[toobluvr]

Got the itch to roll some tubes in my ATC-1 preamp.......more specifically, the phono section.

This pre has been around since early 90's but not the most common piece of gear out there, so perhaps this photo will jog some memories.



Anybody know how this MM phono circuit works?
It uses three (3) 12ax7 tubes,  accessible on the outside of the left side panel.

Specifically, my questions are:

(1) do all 3 tubes need to be identical and matched?

(2) or is a pair required?  If so, what positions?

(3)  or can 3 different tubes be used?  If so, I guess this means the triodes are being split in each of the tubes to obtain L and R signal.

(4)  if singles or pairs are OK, which position(s) has the greatest sonic effect?

(5)  can 5751 types be used?

(6)  can 5751 be mixed in with 12ax7?

Also, any input on what tubes you liked and why is greatly appreciated.

Thanks in advance,

John

[toobluvr]

Noone?

This piece must be rarer than I thought!

 

[lcrim]

toobluvr:
I tried to find some info online that would be helpful to you but failed.  My Eastern Electric Minimax phono section has three 12AX7's in its MM gain section as well.  The center tube is the primary gain stage and the two flanking tubes are the secondary gain section for each channel.  I know that you could substitute a 5751 for any of these but another member tried changing out just the secondary sections for 5751's and was pleased with the result.  Best I can do.

[toobluvr]

Thanks Larry...you get an A+ for effort!!
 

I did all the online researching too...found nothing.

I just took the lid off and looked at the actual circuit.  I'm no technician, but it looks like it might be similar to your Minimax.  The 2 outside tubes have identical parts and layout around them, so this tells me they are a L and R pair, and matched tubes are probably required.  The parts around the middle tube are completely different, so I'm guessing that it is probably OK to use a single and different tube here.

I called Axiss  Audio (Air Tight imported / distributor) on Friday but guy that handles it was out. 
I hope to have more info and confirmation when I speak to him on Monday.

[Browntrout]

If it's anything like my preamp (art audio) then the single tube does the gain for the line level inputs and the two are for the phonostage. I can recommned you take the tubes out and clean the pins and put some silver paste on them, I did this and the difference was very noticable indeed. 
  Nice piece of kit by the way.

[toobluvr]

If it's anything like my preamp (art audio) then the single tube does the gain for the line level inputs and the two are for the phonostage. I can recommned you take the tubes out and clean the pins and put some silver paste on them, I did this and the difference was very noticable indeed. 
  Nice piece of kit by the way.

No Mr Trout.....

The three tubes are phono only....  as per the manual.  They are accessible from the outside w/o even removing the lid......along the side panel.  A rollers dream!
 

The line section tubes (one pair 12ax7) are in a completely separate area inside the unit and only accessible by removing the lid.  And based on the identical parts and layout around them, obviously a matched pair of L and R.

PS:  what silver paste do you refer to?

[Browntrout]

I used a cheap contact enhancer made by Mapleshade and have used it on the pins of all my tubes. It's the cleaning that makes the difference mostly though, my tubes in the preamp were about five years in and had oxidised quite a bit, very very fine emery paper did the trick then some Kontak then the paste.