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[AE0Q]

Disc ceramic caps don't come with any RF current ratings. BUT, typ the higher V rated disc ceramic also handle more current.  That's why Alpha now uses the 18 kv rated discs.

Every Alpha amp in production gets tested on every band for wattmeter calibration and to be sure all functions work, but they are burned-in overnight transmitting dashes on 160m to be sure all the tank circuit padding caps don't fail.  And sometimes they do, a few times with catastrophic results found the next morning :-(

So all those 6kv (or maybe 8kv) disc ceramic caps worked at 1500w for hours overnight, but then would sometimes fail years later in the field, so Alpha changed to the 18kv parts for the TUNE padders.  I don't think any of the higher voltage caps have failed.

Glenn AE0Q

[VE7RF]

VE7RF - thank you for the detailed explanation.  You cleared up several things that I was unaware of.  Since I'm not in a hurry, I think I'll first locate some of the high voltage discs and give that a try.  If that doesn't pan out, I have room in the Alpha for four doorknob caps, but size may limit me to using 7.5 KV for those.  Someone earlier mentioned the 170 pF HEC doorknobs sold by MFJ in their catalog.  I checked the schematics of the AL-1500, etc. and they use two of those as padders in those amps.  I suppose I could use two of those and that might get me close enough. 

In another thread here, we were discussing a problem I had with drifting padder caps in a homebrew L network tuner - I used a 470 pF doorknob.  I replaced that with five 100pF N750 doorknobs mounted exactly as you described and that completely solved that problem. 

Thanks again for your assistance.

73, Floyd - K8AC

Those HT-50 doorknobs used to be rated at 5 kv.  Several years ago, (like 15 years ago)... HEC upped the rating on em to 7.5 kv.
MFJ sells both the 170 pf and also the 500 pf versions  in their online catalog. (google mfj catalog) . I bought 20 of their 500 pf caps..and they all measured 500-502 pf.   The 170 pf versions are within 1 pf.

The 170 pf doorknobs will handle 5.8 amps @ 2.0 mhz. 2 of em will work nicely..and provide for 340 pf of padding.
3 of em will provide for 510 pf of padding..and will also work.   You have to be careful with padders, if you use too much, they will work fine down at 1800 khz.... but might be too much padding up at 1999 khz.  You don't want the tune cap to be at min C when at the upper end of 160m band.

On paper, 2 of em should work.   Current wise, it's almost a wash whether 2 x 170pfs are used.... or 4 x 100 pf's are used.
2 x 170's are good for a total of  11.6 amps.    4 x 100 pf's are good for a total of 14 amps.   Price is the same on both types....and both are N750.  4 x 100 pf's would cost double the money vs  2 x 170's.

On a side note, the cheapest place to buy HEC caps...is right from HEC.  They also offer bulk qty discount pricing, when buying more than a few of the same type. (a helluva lot less vs RF parts).  MFJ is priced in the middle and always has em in stock.

The chinese 100 pf caps in 20 or 30 kv  will probably do the trick. They are huge things.  GL with the project.

[K8AC]

VE7RF: Good info, thanks.  When you said the Chinese HV discs were cheap, I didn't know that what you really meant was CHEAP!  You can get (10) of the 20 KV discs for $1.80 total plus 3-4 bucks for shipping.  The 30 KV units are around $3.50 in total for (10).  Vishay 20 KV caps from Mouser are $2-3 EACH.

Floyd

[VE7RF]

Disc ceramic caps don't come with any RF current ratings. BUT, typ the higher V rated disc ceramic also handle more current.  That's why Alpha now uses the 18 kv rated discs.

Every Alpha amp in production gets tested on every band for wattmeter calibration and to be sure all functions work, but they are burned-in overnight transmitting dashes on 160m to be sure all the tank circuit padding caps don't fail.  And sometimes they do, a few times with catastrophic results found the next morning :-(

So all those 6kv (or maybe 8kv) disc ceramic caps worked at 1500w for hours overnight, but then would sometimes fail years later in the field, so Alpha changed to the 18kv parts for the TUNE padders.  I don't think any of the higher voltage caps have failed.

Glenn AE0Q

Sending slow dashes is a lot higher duty cycle vs sending dots at 50 wpm.... but it's still not a 1.5 kw CXR...... which  would be the  real acid test. It's the I squared X ESR that will cook em.  Double the average plate current, and caps have to dissipate quadruple the power.     I don't think he will have issues with 20/30 kv rated discs.  They are dirt cheap to buy.

[K8AC]

Every Alpha amp in production gets tested on every band for wattmeter calibration and to be sure all functions work, but they are burned-in overnight transmitting dashes on 160m to be sure all the tank circuit padding caps don't fail.  And sometimes they do, a few times with catastrophic results found the next morning :-(

So all those 6kv (or maybe 8kv) disc ceramic caps worked at 1500w for hours overnight, but then would sometimes fail years later in the field, so Alpha changed to the 18kv parts for the TUNE padders.  I don't think any of the higher voltage caps have failed.

Glenn AE0Q

Well, all night running dashes is certainly a higher duty cycle than me calling CQ in the 160 contest for a few hours.  As far as I could tell, this 8100 (I'm the second owner) had the original tuner padder caps for 160 and I used it in the ARRL 160 contest in December with no problems at all.  Do you know what brand of 18 KV caps are being used by Alpha today?  The 20 and 30 KV Vishay caps use the blue material today, while the Phillips 6 KV caps I tried were of the beige material.  One of them failed for me after 5 minutes of testing at 1500 watts out.

73, Floyd - K8AC

[VE7RF]

VE7RF: Good info, thanks.  When you said the Chinese HV discs were cheap, I didn't know that what you really meant was CHEAP!  You can get (10) of the 20 KV discs for $1.80 total plus 3-4 bucks for shipping.  The 30 KV units are around $3.50 in total for (10).  Vishay 20 KV caps from Mouser are $2-3 EACH.

Floyd

You sure abt that ?  The 30 kv units were > $4.00 each  and the 20 kv units were $2 something, but that's for .01 uf ( 10,000 pf values)...and 10,000 pf is a helluva lot more vs just 100 pf.  So u are probably correct for the pricing on 100 pf types.  Silly thing is, their expedited shipping was dirt cheap (and no additional charge for additional items) ..and 30 x .01uf @ 30 kv caps arrived stupid fast.  Like weeks faster vs the items Bought from the east cast of the usa... ( which also had high shipping costs ).   The plan was to hi-pot test em..and also measure their exact C value.  .01uf types are used for bypass..or coupling, so their value is not critical.

[VE7RF]

Every Alpha amp in production gets tested on every band for wattmeter calibration and to be sure all functions work, but they are burned-in overnight transmitting dashes on 160m to be sure all the tank circuit padding caps don't fail.  And sometimes they do, a few times with catastrophic results found the next morning :-(

So all those 6kv (or maybe 8kv) disc ceramic caps worked at 1500w for hours overnight, but then would sometimes fail years later in the field, so Alpha changed to the 18kv parts for the TUNE padders.  I don't think any of the higher voltage caps have failed.

Glenn AE0Q

Well, all night running dashes is certainly a higher duty cycle than me calling CQ in the 160 contest for a few hours.  As far as I could tell, this 8100 (I'm the second owner) had the original tuner padder caps for 160 and I used it in the ARRL 160 contest in December with no problems at all.  Do you know what brand of 18 KV caps are being used by Alpha today?  The 20 and 30 KV Vishay caps use the blue material today, while the Phillips 6 KV caps I tried were of the beige material.  One of them failed for me after 5 minutes of testing at 1500 watts out.

73, Floyd - K8AC

On the alpha site they give a CDX part number.    These chinese ones are blue, regardless of value / voltage. 

Ur right, sending dashes all night would probably weed out any bad caps.... without trashing the tubes + B+ supply.

[K8AC]

You sure abt that ?  The 30 kv units were > $4.00 each  and the 20 kv units were $2 something, but that's for .01 uf ( 10,000 pf values)...and 10,000 pf is a helluva lot more vs just 100 pf.  So u are probably correct for the pricing on 100 pf types.  Silly thing is, their expedited shipping was dirt cheap (and no additional charge for additional items) ..and 30 x .01uf @ 30 kv caps arrived stupid fast.  Like weeks faster vs the items Bought from the east cast of the usa... ( which also had high shipping costs ).   The plan was to hi-pot test em..and also measure their exact C value.  .01uf types are used for bypass..or coupling, so their value is not critical.

Just checked my invoice -  for (10) of the 100pF 30KV caps the total shipped was: $4.86.  For (10) of the 20KV caps, total was $5.31.  Those prices are from Aliexpress.com.

[AE0Q]

On the alpha site they give a CDX part number.    These chinese ones are blue, regardless of value / voltage. 

They came from Mouser but I don't have any of the P.O.s so I don't know the manufacturer for sure, probably Vishay.  Not available now on Mouser or Digikey :-(

[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