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Old 29th January 2013, 04:07 PM   #1
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Default help with hum 12ax7 preamp

Just finished a simple tube pre and I'm getting a nasty humming when I hook up the heater with no voltage to anything else any Ideas?
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Old 29th January 2013, 04:24 PM   #2
kevinkr is offline kevinkr  United States
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Yeah,
You need to apply operating voltages to the rest of the circuit and then troubleshoot.
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Old 29th January 2013, 04:31 PM   #3
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Makes the same sound when I power it on if I unhook the heater it slowly fades away cant seem to figure it out, I've double checked the wiring.
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Old 29th January 2013, 04:34 PM   #4
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Unless the tube is operating, everything is pretty much a bunch of open circuits.
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Old 29th January 2013, 04:47 PM   #5
kevinkr is offline kevinkr  United States
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So you're saying that with B+ applied and the filament(s) powered there is hum and when you remove the filament supply it went away?

Can you post a schematic and a description of your test set up?

If the hum goes away immediately when you remove the AC filament supply then it is filament wiring and filament circuit configuration, if on the other hand it does not diminish or actually increases significantly this may point to an inadequately filtered B+ supply.

There are a number of ways to handle filament hum, DC heating is recommended for low level stuff like phono and tape amplifiers, and with care in construction is not really required for line or power amplifier applications with indirect heated tubes.

The filament circuit should not float, several ways to handle this - if you have a center-tapped filament winding ground the tap, if not use place two 100 oh 1/2W resistors in series across the filament winding and ground the point between them..

Next option is to raise the filament voltage above ground with a resistive voltage divider from B+ with a cap to ground from the center of the divider.. Floating the filament circuit 30 - 50V above ground makes sure that nothing gets coupled from the filament to the cathode. Note that the filament circuit is not floating at AC as the decoupling cap is intended to provide a low impedance for AC.
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Old 29th January 2013, 04:53 PM   #6
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with or without b+ the hum is the same when I remove filament supply it doesn't immediately leave but the hum slowly fades as the tube cools. Could the tube itself be shorted its a older used tube I had?
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Old 29th January 2013, 05:01 PM   #7
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Google Image Result for http://www.shine7.com/audio/12ax7_schematics.gif this Is the schematic just a random one I found on the internet only difference is I have 200v instead of 300v
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Old 29th January 2013, 05:12 PM   #8
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Since you have double checked your wiring, replacing the tube with a "known to be good" one is the very next step in trouble shooting you should do.
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Old 29th January 2013, 05:40 PM   #9
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Quote:
Just finished a simple tube pre and I'm getting a nasty humming when I hook up the heater with no voltage to anything else any Ideas?
How have you referenced the heater supply to ground?

Ok let me explain. When I built a preamp for the first time. I left the heater floating. (I.e. not referenced to earth/ground whatever you want to call it) Then I found out you have to reference it to ground. A quick test would be to connect one of the wires of the heater supply to ground. But the better way is to add a 50ohm resistor on each leg to ground.

Last edited by Bas Horneman; 29th January 2013 at 05:48 PM.
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Old 29th January 2013, 06:06 PM   #10
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Supply rail ripple? That will only appear in the output when the valves are working. Show us your PSU circuit with, if possible, your calculation of ripple voltage.
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