Need help with amp slaving

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by telewacker, Sep 14, 2006.

  1. telewacker

    telewacker Member

    Messages:
    2,265
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    I use a low powered (20 watts or so) vintage class A amp for the bulk of my playing and have been trying to make it "bigger" by slaving it out to a seperate power amp and speaker. I tapped the speaker output on the amp with a 3k resistor and a 300 ohm resistor to ground. The signal was still pretty hot compared to preamp level so I added a pot to the circuit, a 50k A&B I had laying around. No doubt it's not the ideal value or taper, but it does the job and the signal level is fine now. I had a cab built to the same specs as the original amp, and installed the same speaker that I use for these amps, a 50 watt Weber ceramic Blue Dog. The cab is powered by a 2-6L6 cathode biased power amp from a late 50s Baldwin organ.

    It sounds similar to the original, but the tone is harder and brighter, not as warm as the original. I tried the slave output to the power amp of a 100 watt head, in case it was the organ amp that was affecting the tone (I considered this unlikely, as the amp is pretty hi-fi) with the same result.

    To complicate matters I had Wobo build me a double looper with remote foot switches, and the slaved output is so bright and gainy going into the looper that it squeals when I turn the treble up near the normal setting I use, and the input is distorted even when I pull the slave output down way below where I generally need to set it. I don't have this issue plugging the slaved output direcyly into either power amp.

    I know next to nothing about electronics, but my impression is that there is a severe impedance mismatch with the loopers input, but I'm wondering where the loss of fidelity occurs even going directly into a power amp. Do I need a better circuit design for the slave? It doesn't matter if the original amp is connected to a speaker or a dummy load.

    I know other people have slaved smaller amps into big power amps. Do they settle for a close facsimile of the original tone, or is there a better circuit for reducing the signal from the OT and making a more compatible interface with power amps (or especially this looper)?
    [​IMG]
     
  2. gtrnstuff

    gtrnstuff Member

    Messages:
    2,268
    Joined:
    Oct 19, 2005
    Location:
    Middle TN
    Sounds like you're trying the right stuff. I have had spotty results slaving amps into anything other than a dedicated tube power amp such as MESA 20/20, VHT 2.50.2, etc. You'd think an effects return on a head would be the same, but it's usually too bright, low headroom, or both. RE: the looper, are the grounds isolated? Sounds like there might be something leaking signal creating a FB loop.

    Experts chime in....
     
  3. telewacker

    telewacker Member

    Messages:
    2,265
    Joined:
    Jan 10, 2005
    Location:
    Clayton, CA
    "RE: the looper, are the grounds isolated? Sounds like there might be something leaking signal creating a FB loop."

    Does the slave circuit need a cap? I tried all the possible ground and phase combinations.

    I'm going to try a QSC SS power amp and see if it colors the tone any less.
     
  4. gtrnstuff

    gtrnstuff Member

    Messages:
    2,268
    Joined:
    Oct 19, 2005
    Location:
    Middle TN
    Sorry, I was just sort of fishing with that grounding comment, without actually seeing and hearing your rig. The slave circuit "shouldn't" need a cap, I've never had problems quite like you describe and my slave outs have been wired like yours, if I understand correctly. That's pretty much how Weber MASS derives its line out: tapping into the speaker line.

    And all my major ground loop hums with multiple amps have been solved with buffered iso transfomers in the guitar>amp input path.

    I used a Hafler SS amp for awhile. Tight, clean, uncolored, ultimately soul-less for the tones I was after. MESA 2.90 was a bit stiff because I could never crank it. 20.20 is nice, but fan is loud. VHT 2502 is my current fave.
    Good luck with it.
     
  5. teleamp

    teleamp Senior Member

    Messages:
    3,508
    Joined:
    Aug 22, 2006
    Location:
    Central Texas
    The best solution soundwise is to install effects loops on both amps, run the preamp of the 20 watt amp to the power amp of the 50 watt amp.

    MikeY
     

Share This Page