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Filament buss question

Discussion in 'Shock Brother's DIY Amps' started by moosie, Nov 23, 2017.

  1. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    I'm listening again tonight, and the 3.9M sounds best to me. But here's the thing, I don't feel the 3.3M takes it wilder. The opposite. Like 3.9M is a peak, compared to 4.7 and 3.3. The 3.3 difference is subtle, might not be much less wild, but really not hearing more wildness there.

    Also, when I went back and listened to the 6k7 cathode resistor, with the stock 4M7 R37, I hear more 'beating' of the LFO. I guess it could be that I'm playing just bit differently. But that low end on the samples in post #225 bugged me. Lowering R37 seems to have smoothed that.

    I like the sound at this point. I don't have a clear picture as to what these last tweaks did, and that's OK, but I don't know if it's worth doing anything more.

    Clint, is there something more you wish you could tweak if it were on your bench? I'm happy to continue, if you like.

    On the other hand, if you're dead and tired of this, you have my eternal thanks. Well, you have that regardless. :)
     

  2. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Here's more endless arpeggiating... This is Speed on 0, Depth on 100%, with the same components I left off with last night, which happens to be the 6k7 cathode, and the 3M3 R37. But this time I turned down the bass, and I'm using P90s instead of PAFs. I was trying to use similar tonality to @Snfoilhat's best demo, here: http://www.tdpri.com/threads/harmonic-tremolo-amp-2.790875/#post-8071248






    Sounds darn good (not the playing, I was distracted, sorry). Yes, I might want to back off 100% most of the time, but it's nice that it's there.
     

  3. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    559
    Apr 8, 2016
    Oakland, CA
    Sounds great!
     
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  4. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Thanks, man! Your sample was an important model as I got closer. I probably would have called it good sooner, if not for that. Glad I didn't.
     

  5. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Oh, that's quite lovely sounding now. :)

    Oddly enough, the P90 hum makes for a bit of a giveaway that the harmonic part is working. It's not quite as profound as some of the brown Concert amps, but the really juicy ones of those on YouTube seem to be running the later 5 triode version based on age. I'd love to hear this unit paired with my Bassman, with its 4 10s and the juicy midrange.

    To recap the changes for anyone following along, and clarify:

    Changing the V4 cathode resistor shifted the bias even cooler, pushing it closer to cutoff. That raises the sensitivity of the mixer stage to the LFO signal from V5.

    Adding resistance to the ground of the Depth pot lets more LFO signal pass to V4 at all settings, and sets a higher zero level.

    Lowering R37: R37 and the Depth control form a voltage divider network, adjusting how much the LFO signal is reduced before it hits the Depth pot wiper. Lowering r37 lessens the voltage drop, making the signal stronger at the Depth control and at V4.

    The second two changes raise how hard the LFO signal hits V4, and the first make V4 more sensitive to that signal.

    If you're happy, and it sounds like you are, I'd make any temporary value changes permanent by swapping the relevant components out (like the 4k7 and trimpot for a 6k8), retest it, and button it up.

    This has been a real journey, and I've kind of enjoyed it. That last sample made me smile. You are very welcome, and thank you very much for sticking with it. I've learned quite a bit about vibrato circuits and some nice history lessons along the way too.
     
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  6. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Yeah, I'm quite pleased with this result. All done. I'll button it up tomorrow.

    I'll see if I can bang out a better final sound clip, too, for posterity. Maybe use some different pickups, different speeds, reverb.

    I think it was a mistake to use those bassy humbuckers for all of the testing. I just used whatever was handy, and then wanted to keep it consistent. But I should have at least turned the bass down more for testing.

    Thanks a lot for that summary. It's sort of what I thought, but I'm still slowly wrapping my head around all of it. To complete the picture, what would happen if that cathode resistor were increased further? Would we just turn the tube off completely? Chance of a sweeter spot before that happens?


    Clint, thank you. Your generosity, patience, and persistence has me floored. I learned a lot, and got a finely tuned Revibe. None of it possible without your help.
     
    Last edited: Jan 13, 2018

  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    There's a tipping point where the the tube is so close to cutoff that even the slightest signal would cause cutoff, and past that it can be biased to pretty much full cutoff with just a tiny bit of signal flow due to the island effect, where just a trickle of current actually flows around the edges of the grid. There's a fine balance, and I think we found a happy spot in there. Sounds good, and there's a nice wide range available.

    I'm looking forward to hearing it in its full glory. :)

    And again, you're welcome. It was my pleasure, and really made me think outside the box to grasp how this worked (the easy part) and how to tune it (the hard part). The best problems are the ones that teach, and this was one of those.
     
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  8. Phrygian77

    Phrygian77 Tele-Holic

    920
    Apr 30, 2016
    Crawfordville, FL
    I'm curious about the output level, and any high frequency loss, vs straight into the amp. The mixing resistors going to the output without any kind of buffer to bring the output impedance down was obviously a compromise. Anyway, it sounds great.
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I scrolled back through just now. We've been tuning this thing since Dec 18th, and this thread has racked up over 5,000 views. That was a long row to hoe indeed.

    So, I guess that definitely answers one age old question: wouldn't it be faster to just take it to a tech? ;-)
     
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  10. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    What are you thinking? Something like a cathode follower buffer stage on the output?

    I'm kind of wondering how hard it would be to build just the 5 triode version in a small Hammond box now. Use the PT from a single ended EL84 amp for power, etc. Might be a fun exercise for next winter when the snow is deep.
     

  11. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    That's almost never the question I would ask. I like to do things the hard way, I guess. :)
     
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  12. Phrygian77

    Phrygian77 Tele-Holic

    920
    Apr 30, 2016
    Crawfordville, FL
    Right, something like the fixed biased cathode follower in the 6G15.

    Hammond box in a tweed style cabinet? I'm still thinking PR chassis and head cabinet. To make the layout cleaner, maybe drill out one of the 9-pin socket holes to octal size on the preamp side of the chassis for the 6V6, and mount the reverb output transformer over there. Then, use some octal to 9-pin adapter plates on the other side of the chassis. Although, that would put some of tubes a bit close to the mains wiring.
     

  13. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    559
    Apr 8, 2016
    Oakland, CA

  14. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    I've got it on a footswitch, and I don't notice any difference at all. The whole unit (reverb and trem) seem to match unity perfectly (to my ear), and I don't notice any tonal loss.

    I don't know that my ears are all that great, but in general, I'm easily be bothered by the treble loss on an extra 18 foot of cable with no buffer.
     

  15. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Here are a few more clips. Boring music, badly played, but highlighting different pickups. This is a Gretsch 6120 with T-Armond single coils:









     

  16. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    And finally, a couple with my Tele, and one with the 335.





     
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  17. Phrygian77

    Phrygian77 Tele-Holic

    920
    Apr 30, 2016
    Crawfordville, FL
    Reading back through all of this plus the Harmonic Tremolo thread, my head hurts now.

    I'm wondering if just lowing the value of the 4.7M resistor coming from the plate of the oscillator might have solved the problem alone.

    I'm in the planning phase for a 6G4 (5G4) Super build. I'm beginning to think that maybe I should build the thing like a 6G4A and sub in the 6G4 tone stack, instead of trying to build a faithful recreation of the early 1960 6G4. I'm not going to use a 500kL volume pot, for example, anyway.
     

  18. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Just speaking about my build, I think we could have left the V4 plate resistors at the designed 220k, instead of lowering to 100k to match the early circuits. I don't think it had much effect either way.

    Adding the 2M resistor to change the Intensity pot sweep was a good change, as it effectively shifted the pot sweep to a more useful range.

    Reducing the 4M7 resistor was needed to increase LFO drive, but I still think the most critical tweaking occurs in the V4 cathode resistor. If you read Leo's patent application, he mentions this:

    "The high-value cathode resistor 67 [in his patent schematic] has been found to be extremely important in creating greater tremolo (differential amplification) action in the triodes 61 and 62, so that the tremolo-frequency waves need only have relatively small amplitudes."

    He follows this with more detail of the action of those triodes.

    I think the 4M7 resistor change addresses basic intensity, but tuning the V4 cathode resistor is what makes the unique 'chewy' nature of the harmonic tremolo.

    4-Triode Patent Schematic.png
     

    Attached Files:

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