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Earth Sound Research amp needs some love

Discussion in 'Amp Tech Center' started by rojo412, Aug 8, 2019.

  1. D'tar

    D'tar Friend of Leo's

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    Rojo,

    What tubes do you have in there? 20mv will be pretty cool. You can heat that up a little and take it off the limiter.
     
  2. rojo412

    rojo412 TDPRI Member

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    They are 5881s.
     
  3. Wally

    Wally Telefied Ad Free Member

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  4. D'tar

    D'tar Friend of Leo's

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    Right on the money Wally!!!

    LikeX2:)
     
  5. rojo412

    rojo412 TDPRI Member

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    Took the amp off the limiter, fired it up, fuse holds fine.
    Plugged in a guitar, it makes sound. Like, LOUDLY! And guitar sound, that is, not yucky sounds. It's pretty freaking clean!
    The reverb was kinda crusty, but spraying the contacts and jostling the wires fixed that.

    So yeah, the amp works! The immediate fuse blowing issue was likely the rectifier shorting in that popped diode because beyond that point, the amp never lit the limiter again.
     
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  6. Wally

    Wally Telefied Ad Free Member

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    Did you test the diodes with a meter? That would be informative. Have you taken a look at those tube pinouts? If not, now would be a good time to do it in order to reinforce what you have been working with here ...with guidance. You know....which pin is the control grid in those power tubes. Plates? Screen grid? Cathodes? Heaters. How about the novels?
    As I noted earlier...I admire initiative. However, repair by instruction is different than learning to figure out things on one’s own. One of those methods never leads to understanding, ime.
    D’tar, I learned something here about that rectification. Thanks. Fwiw, the Peavey I have here has only two HT leads...no CT. It has four diodes in the rectifier circuit. I may sit and draw it out and compare it to the schematic just to reinforce things in what is left of my mind.
     
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  7. rojo412

    rojo412 TDPRI Member

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    I'm definitely going to provide as much info as I can, whatever anyone wants. It's crazy old weird stuff like this - stuff that doesn't have anything about it presented on the internets - that needs to be documented.
    With the way things show up in a search, the more info that pops up for people in such a situation as this, the better.

    And I've learned so much in a week, I can't even tell you how thankful I am to have the support and knowledge of everyone here. It's honestly been so rewarding to do any of this. It took me reading things repeatedly, and I'm going to continue doing so until it sinks in, but it's been very informative.
    Thanks to everyone here, seriously.

    The quest for knowledge continues. I'm hoping to build something from scratch here soon, that's going to help bolster a lot of the info which I'm still digesting here. I most certainly hope to be on this forum some day, having learned more and having more experience under my belt, helping someone new here solve a problem.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    ...
     
  9. rojo412

    rojo412 TDPRI Member

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    - The diodes are measuring something like -1 vdc on one end and 472 vdc on the other.
    - CT voltage at the STANDBY switch is 473 vdc
    - Plate voltages (#3 pins) are 467 and 468 vdc
    - Resistance from CT to Plates is 140.2 and 124.5 ohms
    - So that would mean the bias current is:
    6/140.2 = .043
    5/124.5 = .040

    Does that seem correct?
     
  10. peteb

    peteb Friend of Leo's

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    The numbers look good
     
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  11. D'tar

    D'tar Friend of Leo's

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    This is very true!

    "Teach a man to fish" right Wally!

    I tend to serve up fish dinner. But you are doing the cooking!!!
    One will realize quickly if the work will keep their interest beyond right now!
    If you now love the taste of fish.... Then it's up to you to continue your education!!!
    If you don't like the fish, well, at least you know what it tastes like and move on.

    .02:)
     
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  12. D'tar

    D'tar Friend of Leo's

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    Rojo... What MPD% does this translate to? Is it safe for your 23w 5881s tubes in P/P, adjustable fixed bias? Does it sound good?
     
  13. rojo412

    rojo412 TDPRI Member

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    D'Tar, I'm not sure what the MPD% is. Frankly, I'm still not sure I'm using the Tube Bias Calculator properly. I've done what the instructions say, but am still a little in the dark.

    FWIW, though, the amp sounds good.
     
  14. peteb

    peteb Friend of Leo's

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    6 volts and 5 volts are the DC drops at idle on either side of the primaries of the output transformer.


    The calculations work out and the idle is a little hot.



    The Watts being dissipated in the tubes at idle is


    468 V * 0.043 Amps = 20 Watts.


    In class AB push pull, the current draw will go up with signal, so the idle is set at less than 100% max plate dissipation of Watts for the tube type.


    I am more familiar with the 6L6 tube and I was figuring 20 W / 27.5 W = 73% of max plate dissipation. 73% is fine, the preferrred range is 50-70% so 73% is in the range, although a little bit high.


    For the 5881 you have


    20/23 = 87%


    The tone is probably fine if not outstanding but the tubes will wear out faster in that condition.
     
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  15. D'tar

    D'tar Friend of Leo's

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    Rojo, if you are using Robs calculator

    select your tube type, input your plate to cathode voltage and hit calculate.
    This will bring up calculations and suggested limits. here are your #'s

    upload_2019-8-19_14-2-4.png

    Note** when you change bias your plate voltage will change so keep everything updated. You would be looking at class AB Fixed Bias for this amp with a max of 35mA for the current plate voltage=70% MPD (Max Plate Dissipation)

    Your math looks good and it is easy to finish out as Pete showed above. If you continue with the calculator then input your plate current in mA and hit calculate again to see the following...

    upload_2019-8-19_14-7-27.png

    Again, this is pretty hot for fixed bias. Remember your plate voltage will rise when lowering bias and will lower when increasing bias.
     
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  16. rojo412

    rojo412 TDPRI Member

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    Okay, thank you very much for the breakdown. Again, helps immensely!

    - Adjusted the bias pot, which changed the plate voltage to 498.
    Screen Shot 2019-08-20 at 1.21.42 AM.png

    - Measured the mA at the cathodes, got 32.7 and 29.8.
    Using the higher of the 2, plugged that in, got this:

    Screen Shot 2019-08-20 at 1.26.24 AM.png

    That means I'm just slightly hot, correct?
     
  17. Wally

    Wally Telefied Ad Free Member

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    Rojo, that is not hot bias, ime. That 70% is not a hard and fast rule. There are some who hold that 85% is doable....and I have witnessed that with no problem.....and even higher. Some people like the bias much cooler than 70%.
    What happens as the current draw is increased from 70% is that the ‘reward’ in the change in Sonics decreases as the wear and tear on the tube increases. I personally might not find a reason to push beyond that 70%, but some do. I also do not worry a great deal if the dissipation is above that 70% point.
     
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  18. rojo412

    rojo412 TDPRI Member

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    Thursday is band practice. I'm gonna bring this for a field test and see how it goes. If everything works out, this will be replacing a rather pedestrian, "solid state with tube preamp" amplifier that my guitar player has, which has fallen into a state of disrepair.

    In the mean time, I'm gonna track down the mechanical buzz in the speaker area.
    Again... thanks everyone!
     
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  19. D'tar

    D'tar Friend of Leo's

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    :rolleyes:

    Sound clips are always welcome!!! Amp "alone" and amp "in the mix".
     
  20. rojo412

    rojo412 TDPRI Member

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    I recorded a clip, but it won't let me upload on here. I tried MP3, AAC, and WAV.
    Either TDPRI doesn't host that or I don't have the clout yet.
    :oops:
     
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