Andy Evans built a pre-amp with the 4P1L and was delighted with the sound of it albeit the 4P1L was running below its optimal operating point: 15mA given the limitations of the 126C interstage transformer.
I went to my workshop to test this configuration and looked at biasing 4P1L with fixed bias and driving it with 1Vrms or more to see what the results were:
So here is the first test at Vg=-4V, Va=74V and Ia=15mA
(all tests were done with the 100k input impedance of the Pete Millett Sound Card interface as the secondary load of the OT. 4P1L had both filaments in parallel and If=600mA)
You can see a richer harmonic profile with the OT and distortion is around 0.13% when driven with a 1Vrms providing an expected Vo close to mu (Vo=8Vrms)
The distortion gets very high when output voltage is higher than 9Vrms:
Now if we bias the valve at a more convenient operating point:
We get a slight improvement in THD down to 0.11%. However the distortion above 9Vrms is still high:
So what if we compare the performance of the OT against the CCS?
As we can see from above the distortion is halved. Now if we look at how well this valve could perform if biased in a better operating point, we can see that distortion can be reduce down to 0.03%
Minimum distortion from a CCS (or gyrator) doesn’t mean that it will sound better. Clearly the OT doubles the THD of the CCS equivalent circuit. Gain here is nearly same on both as OT is in 1:1. Only way of judging both is to do a listening test….
(which is what I’m planning to do next)