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  1. JJman

    EL84: Class A PP at 252V, right?

    Wrong. At least with 253V on the screens at idle. I wanted to get rid of the crossover distortion I was seeing on my '67 Harmony H415 2 x EL84 amp. The scheme shows 299 net volts on the plates and mine was running 360-375 depending upon the idle. So I added a 500 ohm 10 watt dropping power...
  2. JJman

    What 6v6 is this?

    Base is tan in color.
  3. JJman

    Cathode bias & Bias Trem & Post PIMV

    My project head has cathode bias and bias vary trem. Having both is not new and I was able to reference different schematics. Including a post PI MV was always part of the plan and I used the common approach of a dual pot to ground. It worked and sounded good but the trem effect would also...
  4. JJman

    Ultra low hiss = crosstalk

    I picked up this 12ax7 tube recently and noticed it had no hiss at all in my homebrew Champ. I was really looking forward to using it in my high gain head. While running a signal generator into the Champ I could hear significant sound with the volume fully down. So I changed the volume pot but...
  5. JJman

    12ax7 pics, auction buys

    Went to an auction in Wall NJ on Sat. Lots a cool old stuff. Wackiest was a "watch calibrator." Got a tube caddy with a Tung Sol ad on it. Was trying to convince myself that I had to have the drug store tube tester with stand. Picked up a 6pc lot of 12ax7s. 2 boxes were Mullard, 2 Dumont and...
  6. JJman

    Is it a Champ or Vibro Champ cab ?

    My VC's chassis has a 73 OPT and 74 PT and pots. The tube chart has only 3 tubes on it. From the pics, can anyone verify that the cab is from a Champ and what year it is? Were the Champ and VC cabs exactly the same other than the tube chart? Last pics are the solid sheet of aluminum...
  7. JJman

    Brown Turds vs. Blue Turds

    I recently checked all the brown turds I pulled from my '71 DR for dc leakage and capacitance. Every one tested essentially perfect for both. I have my '74 VC on the bench and I tested the 2 blue turds for dc leakage and they don't leak. However they both tested about half the proper...
  8. JJman

    Cap across the PI plates

    I was messing with my ’71 SFDR today and figured I’d try to tame the highs a tad. (Bright cap already gone.) I wanted to reduce only the highest highs rather than all the highs the way the treble control would. I tried a cap across the PI plates since I’ve often seen this on schematics. I...
  9. JJman

    Who made this 6v6 ?

    The 6v6 has a screen that lights up when DC is on. Who made it?
  10. JJman

    Crossover Distortion During clipping

    I had crossover distortion on the output of a home build with a 5e3 output but with a LTPI. I read up on it and it’s caused by the ramifications of driving the grids somewhat positive which charges the coupler for a period of time. The bias is momentarily pushed down I think while the coupler...
  11. JJman

    TV's "How It's Made": Electric Guitar Amplifiers

    Science Channel- 8pm eastern Sunday 10/25. Warning: This episode contains ribbon cables, robotic component insertion and wave soldering. Viewer discretion is advised.
  12. JJman

    Grid stoppers, clamping, blocking and crossover distortion.

    In my homebrew I’m using a LTPI to drive 2- 6v6 tubes in PP. They are cathode biased running AB at 12watts idle each. The plates are at 350VDC. I’m wondering about the grid stoppers and if they are adequate at 1.5k. I sometimes think I’m hearing a bad distortion in addition to the good...
  13. JJman

    Tweed Deluxe: max clean output 9watts?

    I measured the output of my homebrew just before clipping using a 1k sine wave. I show about 8.5volts RMS on an 8ohm dummy load using my Fluke meter. Also see about 24volts pk-pk on my scope, (which is 12volts pk, which is also about 8.5volts RMS.) Doing the math: Watts=VA, A= V/O...
  14. JJman

    Which Tweed Champ impedance?

    If you had to choose from the below impedances for the 6v6 to see on your Tweed Champ, which would you prefer? 8800 6400 5800 4100 Would it matter to you if the plate was closer to 305volts (5E1) or 340volts (5F1) ?
  15. JJman

    Another grid-to-ground tone robbing cap? (A1270)

    I just spotted a grid to ground .002 (2000pf) cap on the reverb return jack of my '71SFDR. Looking at the schematics it looks like this was added with the A1270 and was not on the previous AB868 or AA/AB763. I removed the ones on the output tubes a long time ago with no problems. Anyone ever...
  16. JJman

    How loud to overdrive an SM57 ?

    Would an amp need to be extremely loud to cause distortion from overdriving an sm57 ? I'm asking about the mic itself being forced to distort, regardless of the unit it is plugged into. The specs sheet...
  17. JJman

    Is this a "crossover notch?"

    When clipping is reached, I’m seeing a “notch” on the output of my project amp. The output is tweed Deluxe and the PI is a LT type. It happens with or without NFB connected. It develops when I turn the volume up to the same point that both PI plates begin to clip (from cutoff.) I don’t...
  18. JJman

    One triode for parallel loop?

    I did some tinkering with the perfect-world no-buffer approach: Definitely need at least a boost after the return. The approach “worked” but the wet path was too quiet as feared. Also confirmed the need for mixer resistor(s) before...
  19. JJman

    Plate resistor hiss bypass cap: Value?

    I want to experiment with bypassing the plate resistor on the 1st stage of my hi-gain amp. It's a 100k with a 1500ohm cathode. Plate has 160V on it. PS node is 240V. What value range of caps should I try? Any success stories?
  20. JJman

    My Signal generator has DC on it's output ;(

    I noticed that my signal generator has 4.5VDC on it's outputs. :NUTS It a cheap DIY kit "Elenco Elec Model FG-500." This was messing with the gain of the 1st triode when using it into my amp on the workbench. So I checked Mouser for a new chip since it's outputs comes right off the chip. I...
  21. JJman

    DMMvac vs. SCOPEpk-pk vac: Pic

    My scope and my DMM match each other perfectly for DC. My scope shows 0.5vac when using it's calibration terminal (square wave.) My DMM shows 123vac on the house line. All those are ok. When measuring vac, the two differ by a factor of 2-3 times or so. I read that pk-pk vac should be more than...
  22. JJman

    Long Tail PI: Later clipping

    I'm using a 470ohm > 20k > 5k-presence > ground for the PI's cathode of my build. LTPI type. It gets a strong signal on it's input. I want stronger clean outputs from it before the PI’s outputs start to clip. What new values should I try? Or maybe I'll need to increase the plate voltages by...
  23. JJman

    What do these resistors do?

    What do the two "33k-100k" resistors do in this PPIMV ?
  24. JJman

    Preamp tube clipping. Top vs bottom of wave.

    If a preamp tube clips asymmetrically, is there any difference in the sound of it saturating (clipping the top of the current wave) vs. reaching cutoff (clipping the bottom of the current wave?) Assuming the same input signal and %age of the wave being clipped. I ask because I have a 3rd...
  25. JJman

    Shield the BOTTOM of the Reverb Pan? -SFDR

    My '71 SFDR starting buzzing a little as the Reverb is turned up. It does not matter what environments/outlet the amp is connected to. The buzz is on the grid of the post-pan tube. Unplugging the pan on that side eliminated the buzz (and the reverb of course.) Unplugging the driver side does...
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