5E8A Low Powered Twin Build Thread

Discussion in 'Shock Brother's DIY Amps' started by guitarbuyer, Dec 18, 2013.

  1. laird

    laird Tele-Holic

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    To get the bias voltage to go more negative (and lower dissipation) you can replace the 15K resistor between the bias pot and ground with a 22K.

    BTW, NOS Tung Sol 5881s can take well over 400v on the plates for years and years... most Bassmans ran 5881s between 440-480v. As long as you can keep the dissipation under control the B+ voltage won't be a problem.

    -Laird
     
  2. guitarbuyer

    guitarbuyer Tele-Meister

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    Laird,

    Thank you very much for the response. I was having difficulty trying to sort out the exact component that would give me that range.
     
  3. guitarbuyer

    guitarbuyer Tele-Meister

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    I can’t believe it’s been over 7 years since I built this. I only got thru it because of this forum. Still play this amp every single day.

    Anyway, I finally got around to buying some proper NOS RCA 6L6GC tubes. My issue is that at max adjustment on my pot, they are seemingly biased way too cold. Assuming I am calculating this correctly - measuring with one of those purpose built bias probes -

    30w / 404v = .071 x .7 = .051ma

    At my max adjustment, I am at .033. To be honest, the amp sounds fine but I’m guessing I can squeeze some more goodness out of it by increasing that value.

    if the above is correct, I’m guessing I need to change out that 15k resistor for a smaller value, right? Any idea what that would be?
     
  4. guitarbuyer

    guitarbuyer Tele-Meister

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    I’m guessing I could also swap out the pot also. Going back thru the thread, it appears it is 25k. Probably put a 50k In there instead and get a greater range. I’d prefer to just replace the resistor since I have them laying around.
     
  5. 2L man

    2L man Tele-Holic

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    You could replace 15k with about 8k and install another 8k between diode anode and trimmer leg. This would change the adjustment range hotter but keep adjust sensitivity the same.
     
  6. guitarbuyer

    guitarbuyer Tele-Meister

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    Thanks. So I’m replacing both of those. The 1k and 15k resistors with 8k. Can’t find any of those around of course. Will have to order.
     
  7. guitarbuyer

    guitarbuyer Tele-Meister

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    This is wrong. I’m replacing the 15k resistor and adding another 8k to the pot leg. I think I have it now.
     
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    You might get away with going below 10K on the 'tail' (shunt) resistor, but Doug Hoffman suggests staying at 10K or more to avoid too much current loss to ground. I can't read the size of the 'head' (range) resistor in your drawing, but if your bias is too cold, you want to reduce your negative voltage, and as Doug explains the standard way is to increase the size of that range resistor.
     
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  9. guitarbuyer

    guitarbuyer Tele-Meister

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    Thanks. Should I do a pair of 10k or skip the shunt altogether? I don't have a good understanding of this stuff. But I'm thinking if I just replace that 15k with a small value and call it a day. Thoughts?
     
  10. 2L man

    2L man Tele-Holic

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    If you have 10k resistors you can use two. Starting from bias power supply voltage which I assume is -50V and potentiometer is 25k.

    10k resistor, 25k potentiometer and 10k resistor moves Grid voltage adjustment range towards cathode voltage which increase tube bias current.

    New adjust range comes to the ratio of resistances.

    Hot end bias: -50V : (10k+25k+10k) x 10k = -11V

    Cool end bias: -50v : (10k+25k+10k) x (10k+25k) = -39V

    So potentiometer is not as sensitive to adjust. If you use only 25k potentiometer over 50V it comes more sensitive when range comes 0...-50

    Now you have hot end -50 : (25+15) x 15 = -18V which is too much

    On cool end now there is possibility to go to -50V.

    Even better range comes if you change current 15k between pot and bias supply and 10k too.

    Then range comes -8V...-29V
     
    Last edited: Jun 7, 2021
  11. guitarbuyer

    guitarbuyer Tele-Meister

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    Thank you for the detailed explanation. I will do a pair of 10k. Will report results.
     
  12. guitarbuyer

    guitarbuyer Tele-Meister

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    Since I'm a moron and in the interest of not blowing things up -- finally got my order in with the resistors. I drew my proposed circuit in with the two new 10k resistors I have underlined below. This is the correct spot for the new resistors?

    [​IMG]
     
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  13. guitarbuyer

    guitarbuyer Tele-Meister

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    You only live once. Went ahead and did the above. All seems to be well. Plate voltage at 390 and cooking along a tad cool at .050 ma. That is bout right in the middle of the pot so have plenty of leeway next time.

    Thanks for all the help over the years.
     
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  14. mgrossm

    mgrossm TDPRI Member

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    Anyway you could post a pic or 2 of the bias as it looks now?. Im about to build a 5e8a and would really appreciate seeing it up close.
     
  15. guitarbuyer

    guitarbuyer Tele-Meister

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    I can do that tomorrow for you. But, the drawing in post 52, is exactly what I ended up with. It works perfectly.
     
  16. mgrossm

    mgrossm TDPRI Member

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    May have to use a 10k pot as its all i can find so now not sure what resistors
     
  17. guitarbuyer

    guitarbuyer Tele-Meister

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    Smarter people than I will be here soon to give you good advice. It was the only way I got through it.

    The 10k pot will narrow the useable range. 2l man in post 50 did the math for us. If you know where you need to be, you’ll be able to calculate the resistors needed to get there with the 10k pot.

    Good luck my friend. I adore the amp I built and it sounds wonderful.
     
  18. mgrossm

    mgrossm TDPRI Member

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    I think I figured it out with a 50k pot. Would love some pics to see what got grounded where around the end of your 16uf caps at the end of the board i.e., the 100uf cap etc
     
  19. 2L man

    2L man Tele-Holic

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    Bias circuit can be thoucht having two sections: First is bias negative supply voltage which can be about 50VAC when PS has a bias output. Or then bias is taken from high voltage and this force us to use different value voltage dropper resistors.

    Then second, bias has an adjustment and there we can use both resistors on both side of the potentiometer to define the adjustment range. Different tube types reguire different bias points where many variables effect but if you are going to use only one tube type you can use Universal Loadline Calculator to get approximate g1 voltage, which should come when potentiometer is in the middle.

    I have used bias circuits from amp schematics which I know to function and sometimes I have not had to change anything. Or I have changed only one resistor eiher side of the pot to get pot better centered if I have only one pair of tubes I can't say do they represent average?

    If I take bias from HV I have used two resistor/capacitor filtering to drop and stabilize the voltage to about -30...-50 VDC so first calacitor needs to have 160V rating and second can be 100V or 63V. This because one amp which had only one capacitor filter the voltage did fluctuate. Perhaps it does not effect to anything posiyively and perhaps I have lost important guitar amp sound effect building too HiFi? :)
     
  20. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Merlin points out 10k is always big enough; the range depends on the balance of head and tail resistors. Then the choice of resistors comes out of a simple voltage divider calculation. He also shows you how to calculate the range needed depending on your output tubes, how to build a better bias circuit, and a lot of other nice bias tricks…

    http://www.valvewizard.co.uk/bias.html
     
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