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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    At 11:27 pm on Christmas Eve, I'm just settling in with a cup of cocoa.... to read a bunch of patent documents and schematics from the mid-20th century. :eek:o_O:D

    We had half the family over yesterday (four grandkids), and the other half (two grandkids) tomorrow. Today was quiet. I had a nice quiet dinner at home with my wife, and we watched Ice Age, and then I mostly slept through some ancient Christmas movie she likes. She toddled off to bed, and now it's time to get crackin' with this wonderful warbly circuit :)


    Have a great holiday, you all!
     
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  2. clintj

    clintj Friend of Leo's

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    I would totally get that Siglent. That's roughly on par with mine, at significantly less cost. 50MHz is well above what you need for audio work, but it does get you better resolution while doing it. And, the auto trigger function most digital scopes have is amazing. Connect a lead, hit a button, and it auto sets the ranges to get a nice view with minimum tweaking.

    Basically what you need at a minimum is the ability to adjust time, voltage, and trigger, and a couple MHz of resolution. Those are the most basic controls any scope should have, and all you need to capture a waveform.

    I seldom wish I had a 4 channel. It can save time when hunting for an intermittent breakdown in a complex amp, but aside from that you can check most problems and take needed measurements with a 2 channel. I'm content to backtrack with a 2 channel, and I'm seldom really pressed for time on repairs.
     
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  3. clintj

    clintj Friend of Leo's

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    As far as probes, if you have the ability to tell which side of a capacitor you're on, a pair of standard probes is fine. The cap will block the high voltage DC for you. A x10 probe lets you easily snoop the PI outputs, for example.

    I do have a x100, 1000V probe for snooping around power tubes but it doesn't get used as often as I thought it would. Usually looking at the OT secondary tells you a fair bit about power tube balance and operation, and rules out problems like crossover distortion.

    Since shipping will take a couple of days, there's some great YouTube channels on basic to advanced scope use. You want to watch things like how to adjust the scope to view a waveform and adjust view, set trigger, etc. Also look up one on identifying mains grounded vs isolated ground equipment. If you clip the probe ground lead to an isolated ground chassis, expect pyrotechnics when the ground lead evaporates. (Your Revibe is a mains grounded chassis)
     
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  4. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Siglent is on the way. Friggin' Amazon, so easy, so dangerous... :)

    Thanks for the advice, and the tips. Heading off to look for some how-tos...

    Unit arrives Thursday, two days after I build my signal tracer LOL.
     
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  5. keithb7

    keithb7 Friend of Leo's

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    Moosie, is a Genius. It's 10 PM here on the left coast. The Mothers-In-Laws are in bed. Our older boys left for the night. Wifey is digging out some stocking stuffers, unwinding. "I just gotta run down to the basement for a minute. I may have left my soldering iron plugged in. I better go check." :rolleyes:

    [​IMG]
     
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  6. Thin69

    Thin69 Friend of Leo's

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    I have an old Tek that works great and the CRT resolution is very good. The only thing I don't like about it is it is as big and heavy as a house. The digitals are nice and light!
     
    Last edited: Dec 25, 2017
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  7. moosie

    moosie Doctor of Teleocity Silver Supporter

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    My most recent memory of a scope is a Heathkit, or similar. When I was twelve, my best friend was a bit of an electronics whiz. Some friend of his dad's had one of these that didn't work. Said if Russ could fix it, it was his. He fixed it in 20 minutes. Between that and the ham gear, he had a pretty credible bench for a pimply-faced 6th grader.


    db_Heathkit2.gif
     
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  8. keithb7

    keithb7 Friend of Leo's

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    Moosie, not wanting to hy-jack your thread. Not sure if I should post my video here or on your other thread. Hopefully this is ok.
    Snuck away for an hour. Here is what I was able to accomplish today. My theory is weak on the subject. - Keith

     
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  9. clintj

    clintj Friend of Leo's

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    That's pretty neat to watch. As far as the drop in amplitude, that's quite intriguing. I'd backtrack to the actual oscillator and see if it happens there or gets attenuated somewhere later. If it's the actual LFO triode, it might be a quirk of the RC network used to create the signal; my knowledge is a little thin there too. Might be a wibbly wobbly timey wimey thing. :)

    One article I found on this said the crossover frequency is around 600Hz. If I had that amp on the bench for this, I would inject a signal and scope the overall output. Start at 100Hz, then 500Hz, then 3000Hz to see what the tremolo effect looks like at each point. The scope would be on a fairly wide time base like it is on your video, so you can see the pulsation of the signal strength.

    This would be a good video to also post to the other Harmonic Tremolo thread. Thanks for taking your time to do that on a holiday like this.
     
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  10. moosie

    moosie Doctor of Teleocity Silver Supporter

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    No worries, Keith.

    I like it here, but I'm glad you posted it in the other thread, too. That thread has a better chance of being a good overall resource on the topic.

    I was thinking along similar lines as Clint. I'd like to see some simple signal. Those grids you scoped... just before that is the frequency split, high and low, and that's the first time the signal and LFO are joined together. With no signal, we're just seeing the LFO on each triode, out of phase thanks to the cathodyne PI (triode #3 in this mess).
     
  11. milocj

    milocj Friend of Leo's

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    Hey Keith,
    Since I was also posting in this thread earlier I'll piggyback on your hijacking posts :lol:

    What kind of probes are you using with your scope? I've been trying to weed through the various scopes and actually scored the same Tektronix scope that you have for $105 shipped...just doesn't have probes. I have a '62 Super with the 5 triode harmonic tremolo and the Revibe that I built years ago. If I can find an easy way to make a video I could do a compassion (I'm just not real video/audio literate when it comes to making that kind of stuff).
     
  12. keithb7

    keithb7 Friend of Leo's

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    @milocj I got my passive probes on E-Bay. They are TP6060 model. I see that on the instructions that came with them. They are adjustable to 1x or 10X attenuation. At 10X the maximum input voltage is 600V. at 1x you are to stay under 300V. Mine came as a pair. 2 is nice for examples as shown in my video above. The connector to the scope is "BNC" type.

    This will probably suite you just fine. http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=132281589986

    I have the same spring loaded hook-type end. They stay firmly attached and are shielded to prevent touching surrounding components.

    I also bought a cheap signal generator like show below. All different wave types and frequencies are available. You can insert the Hz signal into your amp's guitar in jack on the front, then trace the Hz throughout the circuit with your scope!

    http://rover.ebay.com/rover/1/711-5...0001&campid=5338148343&icep_item=252822328392

    1,000 Hz gets annoying through the speaker(s) in short time. I don't have one yet, but I'd like ot score a 100 watt 4 ohm resistor. So I can use it as a dummy load on the OT instead of the actual speakers. That would be nice. You'll need a BNC to 1/4" adaptor to plug the signal generator into the amp. Easily done.
     
    Last edited: Dec 26, 2017
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  13. clintj

    clintj Friend of Leo's

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    I built a 2/4/8r dummy load using a Hammond project chassis (black anodized aluminum, finned) and parts from Mouser, and the schematics provided on Aiken Amps. Cost maybe $50 US, works great, and even after I downrated the power for the smaller heat sink size it'll still handle 50 or 100W depending on which resistance setting it's on.
     
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  14. moosie

    moosie Doctor of Teleocity Silver Supporter

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    OK, I'm back, with a scope.

    V5b grid and plate are properly out of phase. Grid is attenuated as it should be. Plate is strong, and it does have a very small voltage swing of about 2V with intensity maxed. That swing should be between 30-50V.

    What next?
     
  15. clintj

    clintj Friend of Leo's

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    Check both grids on V4. They should be equal and opposite like in the video keithb7 posted. What V5b should be doing is creating an inverted duplicate of the signal after the intensity pot, so if the V4 grids are correctly oscillating like the video everything in the LFO circuit is playing nice.

    If that passes, we start troubleshooting the guitar signal path, which should be much easier now.

    Do you have a way to add an audio signal? Signal generator or such, or even just a way to plug a phone in to the input jack? YouTube has a nice selection of test tone videos to pick from, and I'll guide you to which frequencies to choose from.
     
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  16. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I did so much stuff yesterday, I can't remember for sure, so I'll have to repeat it. But, I'm pretty certain both V4 grids looked right. Fairly equal, and definitely opposite.

    I tried adding a signal with my looper pedal, playing a single A open string, repeatedly. I see it on the scope, just not sure what to do with it. Also, I've downloaded a nice little sig-gen phone app, and should receive a 1/8" to 1/4" male to male jack tomorrow, so I can inject it into the ReVibe input.

    I'm going to eat some lunch, and then repeat that V4 test, and I'll get you a screen shot, hopefully.
     
  17. clintj

    clintj Friend of Leo's

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    Sounds great.

    Once you get that app up and running and verify the grids' cycling, here's the next steps:

    Leave the tremolo off for now.

    Feed the amp a 100Hz or lower frequency, and check both halves of V4 (grids and plates). One should be much stronger than the other, note which one.

    Feed a 5,000Hz signal and repeat. The other half of V4 should be much stronger this time. This will verify the proper operation of V4 and the hi/lo pass network.

    The results of that will determine where we go next.
     
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  18. moosie

    moosie Doctor of Teleocity Silver Supporter

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    Here are the V4 grids, with no signal input. Speed and Intensity around middle of range. Yellow is pin 2, Purple is pin 7. Both probes are on 1x, and I just hit Auto, so I assume the scales are the same for each. V4b (purple) gets the direct LFO from V5a. Note that the reverse LFO from V5b to V4a seems a bit weaker. Not sure if this is a problem, or just imperfect voltage divider attenuation, dropping the level too far, before the V5b gain stage.


    SDS00001.jpg


    Also, I forgot to mention, I built the signal tracer. I plugged the tracer into the input of my Super Reverb, for a handy speaker. With my Boss RC-3 looper pedal feeding a 440 hz guitar tone (and overtones) to the Revibe's input, I was able to clearly hear the difference between hi and lo pass filters. Not sure the levels are right, but they sound OK to the ear. V4a is low pass, and V4b is high pass. I could hear this quietly on the V4 grids, and loudly on the plates. And as the scope now testifies, I could also hear the LFO on each side - I just couldn't confirm the phase without the scope.

    I'll definitely follow up with the more accurate signal tracing in a day or two, once I have the ability to inject specific frequencies into the Revibe. But for now, it seems like this is everything you wanted so far. And everything seems OK, yes?
     
    Last edited: Dec 29, 2017
  19. clintj

    clintj Friend of Leo's

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    That does look good. Those are same scale, and the imbalance could either be what you're thinking as far as the divider network, or just an imbalanced tube.

    When you get the signal input rig set up, reconfirm what you're hearing by scope as far as hi/lo. At 440Hz, you can get an ambiguous result as far as whether the two filters are really working - you might be getting one filtered signal and one full range, for example.

    Also, when you do those hi/lo tests with the tremolo off, also check at the output jack of the Revibe unit. Since you're getting some evidence that the tremolo circuit is at least somewhat working, see if that signal is showing up at the output jack.
     
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  20. moosie

    moosie Doctor of Teleocity Silver Supporter

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    I'm puzzled why we aren't seeing oscillation on the V5b plate. Reduced level comes into the grid, from the LFO generator (V5a). It flips the phase, and amplifies. In theory, it's then going out through the plate, to V4a.

    But there's no appreciable oscillation on the V5b plate. Just a couple volts, with intensity on full, vs the 30-50v swing seen on the V5A plate.

    Since the oscillation at V4a grid should be coming from V5b, I am confused as to why we see it there.
     
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