AMI U47 troubleshooting cold tube
« on: October 06, 2019, 04:48:26 PM »
Hi all,

I am troubleshooting an AMI U47 build that is complete according to the schematic.
My EF800 tube is completely cold when the PSU is powered up and I am trying to troubleshoot.
I tried the tube in another circuit and it heated up fine.

I thought the issue could be a cold solder joint, but the voltage going from anode to cathode on the tube seems to be close to normal, about 115V. The heater is really low though, about 1V from cathode to heater directly on the tube.

The mic capsule appears to be receiving the correct voltages and I can't find any point on the circuit that lacks continuity.

The PSU provided by AMI is putting out about
120V at Pin 3 B+,
60V at Pin 4,
2.2V at Pin 6 heater.

2.2V seems very low for a tube heater voltage. Would that cause the tube to be cold?
Is there a way to adjust heater voltage on the AMI PSU?

Is there anything else I am overlooking that could cause a cold tube and no sound coming from the mic? Broken cap, resistor, dirty components, etc?

Re: AMI U47 troubleshooting cold tube
« Reply #1 on: October 06, 2019, 11:09:06 PM »
Retrace all the heater wiring, back to the transformer.  The B plus (HT) should be ok.  2.2 v is way too low output for the heater.  Check the AC voltage going to the rectifier.  Then start checking the DC coming off the rectifier and working toward the output.  Look carefully for a bad connection or a short...

Re: AMI U47 troubleshooting cold tube
« Reply #2 on: October 07, 2019, 11:56:26 AM »
Thanks rmburrow,

There is a voltage adjustment pot inside the PSU, so I adjusted that and it's reading the correct voltages now.
My tube is still cold, so I am going to retrace all my heater wiring and see if I can find a wrongly connected wire somewhere.
Also going to try replacing the tube and see if it makes a difference.
Capsule seems to be connected correctly because it outputs a weak signal at very high gain on my preamp.


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