[AMRadio] Heathkit DX-60 grid current drops (Larry Szendrei)


Jim Candela jcandela at prodigy.net
Mon May 31 10:13:02 EDT 2010





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Message: 1
Date: Thu, 27 May 2010 12:36:24 -0400 (EDT)
From: "Larry Szendrei" <ne1s at securespeed.us>
Subject: Re: [AMRadio] Heathkit DX-60 grid current drops
To: "Discussion of AM Radio in the Amateur Service"
    <amradio at mailman.qth.net>
Message-ID:
    <61478.24.97.227.120.1274978184.squirrel at mail.securespeed.us>
Content-Type: text/plain;charset=iso-8859-1

As Jim says, you may be losing drive to the final grid because the drive
signal is going away. But that's not all that can go wrong. Make sure the
final grid bias resistor hasn't drifted off-tolerance or changing value as
it heats up. If it is, replace it. I have also had bad final tubes (pair
of 6146s in a DX-100) create the symptoms you describe. Originally I
thought for sure the problem was with the drive signal, but that wasn't
what it turned out to be.

73 es GL,
-Larry/NE1S

Reply by Jim, WD5JKO:

Adding to what Larry says, it could be that the 6146 is running too hot. By hot I mean the filament voltage is too high, and a fragment of the cathode oxide has detached and landed on the control grid. If this is the case, with RF excitation, the fragment gets hot and becomes an electron emitter which will cause the grid current to gradually drop, and then reverse. This happens frequently with the 4D32. With the 4D32, lowering the filament voltage from 7'ish to around 6 volts usually makes the tube behave better. I've not seen this with the 6146, but it could happen, especially when running a vintage BA at today's higher AC line voltage.

Jim
WD5JKO


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