After testing 4P1L with the 126C OPT, Paul Leclercq suggested a different operating point to see if distortion above 8Vrms on the OPT version was due to grid current. So firstly did a quick test with the CCS and found that -15V and 170V was a good operating point for 15mA. The 4P1L performs brilliantly with CCS and THD is only 0.05% when driven to 10Vrms.
Unfortunately distortion with 126C grows exponentially above 9Vrms. OPT is tested floating and not sure if is the Pete Millett interface a limitation with it.
EDIT
Andy Evans suggested that secondary green should be negative so phase is 180. Aparently there is a thread meantioning the distortion introduced by the way I was using the OPT.
Anyway, problem sorted! Look at now how nicely the 126C can perform at 10Vrms:
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
4P1L test circuit
(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)
4P1L test 1
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:
4P1L test 2
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:
4P1L test 3
So what if we compare the performance of the OT against the CCS?
4P1L test 5 (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%
4P1L THD 6 (CCS) minimum distortion figure
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….
