Ampeg amp with 6SL7 preamp tubes. Please help me!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by straightblues, Jan 25, 2012.

  1. straightblues

    straightblues Member

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    I just got a converted Hammond organ amp. It was converted into an Ampeg M15 circuit. It has two 6L6 (KT66) power tubes, three 6SL7 preamp tubes and a 5U4 rectifier. The amp has tremolo. Here is the only schematic I could find (it doesn't have voltages.)

    http://www.freeinfosociety.com/media/pdf/3366.pdf

    This amp sounds really amazing. There is only one issue. It isn't very loud. I am talking Champ or Princeton loud. My 64 Fender Deluxe is waaayyyy louder. I will also say the amp is quiet at idle.

    I have had amps in the past with bad filter caps that robbed volume, but they also didn't sound that good. What do you guys think might be wrong with it and what should I do? Since it sounds so good, maybe I will just accept the lower volume.

    I just checked the voltages. The B+ voltage is 480 volts. The screen voltage is 380 volts. The voltage at V1 is 95 volts. The resistor going to V1 is 270K which is what is shown on the schematic. I see some other Ampeg schematics with 6sl7 preamp tubes with a 120K resistor in the same spot. (maybe the schematic is wrong??) The schematic I have referenced above doesn't have any voltages on it so I am not sure what they are supposed to be. BTW, I have KT66's in it instead of 6L6's so the 480 volts should be ok right?

    I think 6SL7's should run at about 130 volts in these amps is that correct?

    Thanks for any help.
     
  2. Blue Strat

    Blue Strat Member

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    Are you sure the 6SL7s are good? This is almost impossible to do without knowing whether it's wired correctly (a lot of people deny this is possible), what condition the tubes are in, the bias current of the power tubes, etc. etc.

    What's the cathode voltage for the KT66s?
     
  3. donnyjaguar

    donnyjaguar Member

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    I don't think this design has enough gain for a typical electric guitar output. Maybe you're expecting too much?
     
  4. straightblues

    straightblues Member

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    Well it is a guitar amp. Yes, I know Ampegs don't have a lot of gain and I play fairly clean. What I am concerned about is overall volume of the amp. My Tweed Deluxe is twice as loud as it.
     
  5. LPMark

    LPMark Member

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    That screen voltage is way lower than what you would expect. That will greatly reduce gain of the outputs. 100V drop across the 1K resistor is a red flag.

    That 1K decoupling resistor [check it's value] at the screen supply is shown as a 10W and would be dissapating that amount if it's dropping 100V... = 100ma which is way more than you would expect from the screens plus three 6SL7's, something is drawing too much current in the preamp or that resistor has gone way high or a filter cap is leaky or bad.

    Measure the cathode voltage of the 6SL7 tubes, they shouldn't be much over 2V, [V2 switch closed]

    What happens if you pull V2 the tremelo tube?

    Mark
     
    Last edited: Jan 25, 2012
  6. teleman1

    teleman1 Supporting Member

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    We have TWO 6l6's. That's a lot of power and every time I have been around 6L6's, the amp has some decent power. Now its from an Organ. Two 6L6's would blow a family out of their seats in an Organ. Is there by chance some sort of govenor on the circuit? I bet the loudest the organ could go did not put it into OD. Something might be limiting the output? Just shooting ideas
     
  7. donnyjaguar

    donnyjaguar Member

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    You can tell by the choice of component values the designer was struggling for gain and could only use 5 tubes total. :( I'd just re-jig the whole amp myself.
     
  8. 900z1

    900z1 Member

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    you said "The voltage at V1 is 95 volts."

    these tubes typically max rating is 300v. I'd bet you could get better headroom and volume by bringing up the v1 plate voltage.
    Are all the preamp plate voltages low ?
    there may be another dropping resistor on the power rail making all the preamp voltages low ???
     
  9. straightblues

    straightblues Member

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    Thanks for the input guys. I am not a tech, just a guy who knows enough to be dangerous. I found out the conversion with done by Bill Carruth. I talked to him and he is trying to find his schematic with the voltages to maybe give me a clue. I will report back when I find something. Any other thoughts would be appreciated.
     
  10. Keyser Soze

    Keyser Soze Member

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    Check the .05 uf and .1 uf caps in the trem circuit - if one is leaking DC that could explain the very low screen voltage.

    If you cannot check the caps directly you could just try disconnecting the trem from the power rail [insert usual warning about filter caps and high voltages HERE.]
     
  11. LPMark

    LPMark Member

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    The math tells the story, if your readings are correct and you have 100V dropped across the 1k screen supply resistor than either that resistor is way higher in value than 1K or your'e pulling 100ma!!! through that point. That is extremley abnormal. Three 6SL7 can't sink that much current.

    If you pull all the tubes and remeasure voltage and it is still low then it is probably that filter cap. With no tubes the voltage drop across that resistor should be next to 0V. If that is the case then put in just the power tubes and remeasure.

    Then repost your findings.

    Mark
     

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