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8mV of noise causing havoc in valve pre amp.

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I have managed to get the noise down quite a bit in my pre amp.
I am now down to 8mV AC hum in the first stage.
The 8mV becomes 80mV after the next stage which is pretty useless.
I have changed heater supply to DC and that is rock solid.
I have to connect one side of DC heater supply to zero volts to reduce hum.

I had a transformer very close to the first stage so I have removed it and put it on flying leads so it cant affect the circuit.

I thought perhaps it was star grounding issue but I have made sure all ground are star-ed.

I have ordered a couple of different valves that hopefully should be lower noise, a 12au7 and a Ruby low noise 12ax7.

The second stage works fine if I short out its input, there is zero noise on the output.

An externally hosted image should be here but it was not working when we last tested it.
 
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How about shorting the input, does that affect the hum? Does a tube shield help? Is it 50Hz or 100Hz?

I have the volume control set to zero.
It is 50Hz.
Shorting out the grid stopper alters the shape of the waveform quite a bit.
Without grid-stopper it is reasonable sinusoidal.
With grid stopper it is a very badly shape sine wave.
Its about 100mV peak to peak.
An externally hosted image should be here but it was not working when we last tested it.
 
Shorting the input to the second stage (if done at the pot input) doesn't actually short it... it simply takes the output noise of the first stage and creates a voltage divider via the two 100K resistors meant to mix the two inputs. The output from the voltage divider drives the input to the second stage. If identical noise is induced to both stages, the input (via the voltage divider) to the second stage is basically out of phase and the end result is likely less noise from the second stage. Without seeing a mechanical layout of the circuit, it's difficult to suggest where the problem is.

Regards, KM
 
I don't have much experience with tube circuits but I would try a 5-20H transformer type choke on the H2 rail making a RCLC filter after the rectifier. They can be bulky and expensive. If that did not work the next thing I would try is running both anodes off the one rail. Meaning that from HT I would further filter and decouple the voltage and supply the input side anode with that. This might reduce the hum a little as it would see more filtering. I might have to change the resistors to get the required voltage though and also use less capacitance.
 
AC heating on a 12AU7 should be fine if done correctly. The OP did state that the hum output is 50Hz. That "should" rule out the B+ supply. The original schematic does show AC heater operation with a positive lift on the heater. As the heater supply has been changed to DC (and a change in hum), it could be nothing more than a poor tube that is leaky between the filament and the cathode.

On another note, using a 10K grid resistor on a 12AU7 is of no real value. It doesn't have that much gain. The fact that shorting it makes a significant change in output noise could be an indicator of a bad tube. Has the cathode current been checked by measuring the voltage across the cathode resistor? See if it matches the published plate curves.

Regards, KM
 
There is less than 1mV of ripple on the anode supply voltage. My scope cant even detect any ripple.

I tried tonight rearranging the filter capacitors making the primary capacitor first so as not to let ground charging impulses affect the audio ground. This made no difference.
I thought I had been careful connecting all the audio grounds together then connecting it once to the RC supplies.
If I short out the 1meg input resistor the 8mV hum reduces down to about 4mV and is a very rounded waveform.
 
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