Hum in my 50w amp.

[mpayne]

With the help from this forum I got my first home built amp up and running and it sounds great.  I put it in a cabinet and now it’s got a hum coming out of the speaker that was not there before.  I’ve taken it out of the cabinet and the hum is still there.  The hum is not as loud as a guitar playing through the amp, but it’s loud enough for me to know it shouldn’t be there.  The hum’s volume tracks with the master volume so that tells me it’s in the preamp.  The hum also tracks with the treble.  If I turn the treble all the way down the hum pretty much goes away.  The bass, mid and presence have no effect on the hum.  I’ve rotated the pre-amp tubes and that did not change anything.  I’ve gone through the hum section in the amp debugging page and did not find anything.  It’s strange because the hum just seemed to “show up” out of no where.  Any ideas what to check next?  The schematics can be found here. http://www.el34world.com/Forum/index.php?topic=9190.0

Thanks again for all the help,
Mike

[Dave]

If it did show up out of no where, then my guess would be that it is a lead dress issue. In other words, a wire got moved around a little bit while you were mounting it in your cabinet.
I would fire it up outside the cabinet and use a chopstick to gently move some wires around and see if you can find the culprit.

Dave

[Leevi]

This will probably not help but is easy to test. Try to ground the speaker to the chassis.
The speaker, if it is placed close to the transformers may work as an antenna.
/Leevi

[mpayne]

My master volume was not soldered down well.  So that helped, but there was still a little more hum than I remembered.  I rotated the three 12AX7 one more time and that did it!  The hum is completely gone and it's very quiet.  Does that mean that one of my pre-amp tubes is going (or gone) bad?  Is this normal?  The amp sounds fine when I play through it.

Thanks for the help,
Mike

[HotBluePlates]

I rotated the three 12AX7 one more time and that did it!  The hum is completely gone and it's very quiet.  Does that mean that one of my pre-amp tubes is going (or gone) bad?

Well, maybe they weren't fully in the sockets. Or maybe there was some oxidation on the sockets or tube pins that was scratched off.

But one thing that is normal is that there is some amaount of leakage from the heater to the cathode. That leakage maybe be exceedingly small, or it could be just moderately small. If there is leakage, it may result in hum being transferred to the cathode, and if the signal is very small and leakage is high, you get audible hum.

So the best plan is to use low-leakage tubes in the earliest preamp stages, where signal is small. Tubes with leakage too great to use here without hum might be successfully used in stages where the signal level is much higher, say, a phase inverter.

And think about the tweed Bassman. The enormous 250uF cathode bypass cap is not only about insuring full amplification down to the bottom of the bass range. It was also about shunting any leakage that occured in that first stage to ground. The 12AY7 was also used because it was developed (and at that time, advertised) to have low leakage and hum.

[mpayne]

Okay, so my hum is back.  I had a cold solder joint that apparently was masking the problem.  I've poked around the circuit board with a chop stick and the hum never changes.  If I unplug the input cord the hum goes away, but I'm assuming that's because the input tip gets grounded.  I've attached a scope capture of what the hum looks like.  It's 60hz, so poor grounding?  I had a PCB made for the circuit board and the whole bottom layer is a ground plane.  I’ve got the PCB, transformers and choke attached to a thin metal plate that is also grounded.  Was that a mistake? 

Thanks,
Mike

[Shrapnel]

If I unplug the input cord the hum goes away, but I'm assuming that's because the input tip gets grounded.

That would indicate there IS NO internally induced hum. It could be a bad cable (Was it connected to anything?), or poor shielding in the instrument. (Single coil pickups? Bad cavity shielding?)

  I've attached a scope capture of what the hum looks like.  It's 60hz, so poor grounding?  I had a PCB made for the circuit board and the whole bottom layer is a ground plane.  I’ve got the PCB, transformers and choke attached to a thin metal plate that is also grounded.  Was that a mistake?

In my not so educated opinion, (I can definitely learn more, and without photos to enhance your description...) probably not, as long as your ground plane doesn't interfere with any other connection.

[HotBluePlates]

The only location of 60Hz inside your amp is the heater circuit and the power wiring leading up to the rectifier. Unless there is leakage from the heater circuit, radiated noise into sensitive wiring (from the heater wiring or from the power transformer or wiring), then there is no available source for 60Hz noise.

But bad cables or pickups picking up hum from flourescent lights or a monitor or from the amp's power transformer itself are your most likely sources.

[sluckey]

That scope capture looks kinda like flourescent light switching noise. It's important to know where your scope is connected. You can get a 575mV_pp signal like that just by laying a probe on the bench if there are flourescent lights nearby. Turn out the lights to see if your hum dissappears when plugged in.

Since the hum goes away when you unplug the input cable, I tend to think the amp is fine.

On another note though... Your half wave bias supply 'could' have 60Hz ripple, especially since you originally had the bias caps installed backwards. I would replace those caps. Bad bias caps could cause 60Hz hum, but it would not go away when you unplug the input cable.

[mpayne]

I think I've got my hum fixed, but now I've noticed that if I turn my presence down too low I get a 8Khz sine wave @30v peak-peak (not mV, volts) comming out of the speaker.  If I adjust the master volume while this is happening the frequency shifts (~7.8-8.2Khz) , but the peak-peak stays the same.  What could cause that?  If I turn up the presence it goes away and everything sounds fine. 

Thanks,
Mike

[Geezer]

Possible "postive feedback".........try swapping/reversing the OT primary wires.

If, after you do the swap, the amp now "howls" loudly, put the wires back to their original locations........the Negative Feedback resistor may be be too low in value, allowing too much signal to be sent back into the circuit. If so, raise the value of the NFB resistor (R13) to stop the oscillation.

Could also be lead dress issues (with the NFB circuit......wires from the OT to the NFB resistor & to the presence pot). Try "chopsticking" these wires while the oscillation is occuring & see if anything changes.