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Bizzaro fix for hum problem needs explanation! Pretty Please


2012-01-25 7:11 pm
In the process of trying to fix a major hum problem in a tube line stage, I stumbled on a weird temporary solution. I think this weird fix is trying to tell me what the underlying problem is, but being relatively inexperienced with electronics and tubes I haven’t a clue.

Background and sad story of woe:
For the output stage of a DAC I choose to use John Broskie’s BCF_9 broskie cathode follower board (to take the balanced output of the DAC and convert it to an unbalanced signal for my audio system).

John Broskie from GlassWare audio design has an excellent reputation for making decent circuits and PCBs but unless there is a missing or broken part he doesn’t respond to emails for technical questions like what follows:

As you can see from the pics the build is nearly complete. I probably should have first tested the BCF_9 board before installing it but I thought I would just go for it.

Couple of days ago I checked the power supply voltages (H+ 12.7 VDC, B+ 300VDC, H- set to 1/4 B+), installed the tubes and was dismayed to have a major hum/buzz from the left channel. Also I would get a repetitive ticking sound.

Right channel is quite. Sound is overall very good from both channels but the hum/buzz/ticking from the left channel too loud to take.

I swapped tubes, tried a different set of 12BH7 tubes, checked all parts to make sure they were the recommended values (see circuit diagram), doubled checked with connections and resistor values with VOM, checked and reflowed every solder connection (twice). Still the left channel hums/buzzes and intermittently ticks (with or without an audio signal input). Completely disassembled and tested still hum in left channel

Pulled the tubes and the noise stops

Tried to trace the hum using an oscilloscope . I think the left channel noise signal is around 60Hz (compared to a signal generator) but not smooth (more saw tooth like and pretty distorted). But this is my first attempt using a scope.

What I did find is that the noise could be readily detected on left channel tube pins (V1 in diagram) 3 and 6.

However when I probed pin 2 or pin 1 the noise stopped. Note: I was using a 1x probe with ground attached to B-/ground. By chance I came upon the fact that the noise also stopped if I touched pin 2 or pin 1 with a test lead/probe that was not connected to anything.

So I tried several lengths of wire soldered directly to tube V1 pin2. To completely suppress the hum/buzz and ticking in the left channel the wire on pin 2 must be about 2-3 ft long and needs to be stretched out (cannot be wound into a smaller coil).

This seems antenna_ish to me with the wire influencing some reactive circuit. There must be a bad part or connection somewhere (V1 + input stage?). But how do I determine which component is defective and/or how to really fix it? Beside trying more voodoo?

This is driving me nuts and I would greatly appreciate your advice before I toss this cursed thing out