Two Similar Band Master Reverb Projects. One with an issue.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mxvin, May 2, 2015.

  1. mxvin

    mxvin Member

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    I did two "Vibroclones" using BMR chassis. One is a 1969 and the other is a 1972. Both chassis were BF'd to Vibroverb specs as close as possible. One is a single 15 cab (69 chassis) and the the other in a 2 x 10 cab (72 chassis). The biggest dif is the 1 x 15 uses a Allen Amps 40MT OT and the 2 x 10 uses a Classictone 40-18006. Both amps are electrically sound and all points measure as they should. Or I should say all the points that I have checked and rechecked.

    The issue I am having is the 2 x 10 lacks oomph and volume with the same volume setting and occurs on both channels. For ex....a 3 on the 1 x 15 one amp is like 5 on the 2 x 10. I realize the speaker will have alot to do with it but if you heard the amp you would know what I am talking about. It lacks dynamics, balls, and sounds sterile. The amp sounds "choked". Speakers in the 2 x 10 are CTS alnicos reconed. Also use a 63 blonde Tremolux cab with reconed Jensen P10Ns. I have tried many other cabs with the same results. Speakers are broken in.

    What I have done:
    Tube rolled power, recto, and preamp tubes. Different bias settings. Different cabs. Different guitars. Have checked and rechecked many of the voltage points. All filter caps are F&T. Tubes are all NOS or good quality new stuff.
    I realize this is an issue with many possible causes. But can anyone shed maybe a little light and lead me in a direction. Maybe just a faulty OT?
    Thanks
     
  2. ScioBro

    ScioBro Supporting Member

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    Are you using the 8 Ohm taps of the Allen OT with the 15"?

    What taps on the Classictone for the 2x10? 4 Ohm?
    ref. shows it has 4/8/16
    If the 10" spkrs are 8 Ohm ea., and you're running the 2 in parallel with the 4 Ohm taps, try them in series and use the 16 Ohm taps.

    In other words, did you try all the secondaries of the Classictone?
    I'm thinking it won't make that much diff., but worth a try.
     
  3. mxvin

    mxvin Member

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    the 1 x 15 is using an 8 ohm tap. The 2 x 10 is wired in parallel and using the 4 ohm tap.
    Thanks for the tip..
     
  4. zenas

    zenas Member

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    If everything else checks out you could swap the output transformers around and see what happens.
     
  5. mxvin

    mxvin Member

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    yeah.....a simple but efffective trouble shooting method.
    why I havent just done that is beyond me.....
    lazy I guess....
     
  6. swiveltung

    swiveltung Member

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    Is the NFB attached to the original circuit designed tap? (ie: if the circuit of the BMR was originally a 4 ohm is the NFB soldered to that?) It should be regardless of what tap you are actually using.
     
  7. mxvin

    mxvin Member

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    hmm....I never knew that. Without looking I would say it is because I did use the 4 ohm tap. Two 8 ohm speakers in parallel.
    However on the 1 x 15 I used the 8 ohm tap and probably soldered the NFB to that.
    Along those same lines I can check the NFB resistor for correct measured value too. I guess, for example, if it had drifted up significantly it would cause a volume drop and lack of "life" in the tone....right?
    Thank you
     
    Last edited: May 2, 2015
  8. Winnie Thomas

    Winnie Thomas Member

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    Check the efficiency ratings of the 10s and the 15. One may be more efficient than the other.
     
  9. mxvin

    mxvin Member

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    I have used these speakers in other amps. Its def "something" with the amps output. If you are used to old Fender's and they way the sound you would be able to tell right away that something just isnt quite right. Aside form the volume issue the amp just sounds flat.....
    Thanks for the input though. Much appreciated.
     
  10. kevmin

    kevmin Supporting Member

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    It sounds like a combination of tube biasing, tube "freshness", and speaker efficiency. If it were me, I would swap tubes between the two amps and bias them up good and see if they are closer in volume. I highly doubt the output transformers will be making that much difference. Both are quality transformers. I have used both with great success.

    Are your plate voltages similar?
     
  11. mxvin

    mxvin Member

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    thanks....Plate voltages and bias is similar. Using 6L6GCs in both and GZ 34 as recto. But the 2 x 10 has a pair of NOS(ish) RCA BPs and the 1 x 15 has Winged C's installed. Not thinking the OT is a problem I am thinking the OT may have a problem. I havent had a chance to TS yet the last few day.

    But I have used several dif cabs and rolled quite a few tubes with similar results/issue
     
  12. Winnie Thomas

    Winnie Thomas Member

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    \

    BTW I used to live in Northern NJ: Stillwater Township at Crandon Lakes, just North of Swartswood.

    I left NJ in 1979.
     

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