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Why are 12ax7 triode sections numbered 'backwards'?

Discussion in 'Glowing Bottle Tube Amp Forum' started by King Fan, Mar 15, 2020.

  1. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    This is totally and completely unimportant, but I'm curious. Although some user-generated illustrations don't, almost all 12a*7 technical diagrams and data sheets put pin 1-2-3 in section 2 (or triode B) and pin 6-7-8 in section 1 (or triode A).

    I know (I think?) the two triodes are identical. Logical reason? Historical reason? or...?

    upload_2020-3-15_12-52-10.png

    upload_2020-3-15_9-45-48.png
     
    Last edited: Mar 15, 2020
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  2. Mikey_Boss

    Mikey_Boss TDPRI Member

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    More knowledgable people will most likely add to this, but IIRC, due to how the tube is constructed, section 1 (pin 6,7,8) are considered lower noise and therefore preferred (or something along those lines)
     
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  3. Engine Swap

    Engine Swap Tele-Afflicted

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    Fuzzy memory here, but I think it is related to humbucking filiments.
     
  4. galaxiex

    galaxiex Tele-Afflicted

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    Because we can.... ;)

    Seriously tho, I only commented cuz I'm curious about this too.
     
  5. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I hope it doesn't matter much. I know for sure I tend to use 'V1a' for, say, the 5E3 normal channel (pins 1-3).
     
  6. Mikey_Boss

    Mikey_Boss TDPRI Member

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    Yup, right. Found it in the data sheet. When 9, and 4+5 are used for heaters and the filament tranny center tap is connected to earth, 6,7,8 are more favorable with respect to hum
     
  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Excellent. Can you share the source (which data sheet) so I can store that away for future reference? Thanks.

    Let me think though. Does knowing the two triodes aren't perfectly symmetrical w.r.t. hum explain why triode 2 has higher-numbered pins? I can almost imagine the numbering rules and standard spacing of nonal pin layout interact with the asymmetry of the heater wiring. Or something. But I guess if so those rules and those interactions would explain the 'backwards' numbering -- so I still don't understand it.
     
  8. Dacious

    Dacious Poster Extraordinaire

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    From the top V1A would be to the left front. Seems to be because the oldest gear tubes went in from the top. When Leo started building amps, first he put them in back; then he hung them from the top, so he could use the same chassis in heads and combos.

    The pinouts are shown from below the chassis, because that's how they were wired. I guess.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I am not an expert in history of valves. Did the older iterations of dual triode such as 6sl7 share this phenomenon?
     
  10. Mikey_Boss

    Mikey_Boss TDPRI Member

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    https://www.google.com/url?sa=t&sou...FjAPegQICBAB&usg=AOvVaw393q_WKWCkUbQoT4ZkxH0T
     
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  11. Mikey_Boss

    Mikey_Boss TDPRI Member

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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So why would a 12dw7 have the 12au7 side on the favorable side for hum? Seems counter intuitive.
     
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  13. Wally

    Wally Telefied Ad Free Member

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    Modern usag3 sometimes names pins 1,2, and 3 as the A triode. Here is a Fender schematic that does this....it confused me when I saw it and made me say Whadahey.....
    https://schematicheaven.net/fenderamps/blues_deluxe.pdf

    All of the old spec sheets indicate 6,7,8 as being the A section or section 1. That is interesting about the hum rejection. If that is so, then it seems one would want to use the 1st or A section for the earlier gain stage rather than as the second gain stage as in those Modern Fenders, correct???
    The Bruce Zinky designed Pro Sonic uses the triodes in this manner...and labels them according to the old spec sheets. The B sections are used as the first gai; in the clean mode and as the first two gain stages in the Gain mode. https://schematicheaven.net/fenderamps/prosonic.pdf in a high gain amp tis might be an important consideration. But...the .blues/hot rod amps are based on the same basic approach as the Pro Sonic, and Fender uses the 1,2,3 triode as the first gain stages. So...does it really make no difference???.
    Vintage .fenders used 1,2,3 as the first gain stage, as well. Again......whadahey?
     
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  14. telemnemonics

    telemnemonics Telefied Ad Free Member

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    10 9 8 7 6 5 4 3 2 1?
     
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  15. kbold

    kbold Tele-Afflicted

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    More important to consider:
    Why does half the world drive on the left, the other half on the right?
    Why do we have metric and imperial measurements (as well as US and British imperial).

    Seems the human brain is too large for requirements, so dumbs itself down to compensate.

    WRT to this post, possibly just some guy sitting at the drafting table, who could chose whatever.
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I have noticed the shared pentode/triode tubes have the triode on what would be the "A" side. The triode would usually be first in a circuit. Food for thought.
     
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  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Rocket science.:lol:
     
  18. JohnnyJumpUp

    JohnnyJumpUp TDPRI Member

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    Simple! That's it! Thank god it's not music theory!
     
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  19. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Ah, but it is. @telemnemonics is famous for his tasty decatonic riffs...

    Back at the triodes, I've learned something cool (and I may wire my next V1 with 6-7-8 first, as per @Wally's helpful note). I guess I still don't know why the pin numbers don't go 'upwards' in the triodes, but I can imagine there's some asymmetry in the internal heater construction w.r.t. the necessary / mandatory layout of pin numbers.

    EDIT: And, duh, just thinking about the 5E3 I mentioned, I realize 4-input Fenders do use the 'quieter' triode as a first preamp section. So for less hum, just plug into the bright/instrument channel... :D
     
    Last edited: Mar 16, 2020
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  20. Wally

    Wally Telefied Ad Free Member

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    King Fan, I suppose the only way to ascertain if there is a preferred order for those triodes is to note operation in one arrangement and then change the input preamp stage to other triode. I am going to think that there may be no difference. I checked the Mesa Boogie Dual Rectifier, which is a ‘clone’ of the Soldano SLO circuit. Mesa uses the B triode...pins 1,2&3...for the first stage of the clean mode and also uses the B triode of V2 for what is the second gain stage in the high gain mode. Of course, Mesa labels those triodes as the A triode, which is contrary to the old school thought. There are no PIN numbers or triode labels in the schematics of the SLO that I have seen. The Peavey 5150 labels the triodes A and B but does not designate PIN numbers. I would think that IF there were noticeable differences in noise levels, these high gain amps would use the B triode...pins 6,7,8....in the earlier stages??? But....then there is Zinky and the Pro Sonic, right? Why did he go against the grain? I don’t know.
    I know enough to be confused...for a second....and then just walk on down the path. My main concern is the modern thought that decades of established terminology must needs be overturned. Many years ago I was confused as to why the engineers who designed these tubes labeled the triodes A and B in the order that they did...but I accepted it. Then I started seeing modern amp designers change the engineering terminology that the designers of the tubes used.
    I have a Bogner Telos open on the bench. I think I will see how he went about using those V1 triodes. Fwiw, this is one well-built PCB amp. 0A1A4AA4-0119-4897-9B13-71DFD1223BBF.jpeg

    note the parallel heater filament supply. And...though you cannot see it, the first two 12AX7s are supplied with 12 volts.
     
    Last edited: Mar 16, 2020
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