Hi guys
Tom says:
>> One design solution to 'secondary breakdown effect' is to
> >derate the dissipation to about 1/8 and parallel additional
transistors.
>> Another solution is to use a power FET transistor. They are
virtually
>> immune to 'secondary breakdown effect'. Motorola and International
>> Rectifier make a 6A, 600v, 150w FET that appears to perform
>> satisfactorily in HV regulator service.
>A little overkill, in the control grid of a small tetrode! Almost any
>HV NPN power transistor would handle a few mA at 50 volts or so!
Secondary breakdown is a funny thing, and you can't always rely on the
curves in the data sheet.. Quite a few years ago now, John, GW4FRX did
an article in RadCom (reproduced in the VHF/UHF DX book that G3SEK
edited) on screen and bias supplies for VHF amps using 4CX250's, and he
used MOSFETs for the secondary breakdown reason. It was commented that
his design was a bit overdone, but was it? Expanding the point I made
earlier about app notes and data sheets always running to the cheapest
solution ( I should know - I've written enough of them!), here we're
talking very much for the 'roll your own' amplifier guy. Putting in
stiff power supplies, under rated components and circuit complexity and
so on is a very small increment in parts cost. I figure that, with a
good bit of begging, you should be able to roll your own for between 15
and 20 cents a watt of output.(This from a guy who had two vacuum
variables as wedding presents!)
So there may be something to be said in reliability terms for overkill
if you're building. My 4-250 amp has been running for 10 years now with
occasional mainetnance - cleaning out the dust etc. Failures have (after
initial rectifier problems) been confined to the input diode switch,
where I used a 1N4008 as a PIN diode. Although they didn't get hot, they
eventually failed, and the failure analysis guys here in the factory
couldn't say why. I now have Jennings vacuum relays on input and output
- I think they're RF1s - little ceramic ones - we had a whole load in
surplus round the ham fests here a few years back, brand new at about
$7-50 a time. Same ones as Surplus Sales of Nebraska wants $75 for! Only
other failure has been a 5R4GY in the screen supply.
73
Peter G3RZP
73, Tom W8JI
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