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Filament buss question

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

  1. keithb7

    keithb7 Friend of Leo's

    Age:
    47
    Jan 9, 2010
    Western Canada
    The best I might be able to do at this time is a phone demo sound clip.
    We sold our house and are in the middle of packing everything up. Zoom H2N is packed. Unsure where it is. Amps are in a pile among boxes in the main rec-room currently. I can access my 6G8A if a phone clip will work. No more bench testing for a while. Let me know if that will do. - Keith
     
  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    It's probably seeing too much leakage through the 4k7 next to it to get a good reading. When you check the resistance, does it take a second or two to stabilize at a reading?
     
  3. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Pretty much anything that the amp can be heard on will work. I didn't know you were moving, sorry to bug you.
     
  4. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    One tip I just got was that the mixer network after V4 can be fickle. That's R48, R49, R50, and C33. That has to blend the two signals, yet block the low frequency beating sound. After that is the divider network, r23 and r24. If those components all test per the schematic, using the audio trace probe and a looper pedal to backtrack from the output jack to the V4 plates is the way to go next.
     
  5. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    I don't what we did, maybe the 100k resistors, maybe I've just listened so much anything sounds good. The sweep on the depth pot still seems lacking, to me. But 100% is pretty good, as these clips will show. At 50% it's still quite faint, to my ear. I'll record that next and let you guys decide.

    One without reverb:


    And one with reverb:
     
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  6. keithb7

    keithb7 Friend of Leo's

    Age:
    47
    Jan 9, 2010
    Western Canada
    Man, that is coming along. Sounding very harmonic trem-y to me. With the reverb on, the trem does seem a little weaker. It sounds pretty dang good though!
    Just the trem, no verb is sounding a lot like my Blonde Twin. What amp is that in? Its great man!
     
    moosie likes this.
  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I need to listen at home (my phone is having a fit with a soundcloud link for some reason) but if it's as good as Keith is saying it might be worth tweaking the circuit a little to home in on it at this point.

    Try a couple of different 12AX7s in V4 to see if one plays particularly nice.

    Recheck the trem signal on V4 grids, too. Swapping that cap earlier may have changed something for the better.

    Subtly tweaking the value of the V4 cathode resistor can have a profound effect, is what I'm hearing from others. The cold, highly asymmetrical bias of V4 is one of the keys to this circuit.
     
    moosie likes this.
  8. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Thanks, Keith. That's my '71 Super with the magical CTS alnicos :D. I'm on the vibrato channel, but verb is on zero, and trem is disabled (no footswitch).
     
  9. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    I'll measure the grids again, before rolling tubes.

    I'll also attempt to measure C30 and R45 (cathode pair).

    I'll take stock of my low-value half-watt resistors, for potential tweaking. Might not have much on hand.

    That C26 .0047 cap... The one I pulled was closer to 4.2nf, and the replacement (orange drop 715p 600v, not Mallory) was closer to 4.7nf.


    Let me know what you think, when you get a chance to listen. In the meantime, I'll roll some tubes through V4 - I have a bunch.


    Yeah! Really interesting, though I'm a bit past my limit of comprehension.

    I was reading the patent application last night, and there are enough differences in Leo's original treatment of what we call V4 that I'm confused how much different the 6G12 (and ours) is from the patent.

    The hi/lo filters are handled differently, with splitting into hi and wide band, then reducing the latter to bass only with negative feedback. Huh?

    But then he takes GREAT care to describe that cathode arrangement. In particular:

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

    He goes on to describe how the high value of the resistor causes (if I understand correctly) the cathode voltage to fluctuate with the grid voltage - ever-changing due to the tremolo frequencies applied to the grid. He's saying this special feature enhances the whole warbly thing, I think. Is he saying that this is unique because the input signal and cathode work together to continually rebias each triode, 180 degrees apart? Or am I totally lost (again)...

    Thing is, he recommends a 15k cathode resistor. Interesting...

    Here's the document once more, worth a link.
     

    Attached Files:

  10. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Listen to my clips back-to-back with @Snfoilhat's latest, after he fixed the missing capacitor in his build. Now his is the five-triode, not four, but they share the same output section, where the guitar signal comes in.

    I hear more *apparent* pitch modulation in his clip, than I do in mine. In other words, his sounds better, to me. What do y'all think?
     
  11. keithb7

    keithb7 Friend of Leo's

    Age:
    47
    Jan 9, 2010
    Western Canada
    I can't stay away from Tdpri. Supposed to be packing! Here is a 5 triode sound clip from my 6G8A:



    My man cave was once a wall of amps...All that's left to kick in 2018.

    [​IMG]
     
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  12. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    I had time to ponder while driving home. A 75 minute commute with no traffic will do that. :)

    I really think now that the bias of V4 is critical. It's like the power tube bias Fenders (PR, for example), where getting too far away from that perfect bias screws with the tremolo depth. V4 has to be right at that tipping point where signal passes cleanly, but the tremolo signal pushes it over into near cutoff and then total cutoff. That's why it doesn't become apparent until the Depth control is more than halfway up.

    Finding that one perfect 12AX7 might do it, or we might need to incrementally step up Rk in baby steps to hit that balance point.

    A 2k trimpot between tube and resistor wired as a variable resistor, or a selection of resistors from 5k and up in small steps to swap in might be the next step if tube rolling doesn't dial it in.
     
    moosie likes this.
  13. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Here are a couple of quick recordings at 50% depth.


     
  14. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Sad to see your empty room. We need a new video once you're settled again. :)

    Clip sounded great! I'm making headway, but not quite there yet.
     
    Last edited: Jan 2, 2018
  15. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Rolled a bunch of tubes, they all sound the same to me. Nothing more chewy at 50%, for instance.

    Before doing that, I clocked the grids and cathode.

    Order: V4a, V4b, and Cathode.

    Depth 100%:

    No signal: 1.48 vpp, 2.04 vpp, and 3.56 max
    100 hz: 1.48, 2.36, and 3.60
    5k hz: 1.7, 2.08, and 3.56


    Depth 50%:

    No signal: 360 mvpp, 120 mvpp, and 3.2 vmax
    100 hz: 290 mv, 620 mv, and 2.54 vmax
    5k hz: 464 mv, 392 mv, and 708 mv max



    Also, the cathode resistor and cap measure 4.71 k, and 2.242 uf.


    I'll be online tomorrow (Tues), but won't available for testing until evening. Infusion day.

    I have a few smallish resistors on hand, but no trimpot. I'm going to order a selection tomorrow morning, and get them shipping.
     
  16. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    698
    Apr 8, 2016
    Oakland, CA
    Your troubleshooting process looks totally sound to me, and the improvements so far are clear. I can't think of anything I can add to help, other than to comment on some of the similarities between our amps. I got my 10M-RA pot from Weber, same as you if I recall correctly. I can only begin to hear the trem around 4; it only sounds good above 6; and it only takes on the characteristic warble above 8. I also don't think it really measured out to 10M. I bought a spare at the time, and it measures 12.5M. So there is potentially a lot of variance in where the controls kick in with a log curve laid across such a big range or resistance, and the factory isn't even consistent in what that amount of resistance is. Great work so far!
     
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  17. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    Thanks! Yours is really sounding good!

    Good to know about your pot sweep. While it would be nice to have a full useable sweep, I really just need it to go a bit deeper, I think. As you say, it's most 'harmonic' above 8.

    I see that Doug Hoffman also sells the pot. He actually calls it the ReVibe pot. Mine from Weber is nearly 11M, and it's just not made very well. I was thinking of getting a replacement, since they could be difficult to source.

    So, higher max value means more depth, right? If the output triodes' cathode is so sensitive, as @clintj feels, it seems the depth would be another moving part in that equation. Wonder what adding a 2M resistor to that pot would sound like?
     
  18. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    they are "RA" pots, so at max or 10, the resistance should be at zero. least resistance should yield most depth. IIRC
     
    Thin69 likes this.
  19. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Strapping a resistor between that pot lug and ground would shift the whole sweep, and slightly raise the trem signal strength at all settings. Might be fun to try.
     
  20. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    61
    Jul 18, 2010
    Western Connecticut
    But that's not what I found. I posted earlier about this. Both this and the 3mra speed pot are only reversed taper, not overall. So resistance is highest on 10, but they roll back 'slow', retaining well over half their value at the 50% mark.
     
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