I currently have a great-sounding, Amperex 7062 in my Stereomour, which seems to have a huge soundstage and a bit less gain than the stock 12AT7. However, I am wondering what the optimum cathode resistor value is, or would like some guidance in terms of how to calculate it. Right now, I believe I have 500 ohm resistors in place of the stock 431 regulators, which hissed like snakes (apparently, I lucked out and got a bad set). The EH 12AT7 seemed to sound better with more current, which is why I ended up with the 500 ohm resistors versus the 680 ohm variants I began with. For the 7062, would I be OK simply putting new 431's in there versus mucking around with new resistors?
Optimum parameters for 7062 driver tube?
- Thread starter Dr. Toobz
- Start date
We did indeed get a cranky batch of 431's some time ago. Any replacement that doesn't have "WS" etched onto the chip will be a suitable (quiet) replacement. Do note that changing the cathode resistor under the 12AT7 in the Stereomour will not change the plate current.
You would find that the 7062 will achieve lower plate voltage in a stock Stereomour. If we assume PJ's optimal plate voltage value is 175V, you have a 500 Ohm cathode resistor, and your C4S is setup for stock, the cathode resistor/C4S combo will give you 1.8V of bias. 1.8V of bias and 3.6mA lands you somewhere around 120 plate volts on the characteristic curves.
3V of bias and 8mA (110 Ohm R1, 375 Ohm cathode resistor) of current looks like a pretty nice region of operation, and this gives you about 180 plate volts. The 300K/3W R2 resistor perhaps should be changed to something more like a 200K/3W. The issue with the above scenario would be the extra heat dissipated by the MJE5731A transistors, which could be dealt with by adding a heatsink to each of them.
The other option is to use more bias (4V) and less current than above (3.6mA, stock). This could be achieved with a 1K cathode resistor to tweak the bias, and you would be running under thermal operation conditions consistent with the design.
In an ideal world, you would try both and report back!
You would find that the 7062 will achieve lower plate voltage in a stock Stereomour. If we assume PJ's optimal plate voltage value is 175V, you have a 500 Ohm cathode resistor, and your C4S is setup for stock, the cathode resistor/C4S combo will give you 1.8V of bias. 1.8V of bias and 3.6mA lands you somewhere around 120 plate volts on the characteristic curves.
3V of bias and 8mA (110 Ohm R1, 375 Ohm cathode resistor) of current looks like a pretty nice region of operation, and this gives you about 180 plate volts. The 300K/3W R2 resistor perhaps should be changed to something more like a 200K/3W. The issue with the above scenario would be the extra heat dissipated by the MJE5731A transistors, which could be dealt with by adding a heatsink to each of them.
The other option is to use more bias (4V) and less current than above (3.6mA, stock). This could be achieved with a 1K cathode resistor to tweak the bias, and you would be running under thermal operation conditions consistent with the design.
In an ideal world, you would try both and report back!
Ok, so it sounds as though the most straightforward way to go about this is to increase the bias voltage by increasing the cathode resistor to 1k. I will likely try that route.
Is there any harm in running the tube at 120V plate voltage? I am assuming this means that the C4S is dropping more voltage than usual, and thus, running hotter than normal. Correct? The tube may be a bit more "romantic" sounding at this low plate voltage than I remember from before, but that would indicate more distortion at the un-optimized operating point. The 12AT7 sounds so lifeless in comparison, for lack of a technical term!
Is there any harm in running the tube at 120V plate voltage? I am assuming this means that the C4S is dropping more voltage than usual, and thus, running hotter than normal. Correct? The tube may be a bit more "romantic" sounding at this low plate voltage than I remember from before, but that would indicate more distortion at the un-optimized operating point. The 12AT7 sounds so lifeless in comparison, for lack of a technical term!
Paul Joppa
Moderator
The risk of running a low plate voltage is clipping in the driver before full output is reached.
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