Improving the driver with a gyrator load
After the early experiments with the 4P1L driver in pentode mode, I decided to look at improving it somehow given advice given. The gyrator load is not a good match for a pentode unless the reflected impedance is low enough to control the gain of the stage. Gary Pimm recommends:
“In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain.A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without “calibrating” the stage on the test bench. Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.”
So I opted for adding a resistor in parallel (RL) to adjust gain, minimise distortion and improve PSRR:
The load resistor is 68K. I optimised the operating point to reduce distortion at maximum swing (i.e. 200V peak to peak). The input impedance of the soundcard interface which is 100K didn’t produce a significant impact on the distortion when measuring from the anode output or in the mu output:
Interesting to see that distortion is now nearly half of previous operating point and 0.27% for 200Vpp is very good.
The screen current is approximately 1.8mA at 81V bias.
Ale, did you do any experiments to see if the results was the same if the resistor were placed after a coupling capacitor? This would not matter so much with a Gyrator as with a current loop. However, it would allow for adjusting the value of the grid-gnd resistor and eliminate an additional component when capacitive coupling were used.
Steven,
No, I haven’t. Should be ok I think when coupling with a capacitor. The grid bias capacitor can then be the RL used in my test.
Cheers,
Ale
Hey Ale,
I noticed you found out yourself about loading the pentode as I recommended. Hope you did your measuring at the follower output(not A). Otherwise load is now 100k from your soundcard parallelled with 68k. Odd order harmonics are high though, but can maybe be helped from distortion cancellation together with the next stage.
Lars,
Measure is from anode, but thought about what you said as well so tested mu follower and didn’t notice a difference in THD. Now that you say so, didn’t look at whether the harmonic composition was different.
The interface impedance is 100K, so not a big impact if in parallel with the current load resistor…
Thanks
Ale
You’ll have to let us know if this actually sounds better, since from your FFT there is more odd order harmonics.
True. It may be a hard contest against the 46 in filament bias. I should retake measures at lower driving requirements as this may be a good driver in other amps
Cheers,
Ale