5E5-A Circuit - 5 MEG Resistor Explanation

Discussion in 'Amp Tech Center' started by Gibsonsmu, Jun 27, 2018.

  1. Gibsonsmu

    Gibsonsmu Tele-Holic

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    What does the 5 MEG resistor driving into the input of the V2 12AY7 do in this circuit? Looks like the low powered twin and a few others have this too? The 5F6-A does not

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  2. peteb

    peteb Friend of Leo's

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    The cathode is out of phase with the preceding stage's grid, so its negative feedback.
     
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  3. jclute

    jclute TDPRI Member

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    I don't know myself and curious what others will say. Maybe adding a little negative feedback to tame that stage a little?
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    ^What peteb said.
     
  5. wanderin kind

    wanderin kind Tele-Meister

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    Leo did not like distortion. Either did that Ampeg guy. Thought it was bad because they were old school, pocket protectors, bi-focal guys who probably studied electronics while in the service. Probably tapped out Morse Code at night hoping to get a few CQ cards instead of cruising chicks at the hamburger stand.

    Those DC coupled cathode followers do weird things to the wave form. So a little NFB was tried, don't know how effective it was, the math for NFB circuits is hideous.Easier just to put a scope on there and do an A/B test.

    here is a nice write up on this circuit>

    http://www.valvewizard.co.uk/dccf.html
     
  6. Gibsonsmu

    Gibsonsmu Tele-Holic

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    I'll check it out. It's just kind of interesting, with a resistor of that magnitude I just wonder how much it really does - I may have to put in a switch for that on my build to see.
     
  7. Wally

    Wally Telefied Ad Free Member

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    If you want to see the effect that that 5M is having, clip in another 5M in parallel for a total resistance of 2.5M. The smaller the resistance, the greater the cancellation. The 5E7 uses a 10M there; and as I recall it has a smoother, earlier distortion than does a 5E5A. 5E7’s are smooth, singing amps, ime.
     
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  8. ocduff

    ocduff TDPRI Member

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    A 10meg there sounds a bit more strident. The Nov '54 5D6 had no global negative feedback there.

    I've built and tweaked so many of those 5E circuits and just never liked them all that much. And I've built them many times thinking I can tweak the next one better, but it always disappoints.

    Im a huge fan of Leo's circuits but the 5E circuits are comparable to solid axle cars with leaf spring suspension.

    The 5E3 is a different story. And once we get to the 5F6, we weren't talking fuel injection and full independent suspension (with its Long tail pair phase inverter and tone stack), if that makes sense. Anything preceding that is sort of archaic, possibly in a good way - but I've never heard a cathodyne phase inverter distort in a way that anyone would consider Pleasant, if not broken sounding, so I don't understand the perpetuation of this design being hailed as anything spectacular and in fact think it's just marketing upon Leo's good name, which I think was made upon his later designs.

    But as I type this I want to build another 5F4 bad, thinking that I can make THIS one sound amazing. ;)
     
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