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RE: 813 set schematic

Ok, let's look at your schematic and see what it says.

The 6SN7 looks like it is running around 2mA.
This would put 132 volts on the plate and through direct coupling, 132 volts on the grid of the 813.

To bias at 300mA at the 300 volt level you need -20 volts cathode to grid. So the cathode has to sit at -152 volts. This runs the 813 at a 300 volt plate to cathode at 300ma or 90 watts on the plate. This is close to the max rating of 100 watts. And it is very non-linear part of the tube curve.

560 ohm cathode resistor at 300mA would produce 168 volts which is in the ball park. The cathode resistor would have to withstand 168 * .3 = 50 watts or 100 watts for safety margin.

However, RCA says for 813 audio, cathode bias should not be used. The direct coupling may offset some of the problem, but they were pretty smart dudes.

This will play and sound very unique, however this is really not how you should run this tube. Get a hammond 126b interstage and you will have a 1000% improvement by providing the 813 another 160 volts and not rip the tube apart at turn on and you won't need 100 watt cathode resistors and you can run the 6SN7 better

There are plenty of actual schematics out there with an interstage, this is for a reason.


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  • RE: 813 set schematic - Chip647 16:36:59 02/11/19 (1)

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