Author Topic: 834P Phono - Tube rolling  (Read 1737 times)

Offline ozmillsy

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834P Phono - Tube rolling
« on: May 13, 2014, 10:28:25 PM »
Made a discovery tonight about my 834P phono stage.   One of those "uh huh" moments.

Sometime ago I changed out a set of 12AX7 for 12AU7 tubes, because I had a tad more gain than I really needed.   The 12AU7's did indeed reduce gain,,,,, but,,,,,,. 

I dont tend to sit and listen to my TT all that much.   I really only use it for recording my LPs to digital.     I started to notice that my recent recordings were a bit weak for bass,  this became quite obvious when I was comparing my digital recordings made from 12ax7s vs 12au7s.    So I went up the road to our mates place yesterday to borrow some 12ax7's,  and see if I can get that bass back.    I was loaned 1 matched pair, and 1 tube slightly different.

The 834P has 3 tube positions.   I had assumed 1 tube was for left channel,  1 tube for right channel,  and the other for something else.    I wanted to confirm which position was which, as its not immediately obvious, to ensure the matched pair went into the correct sockets.     

I did some googling,  and I learned that it's not designed that way at all.     I found this description of the tube positions,,,,,,,,,

V1 - gain stage
V2 - gain/RIAA equalization stage
V3 - cathode follower/output stage

Each tube has a different function.   So I look a little closer at the circuit board of the 834P, the tube sockets for V1 and V2 are labelled ECC83, but V3 isnt.   And the penny dropped.   

V2 in particular is for the RIAA stage,  and by using a lower gain 12au7,  I reckon I effected/changed the equalization.

I did a little more reading,  and found a forum post on AudiogoN saying, for less gain to only change V3.    Another post describing V1/V2 going to both channels.  1/2 tube to left and half tube to right (for each socket),   and having the circuit this way means the tubes dont have to be perfectly matched.   I could use different brands in each socket and they'll "blend",  both L/R channels will sound the same tonally (in theory).   Very interesting, adds even more tube rolling possibilities into the mix.

So,,  I play some DK in Paris with 12au7's,  it sounded OK.   Then put the loaner 12ax7 tubes in (thanks mate),  and bammm, the bass was strong, warm, the whole tonal character changed - for the better !!   I cant believe I didnt notice this sooner.   

Goes to show,  it pays to do a little research on the circuit, and understand what the tubes are doing,   before changing in different tube types.    It's easy to say 12ax7s and 12au7s are the same tube with different gain,    but it appears to also be about how they are used.     Lesson learned.

It also goes to show, I should have taken greater care,  when swapping out the last set of 12ax7s  [kicks own @rse].

Very happy with the result so far. 
       
« Last Edit: May 13, 2014, 10:40:40 PM by ozmillsy »
It's all about the music,, not the equipment.

Offline Tuyen

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Re: 834P Phono - Tube rolling
« Reply #1 on: May 14, 2014, 01:00:07 AM »
I really like your post oz.  Thanks for taking time to share.

It describes your personal experience (in detail) and also lets me (the reader) take away something useful. I feel so anyway.

Sadly, these sorts of threads/posts are getting far and fewer it seems on the audio forums.   People not keen (or interested?) to share ideas/experiences/diy tips  for others to read and possibly make use of in their own systems.

Offline ozmillsy

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Re: 834P Phono - Tube rolling
« Reply #2 on: May 14, 2014, 11:43:50 AM »
Hey T,  cheers.   Sometimes we come to a realisation and feel kinda silly afterwards.    There is a tendency to often keep that to ourselves. 

But IMO, there is a lot of value in sharing our mistakes and learnings.    It's so easy to fall down a rabbit hole, and not realise it.   Using the 12au7's in this application was an example of that, I think.     

I should have spent more time comparing 12ax7 to 12au7 than I did.    I just whacked in the 12au7s on the assumption that it was simply less gain, and I didn't do any critical listening/comparisons (of X vs U).   

« Last Edit: May 14, 2014, 09:58:15 PM by ozmillsy »
It's all about the music,, not the equipment.

Offline springcreek

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Re: 834P Phono - Tube rolling
« Reply #3 on: May 21, 2014, 12:59:34 PM »
Easy thing to do. Glad you got it sorted Oz. Have you listened much since?

Offline ozmillsy

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Re: 834P Phono - Tube rolling
« Reply #4 on: May 21, 2014, 05:24:19 PM »
Yep, quite extensively.  Sounding good with the Mullards.

It's all about the music,, not the equipment.

Offline springcreek

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Re: 834P Phono - Tube rolling
« Reply #5 on: May 21, 2014, 10:39:37 PM »
Good to know