Finally -- Princeton Reverb

Discussion in 'Shock Brother's DIY Amps' started by theprofessor, Apr 9, 2020.

  1. theprofessor

    theprofessor Poster Extraordinaire

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    So it's this one circled in purple that goes to the plate (pin 1), not the other one that goes to the grid.

    Hoffman layout tremolo oscillator detail.jpg
     
  2. tubegeek

    tubegeek Tele-Afflicted

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    The LED trick mentioned in that thread has an additional advantage: trem rate indicator. Definitely worth a try.
     
  3. theprofessor

    theprofessor Poster Extraordinaire

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    Would the red LED simply replace the 1M resistor mentioned in post #381 above, or something else, too? That wasn't clear to me upon reading the el34 post.
     
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  4. tubegeek

    tubegeek Tele-Afflicted

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    It replaces the cathode R & cathode bypass C. You are using the forward voltage drop inherent in a diode to set a fixed bias voltage. Anode of the LED to the cathode, cathode of the LED to ground. Remove the existing cathode R & C.

    If you put the LED behind the pilot light bezel next to the bulb you should be able to see it blink with the tremolo.
     
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  5. theprofessor

    theprofessor Poster Extraordinaire

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    I looked at my stash, and I only have white LEDs. So I went with a plain-ol' carbon comp resistor. I measured 485k in place of the 1M. I'll give a report in a bit. I haven't tried it out yet, as I have to do most of my soldering in the basement.
     
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  6. theprofessor

    theprofessor Poster Extraordinaire

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    Dupli-post.
     
  7. tubegeek

    tubegeek Tele-Afflicted

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    White not really suitable, carry on with original plan.
     
  8. theprofessor

    theprofessor Poster Extraordinaire

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    Well it's definitely better with the 485k resistor. In the future, when I've got the chassis out again, I may put another 470k resistor in there to get the value down to around 239k. With the 485k, the intensity dial really starts at about 4 and with good, noticeable effect at around 5.5. And at 10, it's really nice and intense. I like the extra sweep on the speed control by replacing the value of that capacitor, combined with the extra intensity on the intensity control by replacing that resistor. Perhaps more experimenting to come, but perhaps not.

    I haven't compared their bias points, but the tremolo on my 6g2 clone is still the bomb. The tremolo on this Lenox Reverb is now in the same ballpark, but the 6g2 tremolo is the bee's knees. And that's with no value-changes from the schematic, either for the speed or the intensity. It's more lush and more intense. I'm thinking that some of that is related to the kind of signal that's running through that amp. Less signal loss than the PR. Just stronger and more midrangey all around.
     
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  9. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    The only difference on the vibrato circuit that I see is that 1M resistor. The 6g2 uses a 220k in that position. (I know your Lenox has different cap values as well.)

    Of course the actual value of components can be different. The 20% tolerance of the pots could certainly give different results if they are at different ends of the scale. (I almost always check the pot values and correct them when it may make a difference.)
     
    Last edited: May 24, 2020
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  10. theprofessor

    theprofessor Poster Extraordinaire

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    Ah, you know... when I compared the two yesterday, I was still looking at the 1M resistor to the grid, instead of the 1M resistor to the plate/anode. Thanks for pointing out this difference. And yes, the pots could make a difference as well. I measured my pots on the PR clone, but I don't think I did on the 6g2 clone.
     
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  11. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    What vibrato tube(s) are you using in the 6g2 and Lenox?
     
  12. theprofessor

    theprofessor Poster Extraordinaire

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    In the 6g2, I'm using a Raytheon long black plates, and in the Lenox Reverb, I'm using a Chinese 12AX7B. Since V4 in the PRs is a tremolo oscillator and a phase inverter, I figured that the tube only needed to make the trem function and to change the sine wave and that there wouldn't be much tonal effect. So I didn't want to "waste" a vintage tube there. Perhaps I'm wrong on that...
     
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  13. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Shouldn't make much difference. I was curious as I know you use a 12au7 in the reverb slot. I thought perhaps there were different tube types in each amp. I think that could make a difference. I might be mistaken. :)
     
  14. theprofessor

    theprofessor Poster Extraordinaire

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    The 6g2 uses (2) noval tubes. They are both 12ax7's. The PR has (4) noval tubes. There are (3) 12ax7's and (1) 12at7 for the reverb driver. I think that V4 in the PR and V2 in the 6g2 -- both of which are 12ax7s -- have the same function: both tremolo oscillator and phase inverter.

    That said, a different tube could make a difference. I can certainly hear the difference between various 12at7's in the phase inverter position in a Deluxe Reverb. Definitely. I've got some slightly microphonic vintage 12ax7's I could sub in place of that Chinese 12ax7b in the Lenox Reverb V4 slot.
     
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  15. King Fan

    King Fan Poster Extraordinaire

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    Sorry for another slight derail, but I'm trying to picture this (I've really liked the idea of a trem rate indicator ever since I read that el34 thread). Some kind of mounting that'd shine the LED into this space?

    lamp - 1 (1).jpeg
     
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  16. theprofessor

    theprofessor Poster Extraordinaire

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    No problem! So long as you re-post your tremolo "heartbeat" solution here... :) I think I recall it having something to do around the intensity pot?

    EDIT: I think I found it in my files.

    6G2 trem_thump_fix_diode.png

    It looks like I could just add a diode here to my PR clone. The trem thump is _very_ slight, but I might do this next time I'm in there. I've still got some spare 1N4007 diodes.

    Hoffman detail pots.jpg
     
    Last edited: May 24, 2020
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  17. tubegeek

    tubegeek Tele-Afflicted

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    That's what I was envisioning - I think some lamp assemblies aren't as protected as that one. The kicker is that the LED would have to light up bright enough to compete with the lamp so you could tell it was flashing.

    It may not be a realistic idea. Worth a try maybe? It'd make sense to solder the LED into a piece of perf board so you could mount the perf board somehow and have something to hang onto other than just the LED.

    It looks like you could possibly double-stick-tape a piece of perf on the inside of that upright flat bracket, with the LED aligned with the gap you are looking at.
     
    Last edited: May 24, 2020
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  18. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Maybe a 3MM LED? Slide the legs along the side of the bulb? A little heat shrink?
     
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  19. theprofessor

    theprofessor Poster Extraordinaire

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    To close a loop opened in this thread: I got my outlets fixed by an electrician today. Now all the outlets are properly grounded, or they are converted back to the 1964-spec two-prong outlets (and don't pretend to be grounded when they aren't). All outlets in the room we use most are both ground-fault and arc-fault protected. Not a very sexy thing to spend money on, but I sure like knowing that we're not "rolling the dice" anymore. Very relieved.
     
  20. theprofessor

    theprofessor Poster Extraordinaire

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    I went back into the chassis to do a couple of things today:

    (1) I thought I'd add the diode across the lugs of the intensity pot to see if that helped the tremolo "heartbeat" noise. To be honest, I could _hardly_ hear it anyway, but I thought I'd give it a shot and see about the result. I did it just as in post #396 above. Result: total fail. It made the pulse really bad. I took it out, and it's all perfectly fine again. Now that makes me want to go back and understand why it would have _increased_ the heartbeat noise.

    (2) I added a 25k linear "raw" pot in the ground punchout at the back of the chassis. I couldn't find anything satisfactory to step up from the bushing on the 3/8" shaft pot to the 1/2" hole, so I just used a washer on the back (outside) of the chassis. I had planned on using washers on both inside and outside, but together they were too thick (once you include the 16 ga chassis and also the backplate). I wanted the nut to have more thread to thread into. So I just used the washer on the back. The pot is right up against the chassis wall on the inside -- no lock washer or anything. A skirted MXR-type knob covers the whole washer on the outside, so that's nice.

    I really like this mod. It makes the sweet and refined Princeton Reverb into a tweedy, snotty little thing with a lot of attitude.

    I attached shielded cable to the 6.8k mid resistor on the bass pot and ran that over to the right lug of the 25k "raw pot." The right lug of that pot is connected to the wiper. Then from the left lug, I ran regular 20 ga wire back over to the ground bus where the 6.8k pot was formerly grounded.

    IMG_2369.JPG IMG_2370.JPG IMG_2371.JPG IMG_2372.JPG IMG_2373.JPG
     
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