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The Ultimate Tube RIAA-Amplifier?

Discussion in 'General Electronics Discussion' started by rogerk8, Aug 5, 2013.

  1. rogerk8

    rogerk8

    176
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    Jul 28, 2011
    Hi!

    I just want to show you my latest Tube RIAA-Amplifier.

    It is also supposed to be able to be used driving a PA thru a microphone.

    Or a PA thru a guitar.

    Using this amplifier both vinyl players, singers and guitarists can make their voice heard.

    What do you think?

    Is it ultimate or not?

    Best regards, Roger
     

    Attached Files:

  2. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I'm a bit confused. I can appreciate a love affair with vacuum tubes (valves) but it's usually from input to output. Why the BJT? Why not a Cathode Follower?
    Vacuum tube environments don't marry well with solid state either. Manufacturers learned this the hard way back in the sixties.

    That said; nice preamp! I love all things vaccum tube. ;)

    Chris
     
  3. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    After examining the print further I have more questions. There's no Cathode resistors. How are the tubes getting grid bias? I don't see a heater supply for them either. Admittedly I'm not familiar with European or UK tube models.

    Chris
     
  4. duke37

    duke37

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    Jan 9, 2011
    The input power is AC and goes to the heaters, it is centre tapped with a potentiometer to reduce hum.

    The ECC83 is self biassed with 10M grid resistances and low anode voltage.

    You would need to do a simulation to obtain an indication of its properties. But where do you get the models for the valves operating in this mode?
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I'm working on a backup PC since my laptop crashed, so I don't know if my Tina includes the ECC83. I would guess that it would have properties similar to the 12AX7, 12AU7, 12AT7.

    Chris
     
  6. duke37

    duke37

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    Jan 9, 2011
    The ECC83 valve is equivalent to 12AX7 tube and also B139.

    To be the ultimate, I would take the transformer off the chassis or at least use a conducting belt around it.

    If 1N4007s are permitted, then why not use a full diode bridge, getting rid of the 6X4.
    Some hum could be eliminated by using a 12.6V (or 6.3V) DC stabilised supply for the heaters.

    The high value (10M) grid resistors use grid current to bias the valve to -1V. I wonder how this voltage was measured and whether you can find a simulation model.
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Not on its own. This is grid leak bias so C1 is required as well as signal. IE, there's no (-) grid bias present without signal input. That much I haven't forgotten. :p

    Chris
     
  8. duke37

    duke37

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    Jan 9, 2011
    But this is used in self bias mode. The hot cathode emits electrons at a low energy and with a very high grid resistive load will generate a low bias voltage. With a high anode voltage, the electrons will go to the anode and the grid voltage will be reduced.

    If the resistance is dropped to say 1M then there will be negligible grid bias and a signal would be needed to generate a voltage by rectification but the linearity would be compromised.
     
  9. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I don't believe that to be true. I know of only three modes of VC bias, Cathode resistor, Grid Leak Bias and external source bias. Grid leake bias derives the bias from the grid input signal.

    Chris
     
  10. duke37

    duke37

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    Jan 9, 2011
    Here are a couple of pages out of 'Electronics' by A.T.Starr 1954.

    I assume that this circuit is used to give minimum noise.
     

    Attached Files:

  11. rogerk8

    rogerk8

    176
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    Jul 28, 2011
    Hi Duke37 & Chris!

    The first time I built these kind of amplifiers I used a CW-multiplier and stepped up 12Ve some 10 times.

    This worked great but meant an irritating separate PSU.

    My thought now is to use a standard 15V+15V transformer driven by 12V as noted.

    The magnetic leakage from this transformer will probably disturb the signal somewhat.

    But I am planning to use "remote" twinned and shielded wires for the signal.

    And by using this transformer, all that is needed is that extra commersially available AC-adaptor (12V).

    I have zoomed in on the problems always appearant while using common supplies so I have decided to build mono-blocks (no current-loops possible).

    The reasons are simple. One, a cossor will enable a smooth start (not that important here though because I will leave it on always, i.e no switch) two, the amplifier will simply look more nice than a single tube would :)

    Regarding the heaters, you are right. But I like simplicity and those extra four components you need to use a regulator just messes things up while building "in-air".

    And I can take some hum. Because let's face it, the vinyl records aren't that silent anyway :)

    I have actually measured the grid bias using 10M (and those ~70Va which might be important). They seem to stabilize at some -1V.

    I think the largest gain using Edison-bias comes from the fact that any flaw a cathode capacitor might have will otherwise be amplified u+1 times.

    There might also be a benifit regarding noise (because at least one channel is extremely silent).

    I thank you both for your comments. It made me very happy!

    Best regards, Roger
    PS
    Chris was confused about my BJT.

    Well my reasoning is this.

    An emitter-follower has an output impedance close to 1/gm (normally better than 100 Ohm for small signal transistors).

    While loading the amplifier with no more than some 10k (common line-inputs are 47k), the variations in gm will not affect the signal whatsoever. The BJT will simply transform the high anode impedance (some 50k in my case) to some 100 Ohms and this while only affecting and disturbing only a fraction of the 100 Ohm-part.

    The reason for not using some high-transconductance ECC81/12AT7 is simply because it isn't needed (and it complicates the design). A BJT will simply just do (note, ECC81 isn't that linear either but with the above reasoning...)
     
  12. rogerk8

    rogerk8

    176
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    Jul 28, 2011
    Hi Chris!

    Thank you for your comment.

    The BJT-puzzle is explained above.

    Best regards, Roger
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    No where in that artical did I read "bias", "self bias" or any other kind of bias. The artical doesn't even mention a grid and seems to be focused on VC diode characteristics. Did I miss something?

    Chris
     
  14. rogerk8

    rogerk8

    176
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    Jul 28, 2011
    I loved reading these pages. But I will have to read them some more to fully understand them. Thank you for presenting them for me!

    I would even like to read some more.

    Could you perhaps scan in some more pages?

    It kind of stops where it is getting interesting.

    One more thing, do you perhaps have access to documents deducing why RL should equal 2rp for maximum available output power in a tube output stage?

    I had some papers regarding this, but they are sadly lost.

    Thanks!

    Best regards, Roger
    PS
    If you refere to my design, noise has not been a consideration whatsoever. Only simplicity and nostalgica has.
     
    Last edited: Aug 8, 2013
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Hi Roger. While your amp is not intended as a power amplifier are you certain it's not closer to Rp/2? Following the maximum power theorem I don't see how it could be 2Rp.
    Rp is the plate resistor, right?

    Chris
     
  16. duke37

    duke37

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    Jan 9, 2011
    Roger

    Here are some more pages. When you have fully understand these, you can explain them to me!

    I was just wondering way back whether a simulation model was available for a valve operating in this bias condition.

    Load resistance
    To get maximum power out, you need a load resistance equal to the source resistance. In this case the power dissipated in the source is equal to the power dissipated in the load. In the case of a valve power amplifier this would lead to disaster.

    The load presented to the power valve should give a 'load line' which goes from approximately minimum voltage, maximum current to maximum voltage, minimum current. This is described as a resistance and will be much less than the valve anode output resistance. Power valve data will give the optimum load.

    Edit understand = understood
     

    Attached Files:

    Last edited: Aug 8, 2013
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I ran some sims. I'll post them when I get back from a bike ride.

    Chris
     
  18. p.gabr

    p.gabr

    3
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    Aug 7, 2013
    Good evening all their friends, is my first Post here
    I liked the old construction
    I have worked and I Experimentally, with very good results and I would like to tell you that it is very difficult construction, especially with tube
    If you are interested, give you the shema

    Sorry my english are bad

    good luck
     
  19. rogerk8

    rogerk8

    176
    0
    Jul 28, 2011
    Thank you for the pages!

    I've read them once but did not understand so much.

    I will tell you when I I fully understand them because I will print these extremely interesting pages and study them at a cafe' for about a week. :)

    Then I will get back to you!

    In the mean time you may want to view:

    http://sv.wikipedia.org/wiki/Child-Langmuirs_lag

    which is an article I've written by myself a long time ago. But as you understand, I used books...

    The article is unfortunatelly written in swedish but I think you may find the universal language of equations/mathematics useful anyway

    I have tried to derive the formula for maximum available output power several times. Everytime I come to the same conclution that RL should equal rp (rp being the plate resistance). Which actually isn't that strange (i.e impedance matching).

    But, and I know this is a but, tubes working at high voltages and/or plate dissipation, do not follow those simple rules of impedance matching.

    Because then it isn't arbitrary to choose a plate load, you have to consider the high voltage and/or power dissipation.

    I know it is kind of so, but I seem to not being able to prove it.

    And this bugs me!

    Best regards, Roger
    PS
    My dear housegod Williamson have taught me that RL should equal 2rp for maximum available output power. But those old papers actually proved it.
     
  20. rogerk8

    rogerk8

    176
    0
    Jul 28, 2011
    Hi Chris!

    Yes, rp is the (dynamic) plate resistance.

    And I know for a fact that power tubes should be loaded with RL=2rp for maximum available output power.

    But do not ask me to prove it :)

    See above.

    Best regards, Roger
     
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