Tube swap THD

[tvrgeek]

This is my first attempt at tube swapping. Base is the cheap Meng 10W amp off e-bay. First stage is a paraphase using 6N2's. Second a push-pull, gnf. 6AQ5's.

Did not sound that smooth, so I found some NOS 6AX7 Tung-SOL. Made in USA. Easier than a 12AX7 as they drop in. Forum comments suggested the 6N2's were the biggest culprit.

Measured THD and gain, base line and after at 100, 1K, 4K, and 6K.
Measured with a 7 Ohm resistive load, 2.7 volts out. ( about 1 W.)
Used True RTA for the generator and analyzer.
I found, the gain with the 6AXT to be about 3 dB higher.
Noise was about 4 db lower.
THD across the board was higher. About a max of 1% with the 6N2, and 1.6 % with the 6AX7's. Second order was higher relative to third with the 6AX7's. Lower with 6N2's.

Oddity, both tubes. At 1K, they had spikes at about 900 and 1.1K. only 35dB down. This did not happen at any other frequency. I have not hooked up my sweep generator to see what that looks like. Next step.

Power supply noise sure was obvious, though about 70 dB down. UGLY. 60, 120, 180, and on and on. I suspect I can improve that a tad.
Crosstalk is bad. Poor chassis wiring. Plan on improving that.

So, do these results seem reasonable to the more experienced? A 1 for1 swap is not exactly a perfect test as the gain is different, but before I play with the bias, and symmetry. ( I think that is what the pot is for) just wanted a baseline.

I did not do frequency response yet, so I have not looked at the compensation to the feedback resistor. ( It has none stock) If I want the gain the same, do I adjust this resistor, or is is set by the tube? Like I said, new to tubes, still doing a lot of reading.