Kalamazoo type trem not oscillating

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JJman, Apr 14, 2008.

  1. JJman

    JJman Member

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    I just fired up my new build and all is functioning except the trem. I’m using ½ a 12ax7 into cathode biased 6v6s. (Only) the trem portion of the below schematic is what I used in mine.

    http://www.schematicheaven.com/bargainbin/kalamazoo_reverb_12a.pdf


    I tried it as is, and also with a 1.5k cathode resistor/100u cap combo. The grounded cathode setup resulted in the -.05vdc on the grid and about 106vdc on the plate. This matches the voltages I’ve seen on other links for this same design. The cathode resistor strategy resulted in 130vdc on the plate along with 1.0vdc on the cathode so I know the triode is conducting.

    I tried connecting the switch jack directly to ground (on) and cannot see any trem-oscillation occurring anywhere in the setup. I haven’t scoped it yet (different room) but my fluke usually shows AC even if very low hz like trem. I hear no effect at all at any setting. I also tried another new tube that I know functions. I'm not seeing dc leaking thru the caps on the plate.

    Trem/controlled oscillation remains a mystery to me when trying to fully comprehend a schematic. Should I try going to .01 instead of .02 somewhere? Maybe it’s too “slow?” Any hints?
     
  2. Jaybird03

    Jaybird03 Member

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    I built a Kalamazoo Model 2 clone that has one of the best tremelos that I have run across. I will try to post theschematic from the Kalamazoo 2, It may work give you some ideas.
    [​IMG]
     
  3. JJman

    JJman Member

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    Yea I saw that setup too but it modulates the cathode of a preamp tube (like a Vibro Champ.) Very cool but I'm looking to modulate the output tubes.

    I scoped it today and don't see any oscillations at any frequency on any nodes. I tried changing the "middle" cap from .02 to .01 to no avail. :confused:

    Any ideas what I could try next?
     
  4. vibroverbus

    vibroverbus Member

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    jaybird's schem is good for one schematic drawing principle even if you disregard component values: oscillator circuits are often drawn with circular layouts with one or two 'flyover' points where there's no connection, and the layout of the schematic is harder to figure out. jay's shows exactly how simple the concept really is - just a series of phase-shifting RC pairs directly between the grid and the plate, end of story. many Fender circuits are wired JUST like this but the standard illustration makes it look more complicated with non-connected crossing lines.

    JJ - when I re-did an oscillator last, I wasn't paying enough attention (kind of blindly wiring up and not thinking) and mis-read a flyover intersection as a junctions. any chance you have the top of R37 connected to the plate? because it shouldn't be - it's really just like jaybirds example if you imagine drawing R37 straight down instead of up and connecting the socket to the same ground line as R35/38. don't put the cathode resistor in as you're working off a plate load on your circuit (vs. the cathode load on jaybirds example).

    you should not have to change any component values, and this will definitely work, there's probably just a little bug to track down.

    here's an as always excellent page from Randall Aiken:
    http://www.aikenamps.com/PhaseShiftOscillators.html
    and another from a good guy named JC Maillet with a bit of a menu of trem circuits to choose from, but many bias-mod oriented. :
    http://www.lynx.bc.ca/~jc/tremoloCircuits.html
     
  5. JJman

    JJman Member

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    OOPS. I forgot to install a resistor. [​IMG] It's now oscillating. It was too fast and not strong enough so I increased the oscillator cap on the plate and flipped the values on the intensity pot and it’s resistor. It’s functioning nicely now but I have a related problem I’ll post separately.

    Thanx !
     

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