Turned out after pulling and swapping the tubes it pretty showed one had
the problem and the other didn't. Was looking in a different direction.
I am wondering if there was something else wrong that caused the tube
failure, though. Put the good one in both sockets, output definitely
less than half, but there may be a good reason for that which i'm not up
on yet. Either way, one step further down the path. Will keep the
rectifier board in mind, maybe that's what's behind the lower output.
Know a good source for reasonable prices for these things?
On 10/2/2014 10:26 AM, Big Don wrote:
Our SB220 went to ~60% power - also 300ma plate current in transmit
mode with new tubes and no drive applied.
Amp was built from new kit in 1977 and flogged daily for 35 years,
More recent reduced power RTTY use had overheated innards even with
some enhanced cooling (additional fans on top of case). It just did
not like continuous keying into transmit.
Replaced the rectifier board and filter caps with Harbach parts and
that fixed it.
Think the problem was one side of the rectifier had failed, i.e., only
rectifying half of the sine wave and was not getting enough juice into
the HV for full power....
Good luck, 73
Don N7EF.
On Thu, Oct 2, 2014 at 7:59 AM, k0zx Rich <k0zx.co@gmail.com
<mailto:k0zx.co@gmail.com>> wrote:
Wonder if anyone has experienced this failure mode with this or a
similar amp:
This amp is 80s vintage and since then has been a solid performer,
grounded grid with two 3-500z. But it has a developed a problem
where at power up (no rf drive, ant relay input not active for
tx), the plate current rises to about 3x normal (about 300ma, 100
is normal), stays there for 5 to 10 seconds and then drops to zero.
If enabled to transmit with antenna relay input on, plate idle
current is normal, going from 0 to 100ma.
If apply rf in, it will output but at only about .6 or .7 its
normal output.
After inspecting the schematic am not seeing an obvious source for
the problem.
Any suggestions appreciated
Rich
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