2 Questions on tube replacement

1-yes

2-no

All these tubes can be substituted for a 12ax7!!

Direct Replacements

1. ECC83: This European equivalent of the 12AX7 is virtually identical in terms of electrical characteristics, mechanical dimensions, and heater requirements. It provides a seamless transition and is often used interchangeably in audio equipment.
2. 7025: This is a high-quality, low-noise version of the 12AX7 that offers improved performance in terms of reduced microphonics and hum. This tube is an excellent choice for audio equipment that requires low-noise operation.
3. 5751: Another directly replaceable tube with slightly lower gain than the 12AX7 (approximately 70% of the 12AX7’s gain). This tube provides a slightly different tonal characteristic, making it suitable for those seeking a cleaner, more transparent sound.
4. ECC803S: This is a premium, long-plate version of the ECC83, known for its exceptional sonic qualities and high reliability. This tube offers improved microphonics and stability, making it an excellent choice for demanding audio applications.

@yogiboy covered the important points

just be careful with the lower gain 5751's -- don't mix them with standard gain 12ax7's, or you may get weird imbalances in the circuitry

btw, i have many many nice old stock 12ax7's collected over the decades, feel free to pm me if i can be of help

Thanks to both of you. @jjss49 I have enough now, but may reach out when I need the next set!

Wow! Question answered and help provided within 3 posts. I am impressed.

But wait… there still may be someone that adamantly claims something to the contrary. 

The 7025’s can be icing on the cake if you can find them. I used to get mine on Ebay. Agree with the comment about the gain on 5751’s being potentially problematic. My previous Muzishare x7 did not like those.

Yes, I am usually hesitant to make such bold comments. :-)