Jim Brown wrote:
>Several months ago, Tom Rauch observed that a protection circuit mod
>for the older vintage amps like this one that use 3CX800A7 tubes
>would be a very good idea, since a primary failure mode is exceeding
>the 60 mA (per tube) max grid current rating. Has anyone designed
>one?
They certainly have :-)
>I have three of these amps, and would like to add such a mod
>if I had a good design for it. The way I currently run them is to
>never drive them higher than 30 mA for the two tubes combined, and
>whatever the power output is is what it is.
>
>Several concerns. First, the reset MUST be quick -- a pushbutton
>that resets instantaneously, so that you don't lose Q's in a contest
>or pileup. Second, I note that on SSB (altho I'm a CW man, I do work
>SSB for contests), I often see flashes of overdrive LED when the
>indicated grid current is only 20 mA for two tubes. This is with
>heavily compressed audio (peak and average should not be widely
>separated) and both meter and light calibrated per Ten Tec
>instructions (DC voltage at a suitable point in the circuit).
The information in the user manual for my Triode Board is also free for
downloading and use by home constructors.
The aim is to protect the amplifier *and* to keep you on the air.
A grid current trip takes the amplifier offline in a few milliseconds,
and the reset is completely automatic - not even a pushbutton. If the
cause of the fault is removed, the amp comes straight back online when
next you transmit.
A pushbutton reset is only required for rare and major faults such as an
anode flashover, where maybe you do need to pause and think. Even those
are often recoverable within seconds.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek/boards
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|