• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

help with hum 12ax7 preamp

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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.
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.
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