Cold hybrid shunt tube

[vegaseddoc]

I recently reintroduced my extended FP3 back into my system after messing around with an old dynaco preamp.  I haven't used the FP3 in about six months and is sounds great but I noticed the hybrid shunt 12au7 is cold and doesn't glow.  I did swap the tube for one that I know is working and still no glow/cold.  All of the LEDs are lit on the underside.  I can't remember if this is normal or not.

Jason

 

[Doc B]

Measure the voltages. That is the best way to tell if the tube is working. It should get warm just like the other tubes.

 

[Grainger49]

The heater voltage is best measured at the tube pins.

I have never had a 12AU7 that didn't get warm.  So I'm wondering if it is regulating.  If you have voltage on the heaters check one of the other tubes' plate voltage.  Those are pins 1 and 6.  That is where the regulated voltage appears. 

 

[vegaseddoc]

Thanks for the input guys.  I suspected this was not normal.  I will check voltages when I get a chance within the next few days.  Something is not getting voltage.  Of note,  I noticed that the A side LEDs on the shunt tube are shining much brighter than the rest of the LEDs in the system.  Interesting how good the amp sounds with the shunt not working properly.

Jason

 

[vegaseddoc]

I measured the voltages and 0 volts on the shunt tube heaters.  So I pulled the boards on the shunt tube and the B tube to inspect the jumper to the center heaters.  What I found was a broken jumper wire to the heater pins.  After close inspection under a magnifying glass I noticed the root cause of the problem.  When I original built the amp, I cut the STP wire so poorly that I had just about fully cut through the red wire leaving it connected by only a microscopic thread of wire.  ( I know. I know.  States right in the build manual to be careful while cutting the wire)  When I moved the amp around the house the wire finally broke.  Or the shunt never really worked from the beginning and I just hadn't noticed.  Regardless, I'm up and running.  Sounds fantastic with a 1963 JAN Raytheon in the shunt position and a set of Mullard box plates on the sides. Thanks for the help.