On icepicks and old Gibsons

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by OlAndrew, Feb 5, 2008.


  1. OlAndrew

    OlAndrew Member

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    I've been working on a mid-60s Gibson GA15RVT. It had been heavily and badly modified when I got it, so its been a "learning experience". When I finally got it to work, it had the famous "ice pick" tone.

    These things were intended to be clean,(think jazz cats with L5s), so there's things like the preamps are 6EU7, a tube designed for low distortion, and a transformer phase inverter. This thing's real simple, 2 coils, 4 taps. One side of the primary goes to the plate power supply, the other to the plate on the last preamp tube. The output taps go to the grids of the power tubes (2 x EL84).

    Most of the books by amp gurus say you can't do much with these, cause of this PI design, however, since I had one ,I thought I'd study up about them. Turns out, this "interstage transformer" is used on a lot of really high-end high-fi stuff, as well as a lot of high-end vintage radios. So it oughta be able to push out cleans, it would seem.

    THe difference in the circuts seems to be that the Gibson is running the plate power supply through the transformer, in the high-fi stuff, the power runs to the plate through the usual resistor, the interstage transformer is connected, one leg through a coupling cap to the plate, the other to ground. I tried setting up this way, .047 ufd coupler, and got a much improved sound. Dunno what the theory is on this, but it works, Tried going back to the "hot" setup, and the icepick's back.

    Running the coupler now , and I've still got some problems, (mostly caps in the tone stack. Makes a major difference. Ever looked at the waveform on the input and then the output side of a cap? Real interesting. These things ain't linear at ALL.)
     
  2. Anywho

    Anywho Member

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    I recently had a ga20rvt minuteman pass through here that sounded great and I am the most sensitive person I know when it comes to distaste for thin bright amps. It was the transformer PI, with 6eu7 in the preamp and choke mounted inside the chassis. Maybe its worth comparing the schematics. I also recall there are two different circuits for this particular model.
     
  3. lexluther

    lexluther Member

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    I'm not very techy. I purchased one of these with the7591 outputs. Interstage tranny was damaged so I got one from andy at mojo and put at 12at7@the driver stage and warmed the amp way up.FYI
     
  4. mojo2001

    mojo2001 Member

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    I have a GA15RVT that was recently given to me. The circuit found in mine bears scarce resemblance to the commonly available schematic.

    As for interstage transformers, there are indeed two ways of hooking them up: DC in primary or "shunt feed" aka "parallel feed" where the DC is fed to driver tubes through a choke or resistor and the transformer is driven through a coupling cap.

    Using shunt feed removes the unbalanced DC from the primary, which eliminates core saturation and extends frequency response. This obtains wide frequency range with a less expensive transformer

    An interstage transformer can be designed for wide frequency operation with DC in primary. This requires an air gap like a power choke and a larger core. Theoretically, the argument can be made that this is a better hookup for hi-fi applications because the transformer is biased in such a way that linearity is improved.

    For guitar applications, extended frequency response is not an issue. Your setup with a 0.47 cap is probably rolling off some of the bass anyway. You might fare better with 1-2uf here for more bass. Use an oil cap for a mellower tone perhaps.

    There are misc theoretical arguments and then there is the sound you actually hear with what you have.

    In any case, I rebuilt mine and found the amp to be overly clean and somewhat icy, but a nice practice amp. Got to keep the treble knob at 1 or 2 with my tele or else the ice is picked. Hey it was free...can't look a gift horse in the mouth.

    I replaced the input cap and one other coupling cap with Vitamin Qs for a somewhat less aggressive brand of clean.

    The usual GA15RVT schematic found on the web shows liberal use of Sprague Couplates which are a packaged coupling cap/notch filter for mid range suckout. If your amp has these little buggers (mine doesn't) it might be worth replacing them with a straight cap or rebuilding the couplates with two 220k resistors and two good caps on a little board. The Sprague part uses dreadful ceramic caps and who knows how stable the resistors were over the years.

    I'd like to get a scope on mine and see how that interstage transformer is actually performing. There are many opportunities for that setup to be as peaky as hell. Rising impedance at high frequencies is a potential issue, which could be addressed by a small cap across the primary a la Dr. Z s "conjunctive filter" and prior art of old clock radio designs of the 1950s.

    I have successfully takes some edge off of diode power supplies by using 10 ohm series resistors in line with each diode, installing .05 caps at the output of the diodes to snuff switching transients, and using better spec diodes. Radio Shack alternator noise chokes can also help if switching spikes are an issue. These tricks won't make it sound like a 5Y3 but can reduce wiry & edgy artifacts.

    I'm thinking that I can probably do better than the stock speaker.

    If anybody has practical experience/suggestions with this amp I'd love to hear it. Wish I had a better schematic for mine but right now I'm playing it rather than tracing out the circuit.


    Joe
     
  5. OlAndrew

    OlAndrew Member

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    Interesting and helpful When I got mine, some fool had dropped the preamp plate voltages WAY down, used half of the reverb driver to salvage some gain, used oddball coupling caps and a truly bizarre tone stack. Sounded HORRIBLE. So I got a schematic and rebuilt it to as close as I could get, but with a very simple tone stack; that was worthless, something about running with no bypass caps on the preamp cathode resistors had me turning up to 9.5 to just barely hear the guitar. Much web study and conversations with folks seem to indicate that there's at least two versions of the thing, neither of which bear much similarity to the schematic....good luck, kid. I use this thing for a hobby, I've got it almost to where I can play through it and enjoy it, but there's always been this lingering bit of edgy bite and distortion that isn't the nice kind. And not much bass, the stuff just doesn't live in that box. Running an Eminence 'Lil Buddy', which is quite nice, although if I was doing it over I'd try a Delta Demon. What fun! Thanks for the ideas.
     

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