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I want to use elevated heaters in a Marshall amp. Help.

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  • #31
    Question: Which is better DC Heaters or Elevated heaters??
    That isn't a fair question.

    First, it is a solution, it is done to reduce or eliminate hum from heater/cathode interaction. When my joints hurt, I take Ibuprofen, when they don't hurt, I don't take the pill. Does your amp have heater/cathode hum? It may well not. And in that case it wouldn't matter.

    DC heaters? OK, but generally only the first one or two tubes in the chain are fed that way, the rest get plain old AC. Of course there are exceptions to anything. By limiting it to just a couple, you need not make a high current DC supply. You could do something like Peavey did in the 5150 and run all the 12AX7s on 12v and in series. But you'd need a 36v supply or something, which your transformer would not welcome.

    Elevate? The good news is the elevation doesn't have to provide any current, it is just a voltage reference, so you can just as easily elevate one or all of them. That is as simle as movinbg your heater center tap to the positive voltage instead of ground,

    The thing about either way is that it is mainly those first two stages, where the signal is not yet strong, and they are more sensitive to hum. SO elevating or DC-ing the power tubes or phase splitter or whateveer, will be less effective.

    Now, if you are not aware, you need to keep in mind that hum can come from a hundred places, and EACH source has its own cure. HUM IS NOT GENERIC. In other words if you have heater/cathode hum, adding filter caps to th B+ will do nothing for it. Likewise, weak filters on the B+ won't be helped in th slightest by elevating heaters. That is why when someone asks me about hum, the first thing I ask is this: is it 60Hz or 120Hz hum. 120Hz implies power supply ripple, while 60Hz implies things like heaters or signal grounds. Grounds matter. You can have a five inch hunk of wire from part of the circuit to ground, and it matters which end of that wire you connect other grounds to. I solved a hum problem in an old AMpeg once by moving a ground connection three inches.


    So which is better? Either method works. A method works or it doesn't, simple as that, if you could do both and switch between them, I doubt you'd hear any difference. What makes one way better than the other is which one fits your situation best. How convenient is it to install whatever wiring is needed?


    Just my opinion.
    Education is what you're left with after you have forgotten what you have learned.

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    • #32
      To be fair, heater elevation (Tomer refers to it as heater biasing p155) seems so simple that there’s no reason not to do it by default on a new build. I can’t see why it wasn’t adopted as the standard by the early 60s, as an extension of the move from single ended heaters (using the chassis as one if the conductors) to balanced heaters?

      In addition to reducing heater hum caused by imperfect heater-cathode insulation, it also will reduce stress / failure on the heater-cathode insulation in circuits where the cathode is significantly elevated above 0V common, eg DCCF, LTP.

      I have a hypothesis that heater elevation may have other benefits, eg reduced liability to arcing between output valve anodes and heaters (the dreaded pin 2 to 3 charring) due to intermittent or open OT secondary circuit, internal valve shorts that blow heater balancing resistors. Due to the reduction in potential difference between the heater circuit and back emf spikes, HT. And also the reduction in the ‘hardness’ of that potential difference, as perhaps the heater elevation point is not as bountiful an electron source as 0V.
      OK, that last one may be stretching things somewhat
      Tomer http://tubebooks.org/Books/Atwood/To...um%20Tubes.pdf
      My band:- http://www.youtube.com/user/RedwingBand

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      • #33
        Originally posted by Enzo View Post
        A method works or it doesn't, simple as that, if you could do both and switch between them, I doubt you'd hear any difference.
        I agree with the 'doubt you'd hear any difference', but pedantically an isolated DC heater supply (ie. a battery - and not one made by rectifying AC and having other forms of parasitic noise/hum leakage) has no hum path through the cathode heater interface. Whereas the elevated heater lowers (but doesn't eliminate) the resistive leakage that may cause hum, and doesn't do anything to lower the capacitive leakage of hum across that cathode heater interface.

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        • #34
          Question: Which is better DC Heaters or Elevated heaters??
          You can do both together. In high gain amps I do all preamp tubes DC (12V regulated) but I have a dedicated 15V secondary. In most cases the first 2 tubes will do.

          In addition to reducing heater hum caused by imperfect heater-cathode insulation, it also will reduce stress / failure on the heater-cathode insulation in circuits where the cathode is significantly elevated above 0V common, eg DCCF, LTP.
          That's why I started elevating DC heaters.



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          • #35
            On a similar note, I've never done an uber gain amp build but I do notice that on most designs of this type the first preamp cathode is typically only partially bypassed with a relatively small value cap. I HAVE built amps with some considerable gain and I've adopted the practice of using a fully bypassed first stage cathode circuit and adjusting for the audio signal in other ways. I did some actual "on a switch" testing of partial and full bypass for the first stage cathode and though I didn't notice a big difference in hum I did notice that most 12**7 tubes do make some noise that the full bypass eliminates. It's a sort of low frequency woosh sound that I took for some kind of cathode emission artifact, maybe due to impurities (?) but I can't be sure of that. Anyway. I design with a fully bypassed first gain stage cathode now.
            "Take two placebos, works twice as well." Enzo

            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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            • #36
              I've built 4 Marshall amps in the past (JTM45/100 and a few super leads 12000 series with dual tap transformers for higher B+). I've also retrofitted a superlead with KOC power-scaling). So I have experience with basics and assembly. My interest in heaters is a new thing. I just what to be able to have some options if i go high gain.

              Question: If my PT doesn't have the 2nd tap for DC heaters, can I add in a smaller PT dedicated just to supply DC heaters?

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              • #37
                Originally posted by cluster View Post
                Question: If my PT doesn't have the 2nd tap for DC heaters, can I add in a smaller PT dedicated just to supply DC heaters?
                Yes you can do that.
                nosaj
                Binkie McFartnuggets‏:If we really wanted to know the meaning of life we would have fed Stephen Hawking shrooms a long time ago.

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                • #38
                  Question: If my PT doesn't have the 2nd tap for DC heaters, can I add in a smaller PT dedicated just to supply DC heaters?
                  If you run DC heaters on the first tube only you can use this corrected schematic from Mark IV without adding another PT. I've used this many times and it works flawlessly. For D8 and D9 use Schottky diodes for example 1N5819 or similar.

                  Click image for larger version  Name:	12VDC reg heaters from 6V3.jpg Views:	0 Size:	29.5 KB ID:	936740


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