12au7 in SII?

[bepis69]

Howdy!
A while back I built a Stereomour II which I'm mostly using with high impedance headphones. There's some AC hum but I expect the DC filament supply to clean that up when I get around to building it. Other than that I'm really enjoying the sound of this thing.

I recently realized that my 1960s HP analog oscilloscope has a bunch of 12au7s from various manufacturers.
Would anything bad happen from running a 12au7 in place of a 12at7? GPT suggests that the 12au7 has less gain (not a relevant issue - I'm using a 2.0V RMS source) and reduced current draw. Does this indicate I would need to adjust the CCS current and/or 365 ohm cathode resistor? Or would it more or less work as is but with reduced voltage gain?

Thank you!

[blackharp]

Regarding swapping a 12AU7 for a 12AT7—you’re correct in that the 12AU7 has less gain and lower current draw. Since you’re using a 2.0V RMS source, the gain reduction may not be a significant issue, and it could still work fairly well with the reduction in overall gain.

The 12AU7 has about 1/3 of the gain of the 12AT7, so you’ll get less overall amplification. However, because you're using a 2.0V RMS source, this may still be sufficient for your needs. You may not need to adjust the current source (CCS) or cathode resistor (365 ohms) as they are designed to work for a wide range of tube variances, and the lower current draw of the 12AU7 likely won’t require significant changes.

However, the reduced current draw of the 12AU7 means it would likely draw less current from the power supply, so the bias point could shift slightly. You can try it as is, and if you notice any unwanted distortion or tonal changes, that might indicate the need for small adjustments to the CCS or cathode resistor.

It will likely work but with reduced voltage gain, so the overall output might be lower, but it shouldn't hurt anything in the circuit.

If you feel the output is too low, adjusting the CCS or cathode resistor could compensate for that loss, but if you're just looking for a quick test, trying it as is could give you an idea of how the circuit behaves with the 12AU7.

[bepis69]

Hey thanks for your quick response.

However, the reduced current draw of the 12AU7 means it would likely draw less current from the power supply

Since the driver tube is run with a CCS, won't the amount of current through the tube be the same regardless of whether it's a 12au7 or 12at7? I assume the CCS would just raise its output voltage to compensate for the lower conductance, keeping current constant?

Since it looks like it won't break anything, I'll try it out later and report the resulting plate voltage numbers

[blackharp]

Correct. Since the current through the tube is controlled by the constant current source (CCS), it will adjust its output voltage to maintain the set current, regardless of whether you’re using a 12AU7 or a 12AT7. The lower conductance of the 12AU7 will result in a slightly higher plate voltage, but the CCS will keep the current constant as intended. This makes it a safe swap without causing any damage to the circuit.

The primary difference will be in the gain and voltage swing. The 12AU7 will provide less amplification (lower gain), but since the CCS regulates the current, this will have minimal effect on the functioning of the circuit in terms of current delivery.

The plate voltage of the 12AU7 might be slightly higher due to its lower conductance. But the CCS should compensate to keep the current constant.

Reduced gain may result in a slightly quieter or less powerful output, depending on your source and overall system sensitivity. Since you’re using a high-impedance load (headphones), you may not need the extra gain that the 12AT7 provides.

[Paul Joppa]

You will need about 8 volts bias for two reasons:

1. to get the plate voltage up to around 200v, so it can swing +/- 60v to drive the 2A3 without clipping.

2. to allow for the grid swing increase.

An increased cathode resistor of around 1500 ohms should do the job, without needing any change in the C4S current. You may need to adjust the resistor,  and/or bypass it with a capacitor, and/or adjust the current to refine the operation, but this will get you close. Input sensitivity will be around 4 volts RMS.

[blackharp]

I agree adjusting the bias is ideal. Am I also correct leaving as is should not damage the circuit?

[Paul Joppa]

I can't think of any reason it would damage anything.