Humming Concertina PI

[tompagan123]

Hello all. My latest build has a moderate background hum, otherwise it sounds great.   It's a fixed level hum that seems happens with only the PI and output tubes in place.   It's a concertina PI with fixed bias.   I have another prototype amp with the same circuit that is fine, but I can't seem to figure out the problem with this build.  It is a new layout which I'm not ruling out for obvious reasons.    If I pull the PI tube the hum is gone.   If I install all the tubes the hum is the same as if only the PI and output tubes are in place.
If I shunt the PI plate to ground via a .1 cap the hum disappears.   The hum does NOT disappear if I shunt the grid or cathode to ground (through the cap).   So it seems to be local the plate of the PI. I've swapped the PI tube (12ax7).   Filament and board to socket wiring is clean.   I replaced the filter cap used by the PI.    I tried an artificial center tap. I think I have the problem localized, but have not determined the cause.   


Any suggestions would be greatly appreciated! 


Tom

[eleventeen]

Sounds like you've tried most of the sensible things. Is your B+ feed to the PI stage held off (eg; isolated from) the feeds to other stages via a resistor? You could try that pretty cheaply, just kick a 5K - 10K resistor in series with the B+ feed to the PI. Yes, it will reduce the plate voltage some, but for a test, it's easy enough.


You might also try a 12AT7, eg; a lower gain tube in that spot.

[sluckey]

Could we see your schematic, layout, and some hi-rez pics of the amp?

[tompagan123]

Hi Eleventeen.   The PI power supply node has a 27K down to the preamp filter cap and a 10K up to the screen grid filter cap.  The screen grid filter cap has a choke up to the reservoir cap.   The PI is the only triode in use on that 12Ax7 and pins 1-4 are unconnected   (filament only going to pins 9 and 5). 


I'm guessing that if the problem were upstream from the PI  (bad upstream filter caps, choke induction) then the hum would be heard with the PI tube pulled, but pulling that tube silences the hum.  So I think it's right there staring at me, I just can't see it.

[tompagan123]

Hi Sluckey, I'll get a schematic and pics up.   Thanks!   

[Willabe]

The PI is the only triode in use on that 12Ax7 and pins 1-4 are unconnected   (filament only going to pins 9 and 5).

Pin 4 needs to be connected to pin 5 even if your using only 1 of the 2 triodes in the bottle. 

[sluckey]

Pin 4 needs to be connected to pin 5 even if your using only 1 of the 2 triodes in the bottle.

I agree. This may be the source of the hum.

[tompagan123]

Interesting.  I'll jumper pin 4 to pin 5 and report back.    Thanks!

[tompagan123]

OK, I connected pin 5 to pin 4 and the hum remains.     Below are schematics of the PI.  I'll add links to some pics tomorrow.   Thanks again!  -Tom

PI schematic:

http://i359.photobucket.com/albums/oo31/tompagan123/Reverb%20Twenty/PI%20Schematic-voltages_zpsux9g0x1p.gif

PI Schematic with voltages
http://i359.photobucket.com/albums/oo31/tompagan123/Reverb%20Twenty/PI%20Schematic-basic_zpsubrucjzd.gif

[HotBluePlates]

... My latest build has a moderate background hum ...    If I pull the PI tube the hum is gone.  ... If I shunt the PI plate to ground via a .1 cap the hum disappears.   The hum does NOT disappear if I shunt the grid or cathode to ground (through the cap). ...

When you shunted the PI plate to ground via the cap, did you lift the end that connects to the output tube? If you didn't, shunting to ground on the output tube side of the cap also shorts out the bias (which you'd think would make the tube redplate). Shunting to ground on the PI plate side of the cap also kills all voltage to the PI.

I ask only because I once had an amp to repair with a failed bias filter cap. That amp hummed even with the phase inverter pulled, which was how I identified the bias supply as the problem. But let's assume your bias supply is good, based on your observation is doesn't hum with the PI tube pulled.

The remaining likely culprit is power supply hum being coupled into the PI grid by the fixed-bias arrangement, and passed to the output. Are you certain you have good grounds for all 3 filter caps at/between the PI and rectifier? And a short, direct ground path from the bottom of the PI cathode load to the 3rd filter cap?

Separately, I'm a little curious about the 100kΩ loads, as those would typically be 56kΩ in this circuit with a 12AX7. The split-load inverter won't provide any gain, so there's not much call for larger loads resistors.