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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. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    That was my thought when wandering into this thread. Absent some kind of industry-wide guidance (that I would love to read if anyone has it), it seems like each company would lay out their tube construction how ever it made the most sense to them. (Which could just mean copying another brand's layout.)

    Continuing that thought, I always figured that numbering the triodes "1" and "2" in tube sheets or "a" and "b" in schematics was mostly arbitrary and for internal reference. Of course a schematic wouldn't have a "V1B" before "V1A"... ;)
     
    Last edited: Dec 1, 2020
  2. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I think one can observe some "lessons learned" over the years. I build an amp with the input section exactly per the 5B3 Deluxe schematic and layout, and noticed that one channel always hummed more than the other one. I tried every 6SC7 I had, tried pushing the grid and plate and heater wires into different positions, to no avail. (I can't remember now, but I may have tried switching the grid wires I think that's one good example of a tube where one triode was more prone to picking up hum. Grid-leak bias on the input stages didn't help, and the one-conductor heater wiring made sure that there was hum available to pick up. So I would bet that plenty of the originals also had a bit of this hum, but they were so thrilled to be heard in their western swing bands, they didn't mind in the beginning. :)

    I think the fact that Fender went to cathode-biased inputs and two-conductor heater wiring on more and more amps over time, meant that either he or the players he listened to figured it was a problem to be addressed.

    More directly to your post, it probably does take a high-gain amp for "triode 1 vs triode 2" to be a consideration. Two-conductor wiring made it a completely moot point on my amp, happily. :)
     
    Last edited: Dec 1, 2020
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  3. Wally

    Wally Telefied Ad Free Member

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    You [email protected] that differently than I do. It is clearly stating that Section A..pins 6,7,8...are “the more favorable section of the tube with respect to hum.” Ime, there is no amp designer, builder, tech, or user who would consider more hum to be more favorable.
     
  4. hepular

    hepular Tele-Holic

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    i can see that--but it's fascinating as a language guy, because it's a case in where context is brought to bear so plainly on the reading.
     
  5. Wally

    Wally Telefied Ad Free Member

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    i don’t see any room for misunderstanding the intent of the writer of that information. Context??. Hum....is it desirable or not???? Easy call for me when deciding what is ‘more favorable’. Now, if you personally prefer an amp that hums unnecessarilywhennitmcould be designed to be quieter, then I can understand your interpretation. (;^)
     
  6. Dacious

    Dacious Poster Extraordinaire

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    Mebbe 'Leo did it that way' and 'Jim and Tom Denney copied him'.

    Jim put a polarity switch on the JTM45 even though it was completely useless for England.

    Every important line of guitar amps ever since has more or less Fender-Marshall-Vox DNA so here we are today.......
     
    Last edited: Dec 1, 2020
  7. hepular

    hepular Tele-Holic

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    "favourable for [ ] hum" the bracket could be construed as including either avoiding or producing. it's one of many spots in a variety of learned jargons that are actually left implied.


    clearer would be something like 'since the connections to ground are more likely to induce hum in pins 123, pins 678 should be used to minimize hum'
     
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  8. mrriggs

    mrriggs Tele-Meister

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    It states that it's favorable for hum when the heaters are wired in parallel AND the center tap of the transformer is grounded. That implies that it wouldn't make any difference if the heater voltage is elevated or if the heaters are wired in series.
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Have a look at this:
    http://www.valvewizard.co.uk/heater.html

    "Hum and buzz is caused by leakage current between the heater and cathode, and by the electromagnetic field around the heater and associated wiring. The magnetic field modulates the electron stream inside the valve, leading to hum. The electric field couples into the audio circuit (especially the grid and anode) via stray capacitance. You might think that the stray capacitance would be too small to couple low-frequency hum, and you'd be right, except that the mains supply -and therefore the AC heater voltage- is not a very clean sinewave. The peaks are usually clipped off by rectifier action, and who knows what industrial motors pollute the supply with hash. These ugly corners and spikes contain high-frequency enegry which couples quite easily through even tiny capacitances. This leads to more of a buzz sound than a low hum. There are various way to deal with these noise sources.

    Rule number one is that the heater supply must have a DC connection to audio ground. This may be a direct connection or an elevating circuit (see shortly). This is equally true for AC or DC supplies. Leaving the heater supply floating will result in almighty hum due to primary-to-secondary transformer leakage current, and is a common beginner’s error. AC heater supplies should also be balanced to suppress the EM field. Not only does this reduce the magnitude of the voltage on each wire (e.g., each wire carries 3.15V rather than one wire carrying 6.3V and the other zero), but the opposing fields will tend to couple equal-but-opposite hum signals into the audio circuitry, which should cancel each other out."
     
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  10. chas.wahl

    chas.wahl Tele-Meister

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    That's exactly the same datasheet referenced in post #8, and so far as I know, the only mention if this "phenomenon" that anybody's posted or referenced so far. It doesn't help that no reason is given for the "favourability" of section 2 over section 1, and that the Philips diagram shows conductors within the valve connected differently than GE or RCA or Sylvania show in their datasheets: in Philips the wires of 1, 2 and 3 are crossed, and those of 6, 7 and 8 are not; and the heaters are shown on the side of the valve where 4 and 5 are located. In the GE, RCA and Sylvania datasheet diagrams, heaters are on the pin 9 side of the tube, and the wires of pins 6, 7 and 8 do not cross each other, and are traversed by one heater wire. Unless this is a schematic-only representation of which pin connects to which element, then these appear to be two tubes of the same "type" that are internally configured quite differently.

    This leads me to suspect that the Philips comment a) might not be universal for the valves manufactured by others, or b) Philips may have drawn their diagram "wrong" but constructed their tubes just like everyone else did, in which case, perhaps, this phenomenon may be universal. In any event, it doesn't seem to me that the "conventional" assignment of a particular triode as being less susceptible to hum is necessarily true, in the absence of better information, reasons for such a convention, and trial and error experiment with real-life tubes and a knowledge of how the tubes used in such an experiment are actually wired inside.

    Based on the quotation from Merlin Blencowe in @Lowerleftcoast's post just above, I'm feeling that heater wiring crossing grid wires within the tube, or crossing of the anode, grid and cathode wires on one side of the tube (since pins are reversed in order of the elements they serve), or both, is like as not what's responsible for one triode being more susceptible to hum. But I have no proof of that.
     
    Last edited: Dec 2, 2020
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