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Champ OT: 5k or 8k primary? Has it ever been answered?

Discussion in 'Amp Tech Center' started by gabasa, Sep 15, 2020.

  1. gabasa

    gabasa Tele-Meister

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    I wired up a Champ for my son recently using Hammond's new 1760CP output transformer, which is their 1760C with a paper bobbin. It sounds amazing, btw.

    So..... brown wire or blue? I've never been able to find any consensus on this. I figure that either one is safe to use, and I was going to do some listening tests to compare both, but after wiring it in we loved the tone and left it that way. Fwiw, we went with the 5k brown after doing a lot of reading online.

    The only thing that gets me is that companies like Hammond and Classictone claim to be mimicking old, original transformers that they've measured from real amps. If they are, any old 5F1 pic I look at uses a blue wire. Does this mean that Fender generally went with 8k?

    What's up with all this? Is one more correct? What could I expect to hear differently if I were to switch to the 8K in terms of tone? Would the volume drop?

    Let's get to the bottom of this, lol! Thanks.
     
  2. elpico

    elpico Tele-Afflicted

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    What's the correct ice cream flavour, chocolate or vanilla?

    The difference in volume won't be significant. Just a slightly different tone.
     
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  3. tubedood

    tubedood Tele-Holic

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    My 5F2-A build is 8K to an 8 ohm speaker. General consensus is it sounds great.
     
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  4. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    I debated whether to go 5K or 7K/8K when building my 5F2A. 8K makes more sense on paper, but in this case, you're safe to go with whichever one sounds better. A 'scope would tell you which one makes more power and/or has a more symmetric waveform, if you really wanted to know. :)
     
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  5. Jon Snell

    Jon Snell Tele-Holic

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    Quite simple really.
    Read the valve data sheet on the anode impedance and match it. That will give you maximum efficiency. A mismatch will change the tone due to increased harmonic distortion etc. Too high an impedance means low output before clipping, too low an impedance means current drive issues.
    A 6V6 likes 5k single ended and 7.7k push pull whereas an EL84 4.5k single ended and 8k push pull. Both very similar.

    Brown versus Blue makes no difference until you require phase matching for stereo or NFB loops.
    If the primary or secondary is the wrong way round, NFB becomes PFB and the amplifier is now a high power oscillator.
     
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  6. elpico

    elpico Tele-Afflicted

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    There's no reason whatsoever to expect "most efficient" or "most power" to be the same load as "best sounding" though. And even if it was, all your napkin calculations and load lines are totally imaginary anyways because you're doing them for an 8ohm resistor instead of a speaker.

    The impedance of an 8ohm speaker is just about anything but 8ohms. What does your load like say for 62k? Because that's what an 8ohm to 5k transformer is going to actually reflect a lot of the time. And that load isn't a "line". It's a moving object with weight and momentum that stores and re-releases power back into the transformer.

    Tl;dr - there's no "correct". Try both, pick your fav.
     
  7. jimgchord

    jimgchord Tele-Afflicted

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    I prefer 8k but if you look at the vintage spec classictone its more in the 15k range
     
  8. elpico

    elpico Tele-Afflicted

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    Food for thought


    [​IMG]

    [​IMG]
     
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  9. screefer

    screefer Tele-Holic Silver Supporter

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    Do you have a link to the 1760CP?
    No can find info.
    Thanks
     
  10. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    It's worth noting that the ideal load depends on the operating conditions of the tube (plate voltage and idle dissipation) -- lots of tube sheets use 250V as their example, but most of us are working at least 100 volts above that.

    RCA's example operation of 315V into an 8.5K load is much closer to what we're doing with Champs and tweed Princetons.

    But good point on the speaker impedance, too. I'm sure that's how a 5K load still works so well in old Champs. :)
     
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  11. D'tar

    D'tar Friend of Leo's

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    upload_2020-9-16_11-35-33.png
     
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  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The old Gibson and Fender offerings had power transformers with around 40mA HT secondaries. IMO, they used OT's with a high primary impedance (~17k) so the 6V6 amperage draw would stay near the capability of the PT. Given the B+ of the Champ, a 5k primary on a 6V6 would take the amperage draw north of the 40mA PT capability.

    Old Champs with a 17k primary impedance still seem to sound good to my ears. Some of us seem to chase *that* vintage Champ sound. Sure the power output is decreased when the primary impedance increases but does that matter when you are listening to a '50s vintage Champ?

    The point I am trying to make is there are several reasons for choosing the primary. In your case, you are not bound by the capability of the PT, so tube longevity and sound are your main considerations. IMO, tube longevity on a Champ is not much of an issue. That leaves you with how it sounds. I would listen to both 5k and 8k. In addition an 8ohm speaker on the 5k:3.2ohm taps will reflect a primary impedance of 12k5 and... a 4ohm speaker on the 5k:3.2 taps will reflect a primary impedance of 7k so you are not held to just the 5k or 8k primary impedance. Let your ears be your guide.

    The valvewizard has some pages on OT primaries for reference.
     
  13. Wally

    Wally Telefied Ad Free Member

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    Lower, would that increase in current draw with the 5k primary versus the 8k primary yield an increase in compression as the PT struggled to meet the demands put upon it by that higher current draw?


    gabasa, what sonic difference between the two primary impedances did you perceive?
     
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  14. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You would think it might and maybe that is exactly why the vintage under powered SE amps have their charm.

    I am still uneasy with thinking of an SE amp compressing in this manner though. It would be a different story with a PP, where it would compress when more was asked of the circuit and then be able to recover. My concern would be for the poor SE PT constantly having to give all. Once it reaches an unsustainable heat range it would fail just as so many have. I think about what muchxs has said about abusing these amps. Users installing bigger tubes or biasing them hotter just because the next champ has a hotter bias. They must be at the limit already. Heck the higher wall voltage may push them over the edge.

    Sorry Wally, I can't give a more definitive answer.
     
    Last edited: Sep 17, 2020
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  15. Wally

    Wally Telefied Ad Free Member

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    Side note: muchxs has not been heard from in quite a while...June 1, 2020 if my search was correct. I have him in my thoughts and hope all is well with him.
     
  16. gabasa

    gabasa Tele-Meister

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    Thanks everyone for the info. One of these days I'll connect both wires to a toggle switch and just toggle between the two to see if I can hear a difference.
    I haven't tried comparing the two yet.

    I've got Hammond's upgrade 291AX PT installed, it's great and supposed to run cooler than their standard 290AX.
     
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  17. gabasa

    gabasa Tele-Meister

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    It's a brand new product from them. When things were shutting down last March, I pestered them a few times and convinced them to make a paper-bobbin version of their 1760C.
    If you can't find one, I'd try to contact them directly, because perhaps they need some time to make their way into their paperwork.

    It's a transformer worth trying out. Together with a 291AX power transformer and a 194A choke, the amp I put together for my son sounds pretty amazing.
    I've built with other brands of transformers before, but my ears always seem to gravitate towards the Hammonds.

    I have a contact there that I dealt with, I'll try to get some more info about the availability of the 1760CP from this person and I'll let you know.
     
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  18. Wally

    Wally Telefied Ad Free Member

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    Imho, in order to make some sort of valid comparison, one would need to have two speakers with identical specs but for the impedance and load them in two identical cabs. If you simply use the one speaker in the amp cab and change the OT tap, there will be a mismatch and that has an effect that is not a valid comparison of what the actual difference in between the two OT secondary impedances on matched loads.
     
  19. gabasa

    gabasa Tele-Meister

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    I'm using the 8 Ohm secondary into an 8 Ohm speaker, I'm more interested in the difference between the two different primary loads on the OT (with the speaker I've already got ready to run with this amp).
     
    Last edited: Sep 17, 2020
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  20. J. Bonkosky

    J. Bonkosky Tele-Meister Silver Supporter

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    8k primary is the correct load for a 6V6GT with around a 315V plate voltage. It is in the data sheet for that tube. Use 8k and keep the tube happy.
     
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