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  1. J M Fahey

    .68uf presence cap. Help me read this schematic!

    In practice all 4 possibilities: * .68uF * .47uF + .1uF * .56uF + .1uF and plain * .47uF end up being the same, because respective "turnover" frequencies, meaning "where they start to work" are: * 49.8 Hz * 59.4 Hz * 51.3 Hz * 72 Hz because guitar lowest frequency is about 82 Hz, so ...
  2. J M Fahey

    Deluxe Reverb Speaker

    Maybe because it`s *expensive*magic :( And don`t get me started on *weight* :( But if you can move it around (or have muscular roadies ;) ), go for it.
  3. J M Fahey

    .68uf presence cap. Help me read this schematic!

    Well, if you want to have your cake and eat it, you can add a small unobtrusive toggle switch to the front panel (think a pickup phase switch type), leave the original 0.1 uF cap in its place and at will add a 0.47uF cap in parallel with it when you want to. Original schematic: So called...
  4. J M Fahey

    Unwanted Harsh Distortion Please help!

    To really know first what is happening and then where, you (or your Tech) will need scoping it. Otherwise it´s just guessing.
  5. J M Fahey

    .68uf presence cap. Help me read this schematic!

    Weird. I suppose you replace the .1uF presence cap by a .68uF one? Not sure I´d like it because it defeats the Presence control effect , but fwiw and for those who want to know what it actually does: * power amplifier has an "X" amount of gain by itself * said gain, called "open loop gain"...
  6. J M Fahey

    JBL D120 Clones?

    Yes, and VERY good ones. In fact, many in the know chose these because they were still made "like old JBL" while originals had changed: fact is Spain was economically isolated from the rest of the World (very high Tariff barriers) for decades...
  7. J M Fahey

    Marshall Footswitch alternative?

    You might rehouse it since it´s really simple or get the schematic (or "lift" it from your current one and build a clone, again in a smaller box. If you are not into DIYing, maybe a friend is.
  8. J M Fahey

    Installing Speaker Out Katana Air - Need Help

    Chiming in late, better than never ;) 1) as mentioned above,NEVER EVER join 2 amplifier *outputs* together. Guaranteed damage/destruction 2) do not conside even negative/black wires common. VERY probably each channel uses a bridged/balanced output which means BOTH "+" and "-" wires are HOT...
  9. J M Fahey

    Old Mustard caps possible problem ..... have a look!!

    THAT is an important point,which overshadows lesser or even imagined ones such as Mojo or appearance. Personally don´t much believe the "plastics inside getting mushy" theory, not much cause for degrading in the dark, isolated from atmosphere inside a hermetic epoxy cover, instead just think...
  10. J M Fahey

    Is this speaker ok to use?

    The Glues Direct stuff is the real thing and same as roofing sealant, only thicker. Quite surprised at Dr D´s suggestion because Celestion itself never ever uses diluted PVA (which quickly dries, forms a whitish harder surface skin and has NO tacking whatsoever) as speaker doping but the...
  11. J M Fahey

    Shocked when touching attenuator, already ruled out the outlets. What’s next?

    An OT *is* an inductor, conceptually not different from a car ignition coil or a TV flyback. No big deal if a speaker load is connected because it will absorb amp output energy but if disconnected high flyback voltage peaks appear. High enough to shock you at the speaker side and thousands of...
  12. J M Fahey

    Proco 'lil' Rat

    You are right. Just checked: Mouser does not even name it, Digikey states "Obsolete - no date available" No doubt Boutique/Hobby suppliers can offer some, maybe up to 100 or 200, but no established Factory can depend on that.
  13. J M Fahey

    How to clean a dirty speaker?

    No. If anything, it will sink it deeper, while now it´s only surface gunk. You can "wash it off" meaning like with anything dirty, water softens dirt and something mechanically removes it. Not garden hose or dish washer level water, of course, just the absolute minimum I regularly clean dirty...
  14. J M Fahey

    Is this speaker ok to use?

    10" Guitar Second coat, half around and full around: 15" Keyboard notice cone is brighter and takes sealant with some difficulty.
  15. J M Fahey

    Is this speaker ok to use?

    I use Argentine products, but will suggest similar ones available un USA, plus a mini application course he he. The first ones you suggest are either a coloured paste, or have fibers or are flammable solvent based; the one I suggest is "as liquid as water", water based, roller or brush applied...
  16. J M Fahey

    Old Mustard caps possible problem ..... have a look!!

    But what about MOJO?????? :eek: :eek: :eek:
  17. J M Fahey

    Is this speaker ok to use?

    Sorry didn´t find this earlier. Standard speaker edge doping material is same as household "roof/tile leak sealant", only quite thicker. But you can apply it as is, let dry, apply another coat, etc. Who cares if it takes 4 or 5 nights to apply? I am talking the whitish water based...
  18. J M Fahey

    JBL e120 dust screen?

    In those speakers pole piece is "vented" , meaning there is a hole through it, for various reasons, mainly ventilation fo increased power handling (solid dust cap works like a piston and blows air through it) BUT it allows free accesss to the very sensitive gap area, so some kind of screen is a...
  19. J M Fahey

    what to replace next on my JMP to fix this issue?

    Really. Sometimes a customer brings an amp in, says "smoke came out - I see sparking - tubes redplate - whatever" and I smile. Why? Am I a Sadist? A greedy guy happy with the massive damage? No: because it´s relatively "easy". Problem sticks out like a big sore thumb, you replace what is...
  20. J M Fahey

    I fear the same thing happened to both my amps...

    Unlikely same happened to two amps, at the same time, and for no evident reason. So there must be something common to both. As in: already mentioned: * low wall voltage: get a $10 meter and check it, also useful to check cables, batteries, supply voltage and polarity, no need to be a "Tech" to...
  21. J M Fahey

    Doping removed from speaker?!

    Cones ARE grey/black. Black pigment (usually lampblack) is mixed in the paste at the factory, *before* it´s actually felted and pressed. Before that, it was a very liquid black paste inside a bathtub ("black water"), not kidding. It does NOT have a "white core". This is the process used:
  22. J M Fahey

    16 Ohm Mesa V30 Measure 7.1 Ohms

    It´s clearly an 8 ohm speaker. Trust your meter and not the label.
  23. J M Fahey

    Ever get no appreciable difference biasing hot/cold?

    In general, Manufacturers/Engineers bias tubes "cold". All of them: Fender/Marshall/whatever. Biasing amps HOT, or the silly "70% bias" or whatever (with no base in the real world) are a Musician/Guru/Forum thingie. Power amps are designed by printing power tuve curves, and drawing various load...
  24. J M Fahey

    Solid State Class A Guitar Amps?

    Short answer ... no. FWIW the FIRST SS amp I made WAS single ended Class A but: * it was 1966 or so. * it was 2W RMS * it was the power amp section from an AM car radio, think whatever Ford Falcon came with. * it run 1Ampere at idle and worked skin blistering hot, not kidding. And don´t ask how...