Good Morning;
I'm helping a friend repair an Amp Supply LK800 3x3CX800A7 amp which appears
to have a bad plate blocking/coupling doorknop capacitor. It's an HEC HT-50
style 1000pf @ 5KV , X5V temp rating.
The symptoms are as follows: Varying drive power varies plate and grid
current, yet the load and plate tuning have no effect on readings, and there is
no RF output at the antenna connection. The vacuum relay and all connections
back to the blocking cap check good. I'm guessing the cap failed to an open
condition.
I have a box full of brand new HEC HT-50 style 5kv caps, from 50pf to 250pf,
but nothing as large as 1000pf. There isn't enough room between the plate choke
and tuning network, otherwise I'd fabricate four of the 250pf units together to
reach 1000pf.
Anyone here on the list have a 1000pf 5kv or greater cap for sale, or trade for
my smaller value ones, before I place an order?
One more question - he is pushing the hell out of this amp, to the point where
he is easily reaching 2x plate dissipation. The amp had a larger Dahl
transformer installed (3KV plate voltage), and plate current is around 2A+ when
he pushes it hard, so I'm wondering if the cap blew from being stressed passed
its limits.... According to the chart in the latest RF Parts catalog, the 5kv
cap is rated at 5.6a @ 30MHz, but I recall reading in the past that the RF
current is actually much higher than the calculated DC current, no?
Should I just upgrade the cap to an HT-57 15kv 1000pf cap, which shows in the
catalog as having a higher current rating?
Also, he has the original stock LK800 transformer for sale if anyone is
interested. If I recall correctly, after the diode and cap board, it was
generating approx. 2700v no load, and 2400v loaded. If interested, contact me
and I'll forward his contact info.
Eric
KE2D
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