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Any difference between section 1 and 2 of a double triode?

Discussion in 'Shock Brother's DIY Amps' started by chas.wahl, Nov 30, 2020.

  1. chas.wahl

    chas.wahl Tele-Meister

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    Perhaps a stupid question, but I've seen some schematics/layouts that use the higher-numbered anode/grid/cathode pins of a double triode as the "first" stage connected in a circuit. For example:

    upload_2020-11-30_12-36-14.png
    And also advice to do the same:
    So, is this just a practical working out of wiring configuration and heater, optimized to the socket orientation and location dictated by position and screw holes? Or is there something inherently, internally different about section 2?

    I realize that pins 1, 2 and 3 are generally called "section 2" in datasheets for a 12AX7 or 6SL7; but why is that?

    I also understand that heater wiring might be handled differently for a nonal tube (12A*7) vs octal (6S*7), since heaters are connected to pins across the socket from each other (4/5,9) on the former, and adjacent (7,8) for an octal. But I've seen this switcheroo for both types.
     
    Last edited: Nov 30, 2020
  2. teleMc

    teleMc TDPRI Member

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    upload_2020-11-30_11-2-12.png
    Second section is the lower pin numbers ...
     
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  3. chas.wahl

    chas.wahl Tele-Meister

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    Yes, I understand that, and revised my post to include. But why connect section 1 first, in preference to section 2? Does it matter?

    In the schematic used as an example above, the two 6SN7 tube stages following the preamp both have section 2 connected earlier than section 1:

    upload_2020-11-30_13-13-32.png

    . . . and I did confirm that 1, 2 and 3 are section 2 in a 6SN7 too.
     
    Last edited: Nov 30, 2020
  4. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    Look at how the pins are numbered. Base up. Now look from the top. Assume the 'cut-out' section of the pins on a Noval is toward you, section 1 ( 8,7+6 ) is to your left, section 2 is to your right.
    There's no real difference between the two.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    From what I have read there is a difference. IIRC it was a tube datasheet.
    The proximity of the heater inside the tube contributes more noise/hum to the pin 1,2,3 side.
    When I mentioned this I referred to this thread:
    Why are 12ax7 triode sections numbered 'backwards'?
    Discussion in 'Glowing Bottle Tube Amp Forum' started by King Fan, Mar 15, 2020.

    I think someone in that thread may have quoted a source.
     
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  6. wabashslim

    wabashslim Friend of Leo's

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    Seems like I was once told, many years ago, maybe even by a high school electronics teacher that the higher-numbered pins should always be the first preamp stage for reasons I don't remember now. But I was also once told that a whammy bar will eventually cause the neck joint to fail, so the BS runs wild among the masses.
     
  7. Paul G.

    Paul G. Friend of Leo's

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    Depends on the tube. The common 12AX7, 12AT7, 12AU7, 12AY7 have triodes that are identical.

    There are other tubes that have different configurations, i.e 12WD7 -- one triode high gain, one lower. There are even tubes with one triode and one pentode or three triodes. They are not common nowadays, but I have to mention it.
     
  8. chas.wahl

    chas.wahl Tele-Meister

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    Okay, I've read the "glowing bottle tube amps" thread that @Lowerleftcoast referred to (apologies for not having found it, though I did a search). The poster @Mikey_Boss quotes from a Philips ECC83 (12AX7 in 'murcan) datasheet from 1970, last revised 1999 -- so not necessarily what I would call "vintage", if that means anything.

    I will note the following:

    1. A GE 12AX7 datasheet from 1953 has this "wiring" diagram

    upload_2020-11-30_16-15-40.png

    while the later Philips datasheet has this

    upload_2020-11-30_16-18-47.png
    Notice three things: a) in each diagram, the wiring to each triode is more straightforward on one side (pins arranged in same sequence as the tube elements they connect to, while the other triode has conductors crossing to connect to elements disposed in reverse order as the pins; and b) which triode is straightforward and which is "crossing" is different for GE (straightforward is 678) and Philips (123); also don't miss that even though Philip's sheet is source for the "678 = less hum" note, that's the more complicated side of the diagram. RCA's 12AX7 datasheet has essentially the same diagram as GE's.

    Also, c) look at the heater wiring -- GE (and RCA) show one heater, connected in the middle to pin 9, to which the heater is nearest, and the connections to pins 4 & 5 made across all the cathode/grid/anode connections, while Philips shows two separate heater symbols, with the conductor between them connected to pin 9 on the other side of the tube 9 (conductor run between the elements for the two sections), and the conductors from pins 4 and 5 connected to each of two heaters, without crossing.

    2. A Brimar 12AX7/ECC83 datasheet from 1950-52 doesn't have a "wiring" diagram, unfortunately, but it does note that the getter is connected to anode pin 6. Of the datasheets referred to above, only the 1970 Philips one appears to have the note about 678 section being less prone to hum. And, an earlier Philips 12AX7S datasheet from 1968 has the same "wiring" diagram as above, but lacks the note about hum.

    3. Finally, with respect to octal tubes, the "wiring" diagrams I have for these (6SL7 RCA, Tung-Sol, Brimar; 6SN7 GE, Tung-Sol) all comport with the GE 12AX7 diagram shown above, wherein the "straightforward" wiring from pins to elements is on the section with higher pin numbers (in the case of octals, 456 rather than 678).

    4. I have taken a look in my 1953 copy of the Radiotron Designer's Handbook to see if there's anything related to this topic. While hum due to stuff that happens within a valve is covered, I didn't find anything that seemed pointed at double triodes, with the possible exception of 1) capacitive coupling from heater to any electrode, 2) heater-to-cathode conduction, 3) heater-to-cathode emission (or vice-versa), and a few other things that don't seem to apply to differences between section 1 and section 2.

    I'm not suggesting that the diagrams in question are an accurate representation of the wiring internals of a double triode -- it's not even clear to me whether there's a standard configuration of the elements within such a tube, or whether they vary, like the diagrams do. But based on what I've been able to glean (and understand), I think it's unlikely that datasheets are going to tell the story. Maybe it's better to simply follow the schematic blindly, if in fact it gives one any guidance on this point.
     
    Last edited: Nov 30, 2020
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  9. Wally

    Wally Telefied Ad Free Member

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    And so....Section one..p6/7/8...should always be used as close to the input as possible due to its better rejection of hum?

    It is interesting to note that ‘modern’ usage reverses these two s3ctiins...they call Pin1/2/3 Section 1.
     
  10. Dacious

    Dacious Poster Extraordinaire

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    I think it's because there's a convention in the Western world to work left-right. Viewed from below pin 1 is closest to the left. Usually most devices I've seen plug into the left on a combo (which were the first sort of amps).

    So when wiring things it was discovered short runs = less noise, less cost (copper wire is expensive) so input whether guitar or phono or mic tended to be on the left. Power entered rear right, along with EMF sources like power transformers outputs, power tubes, chokes etc.


    That means 6-7-8 are closer to the right and used first.

    Leo used the same format when hanging amp chassis down inside the rear of his Tweed cabs, then maintained it when the chassis moved to the top and controls moved to top front. He set the IBM computer standard for everyone.

    Jim Marshall turned it on its head with chassis turned upside down in heads with tubes and transformers pointing up.... Still feels weird plugging into the right side of a Plexi.

    That's just my theory - but ask my Southpaw missus, it's a right-handed world......

    Maybe if vacuum tube amps were made in the eastern world first, things would be the other way round ....
     
  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I can't find the reference but I remember reading that in the 12A*7 family the higher pin number triode is quieter and should be used for noise critical areas like an amp input stage. Soldano wired all his triodes this way for minimum hum.
     
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  12. Wally

    Wally Telefied Ad Free Member

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    and so the link that lower left coast gave us states. And, yeah...it would pay serve Soldano well to pay attention to that spec sheet instruction with five gain stages. And...NOW I understand why Bruce Zinky laid out the Pro Sonic as he did. The first gain stage 8n the clean mode is the A sectioned V1...pins 6,7,8. Also, the first added gain stage in the Lead mode is the A section of V2. This first added gai; stage is i; effect the second gain stage of the lead...high gain...mode. The schematic shows us all of these details. The Soldano schematic I find does not enumerate the sections or pins.
     
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  13. NTC

    NTC Tele-Meister

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    But doesn't each triode have it's own heater? One side being quieter does not make sense to me from a purely physical standpoint.
     
  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Maybe I consider these things from a purely different standpoint.
    I don't understand how the two sides could possibly be the same. We know the artist rendition of the above datasheet examples is not accurate but it does demonstrate that the path of the heater is not a mirror image for each of the sections.

    Basically we are talking about leakage current and capacitance which is effected by the materials used to build the heater and cathode. The length of the wire. The thickness of the wire and the aluminum oxide insulation concoction. The imperfection of the insulation. The proximity of the heater to the cathode. The construction of the tube and variances of wire diameter both intentional and unintentional.

    Electrons are very small. The difference of any measurable distance is huge from the perspective of an electron.

    What we are discussing may be picking nits but when considering an amplifier increases a small signal to a large signal, IMO it is good to know where the noise may be coming from.
     
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  15. Wally

    Wally Telefied Ad Free Member

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    One wonders why Leo Fender never paid attention to this aspect. Of course, Mr. Fender never built a high gain amp, did he?
     
  16. hepular

    hepular Tele-Holic

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    but that wording is ambiguous because what is 'favoured' is unspecified: to conclude the 678 are the best pins for reducing hum, the reader has to insert that in the reading. If one inserts "for inducing hum" after the favorable, that would make 123 the preferred pins for reducing hum.
     
  17. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Late to this thread, but I was the troublemaker who started the first one. :D

    Summarizing what I recall and you guys have figured out: as @Mikey_Boss showed us there, heater pins 4/5 and 9 are of necessity asymmetrical to the tube layout, so it makes sense there may be less hum on one side.

    But why number the quieter side with the higher numbers? Our smart friend @Dacious has been at both parties, and I think he's nailed it then and now. When tubes were put in 'right side up' the higher numbers were (may have been?) closer to the input side.
     
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  18. elpico

    elpico Tele-Afflicted

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    Like you guessed those datasheet diagrams have no relationship to the actual construction of the tube. They're not saying "this wire is crossing this one", they're just connecting the abstract schematic symbol for a tridoe to the numbered pins.

    Given the wide variety of 12AX7 tubes that have been made in so many factories in so many different eras I'd be amazed if any rule like this was true of all of them. If a datasheet said "pins 6,7,9 are preferable for hum" I'd believe that may be true for the tube that particular company was making at the time the sheet was written, but what evidence is there that this footnote is some kind of universal gospel?
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Yes, for sure, I have no ideas how the wires route internally from the pins to the heaters. I'm only thinking that 4/5 and 9 are not symmetrical around a circle, or, say, 4 and 5 are not equidistant from 9, so maybe no matter how you arrange the internal array, there's gonna be asymmetric risk of heater hum.

    It might be that the 'Dacious' answer (L-R engineering of inputs, and viewing the numbers from the tube side instead of the wiring side) is all that's involved, and the only reason the A and B triodes don't get numbered with the first pin number in the 'first' triode.

    At any rate, @Wally's right (he always is) that modern usage doesn't always call 1-3 triode B.

    In any event, I refuse to rewire my amps to drag the input leads over to pins 6,7,8!
     
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