Over the holiday I have taken off on another tangent instead of building an amp.
I was trying to get better measurements of tube distortion so I can sort the used tubes I have on hand. I'm focusing on triode strapped pentodes including 6CB6, 6EJ7, 6J51P, 6JC6 and 6J9P.
I built a test box with :
(1) FET with Zener regulator to help filter the B+ comming in
(2) Switch Selectable tube anode loads of FET/Bipolar current source, Cascode DN2540 current source, and Gyrator.
(3) Fully bypassed adjustable cathode resistor.
(4) A Current Source Loaded FET Source Follower to buffer the output signal.
(5) DC input for cathode heater supply
This combination allows me to get measurements that seem to be 20dB better than I had previously made. The two screen shots show examples of previous and current measurements.
Is the data presented in the second FFT realistic for such measurements?
Is the distortion level in line with what should be expected for a current source loaded triode strapped pentode with fully bypassed cathode resistor?
Is the background noise level what should be expected?
In other words am I getting true representative data or do I still have a way to go to be able to accurately measure tube distortion and S/N ratio?
I was trying to get better measurements of tube distortion so I can sort the used tubes I have on hand. I'm focusing on triode strapped pentodes including 6CB6, 6EJ7, 6J51P, 6JC6 and 6J9P.
I built a test box with :
(1) FET with Zener regulator to help filter the B+ comming in
(2) Switch Selectable tube anode loads of FET/Bipolar current source, Cascode DN2540 current source, and Gyrator.
(3) Fully bypassed adjustable cathode resistor.
(4) A Current Source Loaded FET Source Follower to buffer the output signal.
(5) DC input for cathode heater supply
This combination allows me to get measurements that seem to be 20dB better than I had previously made. The two screen shots show examples of previous and current measurements.
Is the data presented in the second FFT realistic for such measurements?
Is the distortion level in line with what should be expected for a current source loaded triode strapped pentode with fully bypassed cathode resistor?
Is the background noise level what should be expected?
In other words am I getting true representative data or do I still have a way to go to be able to accurately measure tube distortion and S/N ratio?