6P1 in triode cathode R value?

[WizardWG4]

I'm trying to optimize a 6N2 6P1 amp in triode mode and am wondering about the recommended value for the cathode resistor. Plate and screen are 244v and cathode is at 11.5v. The cathode resistor is currently 270R. I believe the 6P1 is supposed to be close to a 6V6/6AQ5 and noticed other schematics size up the cathode R when triode strapping.

The amp sounds great but I'm only getting about a 1/2 watt of clean power before clipping. Any help in squeezing some more power out would be appreciated.

 

[TriodeLuvr]

1/2W might be all you're going to get in triode mode. It would help to know the voltage on the 6P1 cathode and anode with the 270Ω resistor installed. In any event, I suspect you won't obtain a significant amount of additional usable power without converting to pentode mode and adding feedback.

Jack

 

[WizardWG4]

1/2W might be all you're going to get in triode mode. It would help to know the voltage on the 6P1 cathode and anode with the 270Ω resistor installed. In any event, I suspect you won't obtain a significant amount of additional usable power without converting to pentode mode and adding feedback.

Jack

I'm getting about 244 volts at the plate and screen and 11.5v at the cathode with the 270Ω. Here's the triode curves from the datasheet.

 

[gadget73]

11.5v / 270 ohms is 42.5 ma, and you're at effectively 232 volts across the tube. That tracks fairly well with the graph as best I can tell without actually drawing lines on it.

If you have a choke with lower DCR than 240 ohms that will give you more supply voltage to work from, which will help power output. Should be able to run this up another 15-20 volts easily and get it closer to 250 across the plate.

per the tube curve calculator, and assuming it's data is right it tells me 0.64 watts at g1 = 0 volts, allowing for transformer losses thats going to come out pretty close to a half watt. You may just be getting all its gonna give. A cap coupled driver isn't going to be able to drive the grid beyond 0 volts relative to cathode.

in pentode mode thats good for almost 4 watts, or ballpark 3.5 after transformer losses.

 

[TriodeLuvr]

I'm getting about 244 volts at the plate and screen and 11.5v at the cathode with the 270Ω. Here's the triode curves from the datasheet.

That's 10W dissipation, which is about where it should be. The curves are very similar to the 6AQ5, as are the dissipation and voltage ratings. Your only route for any significant power increase is to go pentode mode and add a feedback loop. See if you can find a schematic somewhere for a 12AX7 - 6V6 SEP amplifier. That should provide a starting point for the feedback values.

Jack

​

 

[WizardWG4]

Thank you both. It has a 110v transformer and I'm using a variac so I could get more B+ as long as I drop the heater voltage appropriately.

I'm thinking pentode mode with feedback is probably the way to go.

 

[gadget73]

easy enough to stick a resistor in the heater line. If you can do both the resistor in the heater line and bump the B+ by using a choke instead of that 240R resistor (assuming there is room) that would squeeze all there is to be squoze.

If you don't want to run the screen at full B+, breaking that 15k between B+ and the 6N2 into two resistors with a cap in the middle will get that done. I wouldn't make the resistor between B+ and the screen too overly large so it doesn't introduce a bunch of sag but maybe 470-1000 ohms would do OK. Having the extra filtering will also help with noise since supply ripple going into the screen grid will be louder than the same amount of ripple at the plate.