Hi John,
Yep, that looks right. Checked carefully twice now with 2 amps and 4 sets of valves, comparisons in the same amp in the same sessions, today with GerryB. I used the 600ohm input and -40dB output of the Hagerman iriaa rather than include any step-up transformers and reffed at 2v p-p 1KHz on the scope at 0.5v/div showing a span of 20 graduations.
The drop in gain from ECC83 to 5751 is 14/20 on the scope = -3dB. That relates well with the difference in mu of the 2 valve types.
After a straight swap from ECC83 to 5751 there is a slight treble lift. After re-setting the trimmer caps there is a just-perceptible lift (still a net roll-off) which results in a -6dB point at 65KHz instead of 50KHz. I believe the slightly lower plate resistance of 5751 would account for this but we'd have to try a few more different makes to be more sure.
Usage-wise, we are back to that intangible quality - the sound - and to my first post where I did say these may not be better than anything else but are viable alternatives
. I've taken so long to try them as I knew they were different electrically and wanted to know what effect they may have rather than just plonk them in.
I recd some 6072s today which would have been interesting to try, if the gain is enough, but they're not what they should have been so have gone back...
Rich