x 7' sheets of copper in thickness's ranging from  .010"  up to .125" .   I get em to shear what I want, in any width I want.  However, they can only shear the 3' width, due to the  40" max width shearing capacity.  Myself, I hate the look of copper, so silver plate....after any trimming with tin snips, and holes punched etc.

[VR2AX]

No issue with your calculations. Point was that 'willy nilly' mixing 'small' and 'large', possibly of different spec, if spec even exist, is not per se the final result aimed for. Surely even non technical ops can look at a spec sheet of a basic component and take a view of whether it is suitable for placing in a particular circuit..

[VE7RF]

No issue with your calculations. Point was that 'willy nilly' mixing 'small' and 'large', possibly of different spec, if spec even exist, is not per se the final result aimed for. Surely even non technical ops can look at a spec sheet of a basic component and take a view of whether it is suitable for placing in a particular circuit..

As long as you knew the current ratings of each cap, they can be mixed in parallel.
IE:  100 pf HT-50's in parallel with 170 pf HT-50's.   V across em is the same.  Current through the 170 pf caps is higher, since the 170 pf has lower Xc  vs the 100 pf.  It comes out a wash, since the 170 pf types handle more current.  But I get your point.

[K8AC]

Just to close out this thread and issue, I received the two doorknob caps that I bought from MFJ today and installed them in the 8100.  These are HEC 7.5 KV 170 pF units, which measured 182 and 185 on my uncalibrated cap tester.  So, the total is 367 pF compared to the original 375 pF disc capacitors.  The original discs were mounted in a rather difficult to access location and I installed these about an inch from the left side of the chassis.  In testing, the resulting TUNE capacitor setting is almost identical to the settings shown in the Tuning chart in the manual.  Here's how the new caps are positioned:

https://imgur.com/a/VEpTGmD

Note that these are the same tuning capacitor padder caps used in the Ameritron AL-1500, AL-1200 and AL-82 for 160M.  Thanks to all who commented on my original post.

73, Floyd - K8AC

[VE7RF]

Just to close out this thread and issue, I received the two doorknob caps that I bought from MFJ today and installed them in the 8100.  These are HEC 7.5 KV 170 pF units, which measured 182 and 185 on my uncalibrated cap tester.  So, the total is 367 pF compared to the original 375 pF disc capacitors.  The original discs were mounted in a rather difficult to access location and I installed these about an inch from the left side of the chassis.  In testing, the resulting TUNE capacitor setting is almost identical to the settings shown in the Tuning chart in the manual.  Here's how the new caps are positioned:

https://imgur.com/a/VEpTGmD

Note that these are the same tuning capacitor padder caps used in the Ameritron AL-1500, AL-1200 and AL-82 for 160M.  Thanks to all who commented on my original post.

73, Floyd - K8AC

Excellent. The mounting scheme on the pix is superb.  As long as they don't drift when using FT-8 / RTTY / data modes, you are probably good to go.  Both ends of both caps are well heat sunk, so I think it will be a non issue.  On  SSB / CW it will be a non issue.   Those 170/500 pf  @ 7.5 kv HEC caps are always readily available from MFJ.

You probably meant to post under your......  'Alpha 8100 problem'   thread...instead of this ..'padder caps for 1.5 kw tuner'.

[K8AC]

You probably meant to post under your......  'Alpha 8100 problem'   thread...instead of this ..'padder caps for 1.5 kw tuner'.

Yep - don't know if that was a finger check or an old man mind check.

73, K8AC