Filament buss question

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

  1. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Well, it works!

    After doing all my tests, finding nothing wrong, I went through some power up stages. Found a broken pilot light housing (that's the second cracked component from Weber in this kit). But the heaters were getting juice, so I just left it and moved on. I'll order a new one soon.

    I also plugged the tank in backwards, which I seem to do about half the time.

    Once I got all that right, I turned the reverb and vibrato on my 71 Super down to zero. Plugged in the AV Jazzmaster.

    I dimed all the knobs, expecting noise. Nope. Even with all pots on 10, it's so silent I began to hear things as I tried so hard to listen. It's just not making any noise at all.

    I feared it wasn't working, but then I hit the strings and got smacked in the face with the wettest spring drip I've ever heard in my music room.

    I may have something loose at the ouput jack - I thought I heard a crackle there when I moved it. Still need to do that kind of chopsticking check.

    My only concern is that the vibrato depth doesn't start sounding like anything until around 6. It sounds great on 10, but I expected 10 would be over the top. It's not.

    Speed range is great.

    I need to play with the reverb pots more, but it sounds killer.

    I haven't checked the bias on the 6K6 yet (nothing redplating, of course, or I'd have mentioned it).


    So, still some cleanup things to take care of, but basically, it worked first time. And it seems my focus on grounding has paid off. My god it's quiet!



    Thanks for all the help in this thread. Hopefully the next sucker, er builder won't need to be so hesitant. @King Fan? :twisted::D:)


    The other thing, and it may be tied to the limited depth, but the tremolo sounds like great tube bias trem, NOT like the crazy harmonic vibrato that this circuit is.

    Any ideas on deepening it? Think 6K6 bias will affect that?


    20171218_212118.jpg


    I usually use one butt splice, to connect the PT primary return, to the cord neutral. This time I was able to see it coming, thank goodness, and realized there was no way I'd be able to connect the PT secondaries with the two center legs of the bridge recto package. That's why you see two more splices, sitting right atop the filter caps.

    You can also see how I had to install the switch upside down. I couldn't get the board in with the faceplate components installed. Once I got the board crammed in there, there was no way I would be able to solder the switch underneath. I thought about soldering it with some slack, and then spinning it, but I just decided to relabel the faceplate. If it bugs me horribly, I can address some day.

    20171218_223332.jpg




    Here's a close-up of the hum-loop-block network (except the .047uf cap, which is the dog turd running from input jack to the nearest pot shell). That cap, and this blue wire on the right, are the ONLY places the DC and heater returns come in contact with chassis ground.

    I thought I'd read that the resistor pictured needed to be a big 'un, to withstand a full power failure scenario. Then I couldn't find the reference, so I just got a 1W flameproof. Then I found the reference. Merlin's book. He recommends a 5W. It'll fit, but only because I can remove the whole mini-board to install it. It's pretty tight in there. So I have that to do as well...


    20171218_223356.jpg
     
    Last edited: Dec 19, 2017
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  2. King Fan

    King Fan Poster Extraordinaire

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    Moosie, great job. “You built a ReVibe.” Say that over and over!!!! Unbelievable.

    However, if you think I’m tempted by these pics of the entire Mouser catalog tucked neatly into a garter snake coffin, you don’t know my survival instinct. I’ll wait until you go into fullscale production and buy one from you. [emoji6]
     
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  3. D'tar

    D'tar Friend of Leo's

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    And for my next trick... I shall place it all in a 5F1 cabinet!!!! LOL:)


    And quiet too. Awesome job!!!
     
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  4. moosie

    moosie Doctor of Teleocity Silver Supporter

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    :lol::lol::lol::lol::lol::lol:

    Thanks, man.

    I'd love to see the guts of Mark Baier's (Victoria amps) Reverberato unit, which I think is the same thing. He charges $1800, and claims it's the most difficult build they do. I have a feeling it'd put mine to shame in the neatness category.


    If it makes you feel any better, the reverb stopped working as soon as I posted.

    I'll fix that, I'm sure - now that I know it worked once.

    The sweep on the depth pot seems like it should be an easy adjustment, if that's what it needs. I mean, all seems fine, just that the sweep is shifted, doing nothing from 0-6. It's a weird pot that I couldn't find ANYWHERE. 10Meg reverse audio. So, replacing the pot doesn't seem like an option, even if I wanted to.

    Is there a way I can change the sweep, adding more room above 10, by adding a resistor somewhere?
     
  5. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Hey, the tiny chassis wasn't my idea...

    Thanks!
     
  6. keithb7

    keithb7 Friend of Leo's

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    The quality of the “first build” projects around here is amazing lately. Great job Moosie.

    This thread has me thinking about an external effect someday. I have harmonic trem in my 6G8-A however I miss the big boinnggg from the 6G15.
     
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  7. clintj

    clintj Friend of Leo's

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    If you're not getting the harmonic part of the trem, there's likely something awry in the V4 or V5b vicinity, after looking at the schematic. (That's a crazy circuit to follow, BTW). That may also be why it doesn't show up until around 6 or so.

    How are you set for test gear? Do you have a scope or audio tracing probe?
     
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  8. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Nope, and nope.

    I have a nice multimeter :)

    OK, obviously I have some work to do. I need to get the reverb working again - worked fine before, dang it. Then I'll probably chopstick the whole thing, see if it uncovers anything quick n dirty.

    Then I'll be back with questions on this front. When you say 'probe', do you mean something like this?

     
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  9. clintj

    clintj Friend of Leo's

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    That'd be perfect. Probably better than a scope for this purpose, which is checking each stage of the harmonic tremolo circuit for operation. You need to be able to hear if each stage is receiving and amplifying its respective portion of the signal.
     
  10. moosie

    moosie Doctor of Teleocity Silver Supporter

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    OK, I need to watch that vid. Just because I posted a link doesn't mean I know what the heck it's about yet. Or how to use anything I might build.


    To be continued.

    Thanks for all your help, @clintj.
     
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  11. clintj

    clintj Friend of Leo's

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    No problem.

    The first check you can do with a regular old voltmeter. Conditions: no signal, tremolo active, depth and speed about half.

    Probe the plates of V5a and b, look for varying voltage. See if the variance amount and speed changes with the settings. Check the grids of V4a and b for the same thing happening.

    This checks both the regular and inverted LFO drivers for proper operation, and ensures the bias wiggle of V4 is happening.
     
  12. moosie

    moosie Doctor of Teleocity Silver Supporter

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    V4a: 263 - 270V, cycling
    V4b: 273 - 280V, cycling


    V5a: 116 - 182V, cycling
    V5b: 188.6, fixed
     
  13. clintj

    clintj Friend of Leo's

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    Ok then. Interesting result.

    V5, same test, look at pins 6, 7, and 8.

    If you don't see oscillation at pin 7, backtrack through R33, R40, R41, to the intensity(depth) pot. If you do see it there, we'll reevaluate from the other results and go from there.

    V4b might be seeing bleedthrough from V5a causing it to cycle, but doesnt seem to be reaching V5b.

    And one more quick check to satisfy my curiosity: it looks like you're getting about 7V signal at each half of V4, relative to ground. If you connect one probe to each at the same time, do you get close to zero or close to 14V total swing? This is a quick and dirty way to check phase without a scope.
     
  14. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Speed and Intensity set midway. No signal.

    V4 pins (VDC, to elevated ground, just before the hum-loop-block resistor):

    1: 258 to 265.5
    6: 267.5 to 275.5

    2: 1.5mv to 14mv
    7: -0.7mv to +16mv

    V4 plate-to-plate: 1.6 to 16V


    V5 pins:

    1: 116 to 182
    6: 187.7 FIXED

    2: -38mv to -1mv
    7: 4.5mv to 6.5mv (I'm not sure this one is cycling, or just meandering a bit)

    Moving out from the V5b plate, on the other side of C23, we get nice wobble: -6mv to +23mv

    Intensity pot wiper: -6 to +20mv
    Pot on minimum, wiper: 0V
    Pot on max: wiper: -63mv to +80mv


    Re backtracking, the other side of R33 is ground, and the other side of R40 and R41 are the points where I read the V4 grids, above. So they are oscillating.

    Is that what you wanted?



    Screen Shot 2017-12-21 at 10.55.27 PM.png Screen Shot 2017-12-21 at 10.23.31 PM.png
     
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  15. clintj

    clintj Friend of Leo's

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    I'll get back to that tomorrow after the kids leave for school. It sounds, at first blush, like V5a is driving both halves of V4 in phase. That's not right.

    I'll sit down with a hard copy of the schematic, annotate what you found, and come up with a next step to check. I think I see the issue, but need to figure out the cause.
     
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  16. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I have been working on a small combo amp with harmonic trem to replace my first attempt at it from last year. The first amp had a number of issues, but the trem at high settings had a very distinct chorus or univibe sound (I can't recall the exact type of modulation effect to best compare it). The new build is better in every respect, but the trem, while deep and musical, doesn't have nearly as much modulation to it. Not none; it is clearly different from the bias-vary I have built before. I don't miss the chorus sound much, but I wonder if it indicates a mistake in my circuit.

    I listened to the first two clips on youtube that came up for 'Fender harmonic tremolo' and the first was someone's old Pro 6G5a with a crazy distinct modulated sound, and the second was a a Twin 6G8 with deep trem only subtly modulated that sounds a lot like mine. Curious. Of course, there were two different versions (the early 4 triode and the later 5 triode), plus who knows what other differences in the demo amps. But I built ostensibly the same circuit both times (the later version)!

    I'm interested to hear what you learn in your troubleshooting. Thank you.
     
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  17. moosie

    moosie Doctor of Teleocity Silver Supporter

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    You mean this video?



    That's our very own @keithb7, of course. One more example of the best info is always on TDPRI. :D
     
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  18. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Thanks Clint, so much.

    I'll be out of touch during the day tomorrow (Friday), hooked up to a drip line once again. Should be able to get back to it in the evening, and I can try any ideas you might have, or get you more data.

    I'm going to build that signal tracer (actually a simpler BillM version, with just the cap). But I needed to order parts and heat shrink - my junk drawer is getting low. So, that won't be happening until late next week.

    I didn't expect anything to improve, but I did reflow a few solder joints, and I swapped out both tubes. Still the same.

    It makes sense that I'm missing the phasing - it sounds nice, but 'normal', not 'harmonic'.

    Might be nice to put that on a switch, once it's working...
     
  19. keithb7

    keithb7 Friend of Leo's

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    The original 6G8A circuit is shown. I would like to trace this circuit...When I have a few hours! :rolleyes:
    The Harmonic Vibrato section mainly. Maybe I will start a new thread and some folks can help me out. It is an interesting lesson, the all tube harmonic vibrato circuit.

    [​IMG]
     
  20. clintj

    clintj Friend of Leo's

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    That's actually easier to deal with than this one. The first triode, along with the 3 capacitors, creates the low frequency oscillation. That goes to the second triode, which amplifies that oscillation. We've got the basic PR bias vary tremolo at that point.

    What happens next is complex. The LFO signal goes to a cathodyne, which creates equal and opposite oscillations to the next two triodes. This causes them to experience bias vary tremolo, with opposite cutoff times.

    Now, the guitar signal enters the last two triodes and splits via a high pass/low pass network - high frequency content goes to one, low frequency to the other. So what do we get?

    Alternate pulses of high and low frequency sound as we play. The low portion gets cut off, then the high portion, back and forth.



    The 4 triode version is nuts. One tube creates/amplifies the LFO signal, then a portion of that should be sent to both the following triode and the opposite LFO driver grid - that produces an opposite LFO signal since regular gain stages invert the signal. After that the alternate cutoff cycles and hi/lo splitting network are pretty much the same.
     
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