Building Pete Millett 6b4g amp with 2a3 pwr tubes, LS-55 output questions?

Discussion in 'Tube Audio' started by HowardK, Apr 25, 2018.

  1. HowardK

    HowardK New Member

    Messages:
    49
    On the LS-55 output transformers I connect pins 13&15 and 16&18 for 8 ohms, Do I connect the center tap/ground to pins 16&18 for the secondary ground speaker terminal?
     

     

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  2. HowardK

    HowardK New Member

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    49
    Oh!!! here are the pics
     

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  3. soundmotor

    soundmotor super modified Subscriber

    No, you connect on 13 & 18, either one can be used as ground.

    [​IMG]
     
  4. PakProtector

    PakProtector AK member

    Messages:
    3,666
    Location:
    Ann Arbor, MI
    I think that the two of those do not quite agree( the pic and Soundmotor's list ). 13-16 is a 5R-long winding, and you put it in || with 16-18. with 15-16 as CT, 13-18 should be 20R and 14-17 should be something less...

    Get a clip-lead to connect secondaries, a variac to apply some voltage to the primary, and a DMM to measure secondary voltages...then get out the calculator to be sure what is going on, with the primary as a known starting load.
    cheers,
    Douglas
     
  5. HowardK

    HowardK New Member

    Messages:
    49
    IMG_2959.JPG Ok i finished one of the amps today! My first build!!!
    I had 13&15 connected going to the + speaker rca jack. 16&18 going to the - speaker rca jack. And center tap going to ground, which i read was connected to the case. The amp sounds real good (thanks again soundmotor) but i dont know if the output power was strong enough.
    Ive started on the other amp but while i have the other ls-55 out i will check with variac and dmm for correct R. How do i hook the variac up on the primary side to apply some voltage?
     
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  6. PakProtector

    PakProtector AK member

    Messages:
    3,666
    Location:
    Ann Arbor, MI
    You connect the Line and Neutral outputs of the variac to the same lugs you have got the plates connected to.
    cheers,
    Douglas
     

     

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  7. HowardK

    HowardK New Member

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    Thanks Pakprotector,
    Ok i put 25 vac on the 5000r primary and had 1vac between pins (13,15) and (16,18) after that I haven't a clue!

    Just got through putting the last coat of paint on the second amp! Cant wait to hear these as a pair.
     
  8. PakProtector

    PakProtector AK member

    Messages:
    3,666
    Location:
    Ann Arbor, MI
    OK...so take the Primary:Secondary ratio and square it. Divide the plate load by this number and that is the reflected load on the secondary. In your case this gives 8, which as it should is nearly the claimed 7.5R by UTC...:)
    cheers,
    Douglas
     
  9. s-petersen

    s-petersen Scott Subscriber

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    Location:
    Middletown,New York
    So on a transformer like this, are the P1 terminals an ultralinear tap?
     
  10. soundmotor

    soundmotor super modified Subscriber

    You could drive the screens with the smaller Z primary taps but what would the turns ratio be? Not ~43% for "ideal" ultra-linear but I believe much higher.
     
  11. PakProtector

    PakProtector AK member

    Messages:
    3,666
    Location:
    Ann Arbor, MI
    77% U-L is what the 3k taps would provide across that 5k winding.

    I would make note that 'Ideal' is dependent on the tube's operating condition, as well as the tube, and that is *BEFORE* applying what ever the design goals are. For example, if a 30% reduction in power is allowed in trade for the more triode-like output impedance, a ratio of more than 43% would probably look good. If we take the original goals laid out for U-L operation, an 813 is going to need a different tap location than a KT88 or a 6L6. Same goes for a 6V6 or an EL84, or a 5-125B...:)
    cheers,
    Douglas
     
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  12. s-petersen

    s-petersen Scott Subscriber

    Messages:
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    Location:
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    Thank You!
     
  13. HowardK

    HowardK New Member

    Messages:
    49
    I have finished the second amp and the voltage is high on the 6sn7 diff amp section. I checked everything i know to check! I also changed the 2.7k dropping resistor of the B+. The voltage on the 6sl7 phase splitter is a little high but i think will come down after the diff amp section is resolved. The circled voltages are of the second amp!
    As always any help appriciated!
     

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  14. HowardK

    HowardK New Member

    Messages:
    49
    Never mind i found it! Wrong size cathode resistors. Being old and rookie mistake!!!
     
  15. HowardK

    HowardK New Member

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    49
    Both amps running and voltages look good and might i add sound good! My new opportunity lies with the temperature of one of the power trans. These are brand new Hammonds 302ax and a one of them runs 10 deg. hotter than the other, 134 deg f versus 124. Both amps are identical in construction! Should this be addressed to Hammond mfg?
    Thanks
     
  16. Palustris

    Palustris Well-Known Member

    Messages:
    768
    If you are using the Hammond 159Q choke measure the DC resistance for each monoblock's choke and record it; it should be about 120r. Now power up the two amps and measure the voltage drop across each choke. Use Ohm's law to determine the current through the choke. This will either show that one amp is drawing more current than the other or it will confirm that they are both drawing the same current. If one draws more current look for the fault; if they both draw the same current then you could confirm with Hammond that all is OK.
     

     

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  17. HowardK

    HowardK New Member

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    Thanks Palustris,
    Amp1 Amp2(hotter one)
    76.7 DCR 77 DCR
    381.4 VDC 379 VDC
    368.8 VDC 366.7 VDC
    .164 I .159 I

    Does that look correct/good?
     
  18. Palustris

    Palustris Well-Known Member

    Messages:
    768
    I believe that what you have provided is the voltages on either side of the choke. That's fine if that's what you are showing. Now the voltage dropped by Amp 1 is 12.6V so 12.6 / 76.7r = 164mA; Amp 2 is 12.3V / 77r = 160mA. So far so good: the currents are close enough to cause no concern. However, there are two anomalies: 1. the DCR of the choke if it is a Hammond 159Q should be 100r according to the datasheet; 2. according to the schematic you should have a current draw through the choke of 137mA.

    My guess is that the choke DCR is closer to 100r and the current closer to 126mA which is pretty close to the 137mA specified on the schematic. The bottom line is that both channels appear to be passing the same current which was the original reason for the test.

    I would check the DCR of the choke again (or put a 10r resistor in series with the choke to use as a current sensing resistor): if the amp is really passing 160mA, then there is reason for concern as it is considerably more current than the schematic calls for.
     
  19. HowardK

    HowardK New Member

    Messages:
    49
    Sorry i didnt clarify that im using the 193j. I know im drawing more amps than petes! Im running (314-ct-314 vac and 5ar4 rect) higher voltages 358v on the jj2a3-40's plates and 56v on the cathodes which turns out to be 302 plates, correct? Im also using bigger opt (utc ls-55) which have a higher dcr approximately 180. Will them differences make it draw more current?
    Thanks for the response Palustris as I am new to this addictive hobby!
     
  20. Palustris

    Palustris Well-Known Member

    Messages:
    768
    OK, so you are running the JJ 2A3-40 at about 300V @ 75mA. That's fine; I run mine about the same operating point. So all looks good and it should sound excellent. I would monitor the Hammond power transformers though. I had a Hammond 300BX running within specifications fail after about five years.
     

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