Mon, 9 Jun 1997 01:35:21 +0100
In message <PKUw4HABVmmzEwFZ@ifwtech.demon.co.uk>, "Ian White, G3SEK"
>It depends what you call "delicate". The 4CX800 has a rated maximum grid
>dissipation of 2W, which means that at the normal bias voltage of -60V
>you could run as much as 30mA grid current without damage.
>You wouldn't want to do that, for linearity reasons. The tubes are meant
>to operate in class AB1, which is why the ALC operates at the first
>trace of grid current and the amplifier trips to standby at 3mA. Also
>the 91b has a low-impedance grid bias supply, so small amounts of grid
>current do not spell IMD "nutsville".
I'm learning a lot here, so I'll ask another couple of questions.
Regarding the ALC mentioned above, is that the ALC line back to the rig?
If so, I don't have one connected because ETO state in their manual that
one is not required. From memory (I don't have the manual here) I
believe they actually discourage the use of an ALC line. If I'm reading
Ians' comments correctly, then surely an ALC line should be mandatory to
help keep the grid current in check. Or is this a different (internal)
ALC ? Why would ETO discourage the use of something that would help
keep our bands clean from annoying splatter ?
Secondly, is it possible to damage an ETO91b. Strange question, but
given all the in-built protections it seems to be idiot proof. It must
be because this idiot hasn't broken it yet ;-) Seriously though, is
there any particular area that needs special care that has not been
adequately protected. I'm not keen on giving RF Parts a few hundred
dollars for a pair of tubes, so is there anything I need to look out
BTW, mine has the mod installed that prevents damage to the tubes if the
blower stops, which has happened on a few 91b's. I believe it's just a
thermocouple installed in one of the chimneys which trips the amp out if
the exhaust gases get too hot. This happened mine after a couple of
months so it now has a different, smaller (!) blower (87A?) installed,
which has been fine now for 7 months.
Thanks for educating me guys,
Andrew Williamson GI0NWG / AC6WI
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