On 12/8/2023 9:09 AM, Jim VE7RF wrote:
Those oem 220M? caps are each 2200 pf.? ( 6600 pf in total for all 3 of em in parallel).
Actually, they read "222M"; so it was clear they were 2200 pF.
The actual correct component for plate blocking is a? 5 / 7.5 kv rated HT-50/58 sized doorknob.....and 2 are used in parallel...and work good on 160-10m.? ?These days, you can only get the HT-50/58 doorknob caps in 7.5 kv.? They will handle one helluva lot of current on upper HF....per the detailed specs in the HEC catalog.
A 5 kV-rated cap is not likely to withstand 3 kV plate voltage under highly-mismatched circumstances at the legal power limit (such as your antler tuner arcing over during a wind storm). 7.5 kV is not much better, but better than 6 kV, obviously. The higher, the better. Here, with nearly 3 kV plate voltage, I wouldn't have designed in anything less than 10 kV; yet, Alpha put in those 6 kV-rated capacitors. I have been told that my failures are not the only ones that have been experienced in Alpha 8410s. The AL-1500 has a pair of HT-50 doorknobs but they're only 500 pF; 1000 pF is a capacitive reactance of only 88.4 ohms at 1.8 MHz, somewhat marginal IMHO at the legal limit. But W8-no-name apparently originally specified/dictated their use, unless he specified something larger, and then MFJ, in their ever-cheapening-mindset, reduced the original values to something more-affordable; so there they are.
?I have never seen a plate blocker blow up from high swr / no ant, open ant, shorted ant etc..at least not when using HT-50/58 sized caps.
I've "snapped" a 7.5 kV, 1000 pF doorknob years ago on an early 4-1000 amplifier at Ep=4500 volts; the body audibly cracked then arced internally during tuneup of the roller inductor.
These days, I use my dwindling stash of the previously-widely-and-cheaply-available Russian monster-sized doorknobs VERY SPARINGLY, as who knows when, or whether, those will ever become available again? I had a handful of the 1000 pF/10 kV discs and decided this was as good a place as any to try them out as plate blocking caps. I wish I could rember where I bought them; but I know it was within the past several years.
73,
Steve K0XP
