[AMRadio] Johnson Valiant Audio Driver Stage Question


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Sun Oct 14 19:39:44 EDT 2007


A 12au7 is a triode.
The cathode is supposed to go to ground through the resistor
to provide bias for the tube.
If everything is correct, all the current that passes through 
the resistor must go through the tube.

The GRID sets how much current goes through the tube.
A positive voltage on the grid, or a short to the plate,
will give a lot of current.

A bad cap across the resistor wont effect the resistor, although the 
tube might have a hard time if it shorts.
I usually remove the bypass caps to provide some neg feedback.
You trade gain for lower distortion.

The only other possibility is there is a cathode to filament short
and that would not give enough current through anything above
about 100 ohms...

Sounds like a bad tube to me, or the socket is wired wrong
or has a short between the plate and the grid.

Brett
N2DTS
 

> -----Original Message-----
> From: amradio-bounces at mailman.qth.net 
> [mailto:amradio-bounces at mailman.qth.net] On Behalf Of w0ng at comcast.net
> Sent: Sunday, October 14, 2007 2:16 PM
> To: amradio at mailman.qth.net
> Subject: [AMRadio] Johnson Valiant Audio Driver Stage 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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