Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups darrenriley.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Filament buss question

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

  1. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Looming back through the thread, I do see one other discrepancy vs the master plan for this thing, the 6G12 schematic. On the 60s amp, the tube that would be V4 in the Revibe idles much colder. 328 on the plates (which would depend largely on B+), and 3.3V on the cathodes. That would let a much smaller tremolo signal do its magic.

    That cathode resistor measures 4.7k from tube to ground, right? And what is the supply voltage upstream of the 470k plate resistors on that tube?
     

  2. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Cathode is 4k7, almost exactly.

    Re voltage, you mean the V4 plate voltage itself? Because there's nothing else 'upstream' of the 470k resistors...

    Give me a couple minutes and I'll take it once more.
     

  3. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    With tremolo off...

    V4a plate: 272 V
    V4b plate: 264 V
     

  4. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Right at the supply, before the pair of 220k resistors, it's 336 V.


    By the way, I see 320V on the plates, in the 6G12, not 328. Am I looking at the right version?
     
    Last edited: Jan 1, 2018

  5. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Plate loads R46, R47, were increased in rev 4 of the Revibe diagram, by Gehring. Before that, they appear to have been 100k, just like the 6G12.

    What's the cost to try that? Can you guess why it was changed? I'm pretty lost...
     

  6. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Here's the Hoffman version, dated 2015. It still has the 100k plate loads.

    (Gehring's rev 4 is dated 2006.)

    Full four page document attached.


    Screen Shot 2018-01-01 at 12.25.21 AM.png
     

    Attached Files:


  7. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    OK, I swapped out the 220k V4 plate resistors, for the original 100ks. The sound is about the same. Barely noticeable trem at 50% depth, little or no harmonic squish.

    Plate voltage with no signal, no tremolo: 300V / 308V, V4a and V4b, respectively. Up from 264 / 272 with the 220ks. Closer to the 6G12.

    Respective cathode voltage is 3.1V.
     
    Last edited: Jan 1, 2018

  8. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Mistake on my part. The plate loads are 220k, the 470k are the mix resistors. My mistake for trying to do that right before bed.

    The voltage I'm looking for is at the point marked C. I want to figure the plate and cathode currents using Ohm's Law to see if they match up, along with the voltage divider numbers around the depth pot.

    And dumb question, but you do have the Dwell and Mix controls on the reverb side turned down all the way, right?
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Would you be willing to do us a huge favor? Can you record a quick video of your amp, Speed on Zero, slowly bringing up the Depth from zero? A benchmark for what the tremolo should sound like at different depths for Moosie to compare his to would be a huge help. The more I study this circuit, the more I'm wondering how much of the harmonic component is supposed to be present at low depth settings.
     

  10. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Point C I posted before:

    "Right at the supply, before the pair of 220k resistors, it's 336 V."

    That's what's coming right off the filter cap / dropping resistor / power rail.

    I can revert if you like, but I have replaced the 220s with 100s, trying to get the V4 plate voltage up a bit more. I wouldn't normally make a change like this, especially while diagnosing, but Gehring rev 4 is the only place I see the 4-triode with the 220s. Everywhere else has the 100s. My plate and cathode voltages (V4) are now quite close to the 6G12 spec.


    Now, as to your dumb question... No. I don't have them turned down. I have no idea. Huh. I'll do that and take fresh measurements right now.
     

  11. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Thing is, though, Clint, I don't hear ANY tremolo until about the 60-70% mark on the depth. Maybe a bit at 50%, if you listen real hard. And it sure sounds bland, none of that special warble going on, even on 100%.
     

  12. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Well, just zero them out and have a listen. The reverb and tremolo take separate paths through the unit, so there's a possibility of the reverb path being much more dominant than the tremolo at the output and drowning it partially out.
     

  13. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Do you have a way to record a sample of yours? Reverb fully off, speed 0, bringing depth up slowly? I'd like to hear what yours is doing, too. I understand what you're saying, but I can also look at the audio in Reaper to see the full spectrum vs volume output.
     

  14. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    This thing is proving to be rather frustrating. You have the patience of a saint to keep plugging away at this after this long. It's also getting tough to dig through this whole thread to glean info.

    I'm going to put out a call and see if I can get some fresh ideas. In the meantime, have you tried your audio probe, backtracking from the output jack to the V4 plates? Is there any evidence of tremolo in that part of the circuit?
     

  15. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Oh, no, for listening, I've been zeroing the 'verb. Always. I just didn't always make sure to zero them before taking measurements. See, the chassis is reversed on the bench, and I happen to have a mirror on the back wall, so I look at the knobs in reverse... anyway, no, there's no verb drowning out the trem by ear.
     

  16. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    Currently, with all pots zeroed, and tremolo off at the footswitch, using a 10x probe:

    Point C supply: 332-340V (it's jumping back and forth, not sure why)

    V4 plates: same jumping, 300-308 on triode A, and 292-300 on B.


    I will attempt to record a few samples. The only way I know to do it is using my Zoom H2N, pointing it at the amp. Not sure if that gives you enough clear data for analysis. I'll turn off everything else in the room. Give me a few minutes.
     

  17. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    It's you that's the saint, sir. I'm willing to keep going pretty much forever on this. I've always wanted a Harmonic trem, and dang it, I'm going to get it, one way or another. If I can't ever get this working for some reason, then I'll go the 5-triode route, which seems more straightforward.

    But. Any time you want to hang it up, just say so. I totally get it. Can't imagine anyone thinking this is fun (except maybe King Fan and Keith).
     
    Mr Ridesglide and brogh like this.

  18. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    This is going to take a couple hours, perhaps. Need to drag garbage to the street before it gets too dark. Also, I *might* have damaged cap C26 yesterday. I wasn't getting a good reading on C27 when you asked me to confirm it's value, so I desoldered one leg, and it's all very tight in there. Anyway, now I can't hear the LFO beat in V4 grids, with the signal tracer. Doesn't seem right, so I'm going to replace C26 with another on hand. I'll confirm it's value before soldering in.

    I was really careful yesterday with other aspects of the soldering. I had to desolder the pots to confirm values and orientations, and of course the V4 plate loads. As a former software developer, I'm all too aware that testbeds shouldn't be touched. I'm triple checking everything to be sure. I don't see anything else wrong, but I'm sure I could hear the LFO in V4 grids before. And yes, I had tremolo ON, and tried various speed and depth settings, though I should still hear it regardless. I do hear the beating right at the LFO in V5a.

    Be back ASAP.
     

  19. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    One more question, then I gotta go do some real work. C30. What is the value you have installed, and what did it test at? And does it have a solid connection to ground?

    That cap is critical for decoupling the two halves of V4 from each other.
     

  20. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut
    It's a 2.2 uf, and ground connection is good. I'm not sure how to test the value. Do I need an ESR meter? My Klein MM cap setting just sits there, on 0.000. I've never been able to measure an e-cap with that.

    It's an easy part of the circuit, far as access goes. I'm confident all connections are good and clean.



    My LFO issue was me being dumb. Going cross-eyed here. Side effect is we have a nice new cap where we didn't need one, at C26. I wasn't hearing the LFO on the grids through my signal trace, just not enough juice. But it's quite audible on the plates.

    I don't want to raise another possibly unrelated issue, but in case it's somehow related, to my ears, when intensity is at some fixed position, and I spin the speed pot, the thump goes faster and slower through the tracer, as you'd expect. But in the last 10% of rotation, just before the slowest speed, it sounds significantly quieter, too. At first I thought it was just because it's so slow, there's not as much thumping per second. But I'm almost certain. It really does get quiet. I figure I'll scope this, but for now, I'm working on getting you some audio, and one other question you asked about trem on the output jack.
     

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.