[Boatanchors] Johnson Valiant Audio Driver Question

[Al Parker]

Hi Bill,
    When that sort of thing happens to me, I suspect the coupling capacitor 
from the previous stage.  In this case, C90 from the 6C4 3rd audio.  But, 
like Gary says, it's hard for a resistor to burrn up when both ends are at 
zero volts, so ck into that as well.  That resistor (not a bypass) & it's 
bypass cap, are supposed to hold the cathode at a slightly pos. voltage, it 
says +14 in your manual.  That makes the grid, which has a grid-leak 
resostor of 150k to gnd, be biased slightly negative with respect to the 
cathode so it won't run away.  But if that C90 is leaky it'll forward bias 
the tube, both sections, and it'll "burn up".
    I just got a Valiant this week, and am getting, slowly, into it.  I hope 
to have it going soon for the upcoming 160m season.  The Viking 1 that I 
used some last yr has a new owner, if he'll ever come to pick it up.  I 
reworked it's audio system, but not with the hi-fi notions that some go for, 
just for low distortion & decent modulation.  Will make sure the Valiants is 
sounding proper.
    Hope this is some help.
73,
Al, W8UT
New Bern, NC
www.boatanchors.org
www.hammarlund.info


----- Original Message ----- 
From: <w0ng at comcast.net>
To: <boatanchors at mailman.qth.net>
Sent: Sunday, October 14, 2007 2:16 PM
Subject: [Boatanchors] Johnson Valiant Audio Driver Question


> In my eternal quest to bring some life back into my (once total basket 
> case) 1957 kit-version Valiant transmitter, I now have a "gremlin" of 
> sorts living in the 12AU7 audio driver stage (V15).
>
> The 1k, 1/2w cathode bypass resistor R45 (in parallel with a 10uf, 35v 
> electrolytic cap C99) connected from pin 3 & 8 of the 12AU7 driver tube to 
> ground "lets all of its smoke out" after the power is on for about 8-10 
> minutes.
>
> No high voltage is applied, just sitting there on the bench idling and I 
> can watch the resistor burn in the middle. It's weird (to me anyway) that 
> it only happens after the rig is powered up for about 8-10 minutes.
>
> On the off chance the resistor was bad, I replaced it and the electrolytic 
> cap but it burned out again anyway. Checked the chassis grounds and solder 
> joints, all OK.
>
> I took some voltage measurements and discovered that there is ZERO voltage 
> at the 12AU7 cathode after a brief warmup. The manual says there should be 
> 14 volts at pin 3 & 8. When I turn on the filament switch (with the rig 
> cold) there is a slight momentary + voltage on the cathode as the heater 
> warms up but after a few seconds, ZERO voltage on the cathode (???). All 
> other voltages at the other tube pins are in spec. I'm using 117vac line 
> voltage from a Variac.
>
> I've gone thru the wiring to look for any obvious problems, swapped known 
> good 12AU7's but for the life of me don't know why the cathode bypass 
> resistor keeps overheating (until it is actually smoking) and why there is 
> no cathode voltage.
>
> I'm hoping some of you more experienced folks can lend a suggestion or two 
> to point me in the right direction. I'd like to avoid encouraging anymore 
> grey hair on my head if I can, hi.  Thanks, Bill, w0ng
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