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Want to cure my Ampeg Reverbojet of ice pick

Discussion in 'Amp Tech Center' started by Stefanovich, Apr 1, 2018.

  1. Stefanovich

    Stefanovich Tele-Holic

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    Hello amp gurus! I have a 1967 Ampeg Reverbojet (the one with faux wood paneling and copper face plate) and while there are many things I like about it, it is too ice picky. Even my jazz box has ice pick! The amp is pretty much unusable with the tone set above 2. I already had a local, respected tech go through it, and he replaced the filter caps, put in some new jacks, fixed something in the power supply, and tested the tubes. After all this work, the amp sounded exactly the same. I feel reasonably certain the correct circuit diagram can be found at http://el34world.com/charts/Schematics/files/ampeg/Ampeg_J12R.pdf

    I have already tried a speaker swap (to a Cannabis Rex) and while I loved the efficiency of the new speaker, it didn't really change the tone. Apparently, the old CTS speaker has a frequency response similar to the Cannabis Rex.

    The volume and tone pot seem to work as they should. By that I mean there is no scratchiness, the taper is appropriate, and they work through the full range of motion. This leads me to think the fault is either bad design, or capacitors.

    My basic questions are:
    1. Could capacitors in the preamp be faulty, leading to the icepick sound?
    2. Are the RC values in the diagram reasonable?
    3. The volume and tone interact with each other (looking at the circuit diagram this is not surprising). Is there a way to minimize this?
    4. Would using a 5751 as the preamp tube help?

    Many thanks for any suggestions or advice. Amp front view.jpg
     
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  2. TBOT

    TBOT TDPRI Member

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    Wish I could help. That thing is beautiful.
     
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  3. MrYeats

    MrYeats Tele-Meister

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    Maybe run the signal thru an EQ?
     
  4. JD0x0

    JD0x0 Poster Extraordinaire

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    V1a and V1b (both halves of the first tube) use a .1uF (100nF) bypass cap for the cathode. This basically gives you a boost to the highest frequencies, and IME can often make amps harsh. (you can see for yourself by using this calculator.. https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/)


    I'd consider pulling them one at a time to reduce icepick. I think the brightness is just part of the design. You can roll off some highs with the tone control. You could also consider changing the cap on the tone control to adjust the filter.

    V2b creates a treble boost AFTER the tone control, so you can try removing that first, since that's basically just boosting high frequencies after you're trying to roll them off at the tone control, so you have no way of rolling them off.
    It's also likely the amp is staying harsh all the way down to '2' on the tone control as well. Since the tone control cannot roll off the highs boosted at V2b, you're left trying to overcompensate by cutting more highs before this stage, only to have the highest, most harsh frequencies, increased right after that.


    I think a better design would've been to use a .68uF or 1uF on V1a and no bypass cap on V1b. This would give nice sparkly highs that wouldn't be as anemic or icepicky and the tone control would be able to do its job a bit better, IMO.
     
    Last edited: Apr 1, 2018
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  5. chemobrain

    chemobrain Friend of Leo's

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    pretty...
     
  6. Stefanovich

    Stefanovich Tele-Holic

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    Thanks! The explanation makes sense and this seems like a great starting point.
     
  7. powerwagonjohn

    powerwagonjohn Tele-Meister

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    They must have changed things around. I have a 1966 Gemini II and have played other older Ampegs and ice pick is not the way I would describe them. Good luck. By the way, cool looking amp!
    Thanks john
     
    Last edited: Apr 1, 2018
  8. radiocaster

    radiocaster Poster Extraordinaire

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    You might want to mess with those caps in the tone section. Maybe also the .002 after the "dimension".

    If the caps are not faulty, you can simply connect a cap in parallel with the another cap and the capacitance will add (make sure their voltage rating is high enough). You can just try it with croc leads so you don't need to mod your amp. Don't do it if you don't know basic amp safety.

    There's a demo on youtube, sounds pretty bright, especially with the reverb on.
     
    Last edited: Apr 1, 2018
  9. Stefanovich

    Stefanovich Tele-Holic

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    Thanks Radiocaster. I had thought of adding a cap in parallel with alligator clips as an easy way to test, but hadn't considered the .002 cap in the reverb section.

    My amplifier sounds WAY brighter than the one in the video. The amp in the video has some similar tonal characteristics to mine, but just imagine mine has a massive bump at 1000 Hz.

    EDIT: I noticed at the very end of the video his tone control is at about 1.5. That is where I keep mine. I guess they all sound like this...
     
  10. Stefanovich

    Stefanovich Tele-Holic

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    OK. Another question. What about adding a cap in parallel to the feedback resistor as suggested Rob Robinette? Has anyone tried this?
     
  11. Inglese

    Inglese Tele-Meister

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    Generally speaking it's not a good idea increasing feedback at high frequency since stability problem may aride.
    I'll cut some HF out of phase inverter or at power tubes anodes (sorry I did not check schematic before posting this reply).
     
  12. radiocaster

    radiocaster Poster Extraordinaire

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    What about the cap that connects the grid to the cathode on the 3rd gain stage?

    I probably wouldn't mess with it because I have no idea what it does, but I thought I'd ask.
     
  13. Stefanovich

    Stefanovich Tele-Holic

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    Rob does mention this can occur.

    I took the amp out of the cabinet last night (A first for me) and found everything I was looking for in the circuit. Only problem is the amp uses a PCB. Further, the components are close together snug tight to the PCB, making the use of alligator very difficult in some places, and impossible in others. AAARGH!

    I am a bit scared to solder stuff in and out myself because I don't know if my soldering iron gets hot enough, and also because my soldering skills are ummmmm.... sub-par. Happily, the tech who worked on my amp has great soldering skills. All his joints are solid and shiny. I would take the amp back to him, but the cost factor is prohibitive for me at this point in my life. I think my my best course of action is to have a few drinks, lick the filter cap terminals to make sure they are discharged, and have at 'er.
     
  14. Stefanovich

    Stefanovich Tele-Holic

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    Sorry to keep replying to my own thread, but I want to make sure I am understanding the advice so far. On picture A, I have circled the capacitor I think needs to be changed to 0.68 microFarads. It goes from pin 8 of the 12AX7 to ground. I think this is the mod recommended by JD0x0.
    20180402_174709.jpg
    The second mod is the one on Rob Robinette's website. This is a 100 pF capacitor in parallel with the feedback resistor. This is the mod that Inglese warned could cause instability at high volume.
    20180402_174758.jpg
     
  15. JD0x0

    JD0x0 Poster Extraordinaire

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    My main issue with this would be, that the tonal 'effect' will be completely diminished when the NFB loop falls apart. Meaning, if you have the amp turned up, it's possible that you will (temporarily) lose the treble smoothing effects of the NFB network, with the cap in place.


    yes, but I'd definitely recommend try removing that second .1 uF cap on V1B before you mod the first triode. It should simultaneously reduce some of the peaky treble as well as make the tone control seem a bit more effective.
     
  16. robrob

    robrob Poster Extraordinaire Ad Free Member

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