|[AMRadio] Johnson Valiant Audio Driver Stage Question|
w0ng at comcast.net
w0ng at comcast.net
Sun Oct 14 15:25:12 EDT 2007
Thanks for the advice. When I said that there was no HV I meant that no plate voltage was applied to the finals. I must have had a temporary brain-fart. When the filament switch is ON, I do indeed have audio plate voltage.
I'll check ALL the wiring again and let you know what I find. Appreciate the quick response.
-------------- Original message --------------
From: "Jim Wilhite" <w5jo at brightok.net>
> Bill when the filament switch is turned on with the Valiant, HV for the
> audio section is applied as well, so you have voltage on the plate and
> the audio section is hot.
> These voltages are supplied by T2. My Valiant 2 book lists that
> resistor as 820 ohms. For that resistor to go bad, you must have a
> short somewhere or the socket has been miswired by someone. Double
> check that. My resistance chart says you should see about 1K ohms to
> ground with no power applied (everything turned off).
> > 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. ed.
> > 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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