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12sn7 heater-cathode voltage

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alexg

alexg

Member
Joined 2005
I built a 12sn7 linestage, first triode is a grounded cathode DC coupled to the second triode, configured as cathode follower, measured voltage as follows:

Triode 1
* Plate voltage = 121.4
* Cathode voltage = 4.16
Triode 2
* Plate voltage = 254.2
* Cathode voltage = 126.7

Heater Voltage 12.2VAC

Am using a toroidal transformer with 0-12V secondary, both legs of the secondary have 100R resistors to ground.

Sylvania tube manual on 6sn7GT and 12sn7GT regarding Heater-Cathode voltage:

Heater Negative with respect to Cathode - Total DC and Peak - 200V Max
Heater Positive with respect to Cathode - DC- 100V Max; Total DC and Peak -200V Max.

With a cathode voltage of 126VDC on my second triode and my heaters at 12.2VAC, do I need to use a voltage divider to meet the heater-cathode voltage difference?

I tried using a 100K-33K voltage divider and I am getting serious humming problem.

Thanks.
 
EC8010

EC8010

Ex-Moderator
Joined 2003
It's never a good idea to have a significant voltage between the cathode and heater (especially when there's signal on the cathode). Assuming you used one dual triode as the (stereo) input stage and another as the (stereo) output stage, elevate the heater of the second triode to 160V and bypass it to ground at AC with a large capacitor.
 
GordonW

GordonW

Member
Joined 2005
Nordic said:
What do you do where the two triodes in the same tube run at significant difirent cathode voltages...?

My current project has one such a tube, 1.6V on one cathode 145V on the other... wich still falls in the 200V cathode to grid limit specified....
Click to expand...


Split the difference. In this case, you would like to consider the swing of the voltages too. You need to make sure that the highest swing of the bigger voltage, does NOT go over the peak AC difference for the H-K voltage too, on the 12SN7... which, IIRC, is 200v.

So, the way to do that... consider the max voltage swing of the second tube, and add that to the DC voltage. Say, it's a 150v p-p swing... or +- 75V centered at 145v. That would give 220v for the max voltage on the cathode. In this case, with a split difference of 145v and 1.6v (73.3v) would still work... the max AC H-K voltage would be 220 - 73.3, or about 146.7 volts. That's safely within the 200V max swing...

Sometimes, if you're pushing the limit, you have to fudge the voltage to where it meets in the middle somewhere... you may have to push the DC heater offset higher, in some situations.

I've had cases where I was right on top of a max negative DC difference in one direction, and positive max AC difference in the other direction, even after putting the DC heater offset as expeditiously as possible. In a perfect world, that's when you start looking at possibly separating the functions into two bottles...

Regards,
Gordon.
 
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