High gain vs high volume

[Robert bowes]

I have a unison simply phono, phono pre with 12AX7 tubes and a unison symphony SE tube amp. I’m using a Nagoka 500 MM cart. I find with the stock phono tubes the gain is so high that after the 11 o’clock position on my amp it starts to distort. I tried 12AT7 in the phono and the gain has dropped and my distortion starts about the 1 o’clock position on amp. I still like the sound from the high gain tube, is it okay to use as long as I don’t shoot over the 11 position on amp. I am using the 47K ohm position on my phono. Please any advice.

 

[AMR / iFi audio]

I have a unison simply phono, phono pre with 12AX7 tubes and a unison symphony SE tube amp. I’m using a Nagoka 500 MM cart. I find with the stock phono tubes the gain is so high that after the 11 o’clock position on my amp it starts to distort. I tried 12AT7 in the phono and the gain has dropped and my distortion starts about the 1 o’clock position on amp. I still like the sound from the high gain tube, is it okay to use as long as I don’t shoot over the 11 position on amp. I am using the 47K ohm position on my phono. Please any advice.

You can use it with a high gain tube at levels that do not distort heavily. Before it starts to distort, it's all good. 47k is a good setting for the MP-500 so I think you are all set

 

[Mendel]

Hi Robert
i suggest you try 5751 in place of 12ax7, as they are generally considered a direct replacement while 12at7 is not.
5751 have a mu (gain) of 70 while 12ax7 has a mu of 100 ( 12at7 has a mu of 60 by the way).
I believe JJ makes a 5751 but I have not tried it. For NOS (or used tests good) I recommend any U.S. made black plate triple mica tubes from the 1950s such as the Sylvania, GE, Raytheon or RCA. Grey plate Sylvania triple mica tubes are good too, as well as the RCA black plate double mica “Command” series.
No problem using 12ax7 and keeping the volume below 11 o’clock, other than a loss in volume control.

 

[DonH50]

Note the feedback depends upon the tube gain so changing to a lower-gain tube may impact other things like distortion and stability as well as noise. If it is too loud at 11 and fine at 10 I'd stick with the tubes you have.

 

[AMR / iFi audio]

Note the feedback depends upon the tube gain so changing to a lower-gain tube may impact other things like distortion and stability as well as noise. If it is too loud at 11 and fine at 10 I'd stick with the tubes you have.

That's a fair point, but in my experience with tubes, it's really hard to mess up the stability. Usually, the output transformer limits the bandwidth so that even with other tubes, it stays stable under all conditions. On the other hand, it doesn't mean that it can never happen, so a little caution is advised. You can also ask the manufacturer directly.

 

[adrianywu]

This depends on whether the distortion is due to not enough headroom in the phono pre for the cartridge output, or whether the phono pre output is too high for the amp. In general, you should have as high a gain as possible in your phono pre, and attenuate it at the input of the amp. This way, you attenuate both the signal and the self noise of the phono pre. If you use a lower gain input tube at the phono pre, the noise floor of the phono pre will probably stay the same (tube noise, power supply noise etc. is unaffected). That means your signal to noise ratio becomes lower. Just solder a resistor L-pad at the input socket of the power amp.