You can test one tube at a time with the others removed and air flow
blocked. Run 25-30W drive on 80M. A tube with low emission will drift.
Ive found crystalized solder joints on many vintage amps that cause most
drift and tuning problems. Suck it all out and reflow.
A defective cap will affect the tuning but are not a typical failure
item in an Alpha.
Carl
KM1H
----- Original Message -----
From: "Karl Brandt" <nd8dx@yahoo.com>
To: <amps@contesting.com>
Cc: "Gus VO1MP" <gussam@nl.rogers.com>
Sent: Sunday, February 01, 2009 9:28 AM
Subject: [Amps] Alpha 78 Grid current anomily (possible problem)
Hi All,
I recently acquired a 78 and am just getting used to its
characteristics. One thing that I've recently noticed, which to me seems
like an indication of a possible problem, is that under 100%duty cycle
transmissions ie tuning amp in RTTY etc. the Ig will drop some 20 or
30ma, as things heat up. Also at times the Ig will fluctuate 5-10ma.
This occurs in both manual tune and band-pass mode. The decrease in Ig
occurs faster and is greater on the higher bands. For example 15m, to
get to l500 out requires abt 70 watts drive and some Ig "swamping", tune
amp and wait a minute or so to let things cool down. Key amp, Ig starts
at 90ma with an Ip of 1.28A ( meter doubler installed) over abt 10
seconds Ig has dropped to abt 70ma with some fluctuation of maybe 5ma.
While this is occurring there appears to be no change in Ip, plate
voltage, or power output. I have changed to the spare trio of tubes
(used), switched rigs, bypassed the wattmeter and
everything else going straight from the amp to a Cantenna using a know
good coax jumper and got the same results. On thing worth noting is tt
when I first fired the amp up I discovered that idling currents were too
high and subsequently determined tt the biasing zener was bad, replaced
it and tt took care of tt issue. I hv "bounced" this question off N4UQ
whom suggested possibly a problem with the bypass caps before and after
the plate choke or "soft" tube or tubes. I have yet to try changing the
bypass caps and figured before I did anything I'd see if anyone else has
any ideas. Any thoughts, input, etc. will be greatly appriciated.
73 GB Karl ND8DX
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|