PAS3X Phono Pre-amp

I have two PAS3X pre-amps, one I built and one I bought off of EBay. Neither have Modifications. The one I bought had a gritty "clipped" sounding phono pre-amp. The other inputs sounded fine. I read some posts on a guitar blog that a 12AT7 tube could be substituted for the 12AX7. The pin-outs are the same, and the operating parameters are the similar, with the exception the 12AT7 has less gain, 60 as opposed the 100. I made the substitutions on the phono pre-amp. The result of the substitution was much improved audio, excellent mid range, and highs. Noise floor is very low. All the gritty clipped audio was gone. Yes, the gain was a bit less, as expected, but closer to the other inputs. I can find no downside to this substitution. Any comments?

radiojer wrote:I have two PAS3X pre-amps, one I built and one I bought off of EBay.  Neither have Modifications.  The one I bought had a gritty "clipped" sounding phono pre-amp.  The other inputs sounded fine.  I read some posts on a guitar blog that a 12AT7 tube could be substituted for the 12AX7.  The pin-outs are the same, and the operating parameters are the similar, with the exception the 12AT7 has less gain, 60 as opposed the 100.  I made the substitutions on the phono pre-amp.  The result of the substitution was much improved audio, excellent mid range, and highs.  Noise floor is very low.  All the gritty clipped audio was gone.  Yes, the gain was a bit less, as expected, but closer to the other inputs.  I can find no downside to this substitution.  Any comments?

You have turned the PAS to something with more dist and less RIAA like.
Get new 12AX7 and be done with it.

Proper low frequency response of the phono stage requires all of the gain the 12AX7 can provide as well as positive feedback. The 12AT7 will result in considerable loss of bass response.