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Valve (tube) amplifier schematic help

Discussion in 'General Electronics Discussion' started by Solidus, Aug 22, 2012.

  1. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Solidus, interesting comments.

    Yes, hard clipping sounds harsh because of all the harmonics generated by the sharp edges on the waveform. That's why some fuzz guitar sounds can be mixed quite low and still cut through.

    I'm interested in your comments about differences in 12A*7 valves. Have you ever done any double-blind tests to determine whether you can truly hear a difference between them? Apart from genuine flaws that would be obvious to anyone? Not necessarily a challenge; I'm interested.
     
  2. Solidus

    Solidus

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    Jun 19, 2011
    I haven't heard anything other than anecdotal references to such, but something tells me there should be a difference. Whether or not it's pronounced is a different story.

    As I start at the university September 22, I'm looking to have the entire project done by then, rig built, enclosure, everything. I'm not opposed to ordering a bunch of varying 12A-7 valves (12AU7, 12AT7, 12AV7, 12AX7, etc.) and changing them out on the amp to give it an audible go, and we can continue the discussion through to comment, discuss, etc.

    But yes, I would be happy to experiment and play that out once it is built. It would be an interesting project.

    I wonder if the soft-clipping and overdrive nature of valves has to do with the means of action. In solid-state devices, obviously due to the P-N-P and such junctions, most "transactions" of sorts are instantaneous. With valves being thermionic emission and temperature / plate dependent, I wonder if that helps to smooth the spikes of the harmonics. I have no idea for sure, just thinking out loud.
     
    Last edited: Aug 23, 2012
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    What I'm interested in is whether anyone can reliably tell the difference just by listening. I often hear so-called experts talking about these subtle differences, using language that is vague nearly to the point of utter meaninglessness.

    No one that makes these assertions ever seems to back it up with any evidence that they can REALLY hear ANY differences; their perception of differences could be entirely due to confirmation bias. The only way to know is a double-blind test, with someone who claims to be able to hear the difference.

    I'm not saying I think these people are lying. They may well be being totally honest. But that just means they believe what they say; it doesn't prove that it's actually true :)
    Unless you yourself believe you can hear differences, you have nothing to prove with a double-blind test. It's only the people who make that claim who should be willing to back it up, IMO.
    Yes it would, if it was conducted with a panel of these self-proclaimed expert audiophiles.
    As I understand it, the clipping is soft because the knee between the linear part of the response and the hard clipping part is rounder than with transistors. I think this is partly due to the difference between valves and transistors, but probably also due to the nature of how the valves are (over)driven and the design of the push-pull output stage running into a transformer.

    When a musician or audiophile says he prefers valve clipping over solid state clipping, I suspect there are many other factors at play than just valves vs. solid state.
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, to some extent. When I added the output network to my spice run the output started clipping on the positive excursions. I'm guessing that the input Z to the power amp is going to be about 10KΩ but I'm not an expert on guitar amps, so it's a guess. My point being that VR3 would have to be set near minimum to produce the least amount of clipping. This is a balancing act though. I used an input level of 100mVPP because I figured a guitar mic will output at least this level. Reducing the input level below this improved output linearity.

    I live in Delray Beach which is located in South Palm Beach County on the east coast of Florida. It used to be a sleepy little town that rolled up the sidewalks during the summer months. It's now an artsee fartsy yuppie haven that draws tourists 365 days a year. You can barely walk on the sidewalks because the tourists think it's fashionable to dine on them. Fortunately I don't live within the town limits.

    Regarding your conversation with Kris: I too would like to see these claims quantified. I'm not a musician or audiophile but have heard these claims for decades. On a side note, I own a Hallicrafter SX42 that my Dad bought in the mid 1940's. Yes, I could swear that no transistor receiver will sound like this but I can't quantify that statement with anything more than saying it produces a warm sound. It could be just the heat wafting off of 15 tubes. :D

    http://antiqueradio.org/art/hallisx42.jpg
    http://www.ohio.edu/people/postr/bapix/SX42.htm
    Chris
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I think the impedance used with electric guitars and guitar amps is typically around 100k, and signal levels are in the 10~100 mV range. A semi-acoustic with a built-in microphone will be in the 10 mV range, I think.
    LOL Chris :)

    Seriously, I think a piece of art/design like that would have a psychological effect on the listener, and that could blur the line between having a "warm sound" and just giving you a "warm feeling" in general when you're using it. I'm willing to be convinced either way, but only by consistent results from a double-blind listening test.

    Thanks for the links. The ohio.edu page is nicely done. Isn't it amazing to think that these days, almost everything in that big box can be implemented on a single chip that's about 2 mm square!
     
  6. Solidus

    Solidus

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    Well, what I was getting at was while you're stumbling onto one of the largest, most unresolved arguments in guitar-based music, I was more meaning there is probably a difference between valves of one series mainly due to their different characteristics. When a 1 one-millionth of a farad capacitor serves to modify and attenuate the tone, you can see how the slight difference in characteristics of valves arises.

    You and I both, however, seem to agree that whether or not that difference is appreciable is a whole argument in and of itself.

    As you said, the push-pull power stages of the valve amps probably contributes necessary factors. The two modes of guitar amplification (valve versus solid-state) cannot be directly compared because the entire circuitry was changed. It wasn't just pulling a dual-triode valve, putting a transistor in its place and putting a couple resistors in place to drop the power. You then had a shift from the power supplies using power regulator valves to using linear regulators, you had rectifiers in place that previously didn't exist, the whole game changed.

    Where in and of that the signal modification comes, I agree with you, is due most likely to multiple different factors that each, by itself, would not have made much of a difference. One-tenth of a degree change in temperature doesn't matter. One-tenth of a degree change over the course of an hour may make you sweat buckets.

    Ah, Delray Beach, yes. I used to live in a small town at the very southernmost point of Lake Okeechobee. I was always making trips to West Palm Beach and such for various regards, so your stomping ground isn't too far away.

    I could very easily make recordings once the unit is built of each valve's distortion on a common note played on guitar (say, low E for example), but to quantify what causes that is a whole 'nother ball game, and will require a lot more bouncing around of thoughts :rolleyes:

    Eli
     
  7. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Actually 1 one millionth of a Farad = 1uF; not small. Since a Farad is huge we generally work in uF, nF and pF. Tube inter-electrode capacitance is in the very low pF range. Same applies to BJT and FET junctions. However, since Vacuum Tubes and FETs are high impedance devices a very small capacitance will have large effect on high frequencies.

    Chris

    Oh, almost forgot.. You must have lived in Belle Glade or maybe Pahokee.
     
    Last edited: Aug 24, 2012
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Then his circuit should drive it OK if the input level is kept below 80mV

    My Dad's old SX42 has a humorous history. Remind me to recount it in the members lounge. It's a hoot! ;)

    Believe it or not there's a segment of our society that fervently believe that our micro technology is not a product of mankind. Yup, they tell me that our technology is a product of area 51. These people all have one thing in common. Their eduction was devoid of the sciences, :D

    Chris
     
  9. Solidus

    Solidus

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    Jun 19, 2011

    Oh, okay.

    And you're very close - Clewiston. Only a tad larger population-wise :p

    I'm thinking of adding a transformer to put a bunch of LEDs in this thing - I plan on fabricating the enclosure out of acrylic, and the cool factor would be a lot higher if it glowed.

    How would I rig a 8 or 9 LED assembly off a 12V/450mA transformer (it would be a different one than the one for the tube circuit)?
     
  10. Solidus

    Solidus

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    Here is the circuit that I modified to go into the amp. If one of you could look it over to make sure everything checks out, it would be much appreciated.

    The schematic was hand-drawn in Photoshop as I don't have any schematics-drawing programs :p

    [​IMG]
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I don't quite see it that way. A farad is an awful lot of capacitance, and a millionth of a farad is a microfarad, which is a significant amount of capacitance in many contexts in pratical electronics.

    I'm prepared to be proved wrong, but I don't think that differences in characteristics of valves/toobs, especially parts within the same family, are on the same scale of significance as, for example, the difference between a 1 nF coupling capacitor and a 1 uF one in an audio application.
    Exactly. Even though FETs are a lot more similar to valves/toobs than transistors are, there are predictable differences in how they will clip, so I'm not surprised if some people say they can audibly tell the difference between all three. But there are some claims that I would expect evidence for, such as speaker wires that have an "amplifier" end and a "speaker" end, and you're not supposed to connect them in the wrong direction!
    And now the new boy on the block is Class D. (Actually not that new, but it's much more practical now, and has so many advantages.)

    Does anyone have experience with Class D amplifiers? What do you think of them?
    Personally I think my smartphone sounds great (through decent headphones, of course). As long as it doesn't compromise performance in terms of sounding good to me, convenience is my main factor.
    Perhaps it bodes well for the future of our species that it's becoming increasingly difficult to avoid getting a proper education - in developed countries at least :)
     
  12. Solidus

    Solidus

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    I wasn't trying to mean anything quantitatively. I was just pointing out in context to the conversation that a relatively minute modification of the circuit before amplification can make quite the audible difference. I wasn't meaning the literal value of a microfarad, I was meaning the conceptual value of the notion of one-millionth of something.

    That's generally what I feel. I'm all for this warm, reminiscent sound that people supposedly get out of valve-driven amplifiers, but I disagree with the whole audiophile notion. Sure, we all like good-quality sound, but IMO the idea of an audiophile to me brings to mind old, rich guitar enthusiasts that buy way-overpriced equipment and now have to rant about how good it is in an attempt to justify purchasing it.

    But for someone like me, who will be starting to learn guitar as a hobby and for 99% of every other person interested I doubt they would give a hoot about whether the amp is FET or valve driven, and any already-minute difference would be drowned out by effects pedals or hell, playing styles in the first place.

    I'm building the valve pre-amp stage to introduce a bit of a cool factor and a little bit of distortion into my amplifier, so I don't just have bare circuit and two speakers in a box.

    If people don't realize that both ends of a copper speaker wire are the same...I've lost faith in society.

    Fair enough. Most audio that isn't run through a bitcrusher plugin sounds fine to me, as long as it sounds good on my iPod it's good enough for me for routine day-to-day use.

    Let's hope a cure for stupid is found real soon.

    Have you given the circuit I posted last page a look at all?
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Your Bridge doesn't have a filter cap. You're feeding the amp 120Hz pulsating DC.
    BTW, glad to see you're not trying to power it from a 9V battery. The filaments would suck it dry in short order.

    Chris
     
  14. Solidus

    Solidus

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    Jun 19, 2011
    Now could I just cut the rectifier out and run pins 4 and 5 on 12VAC? I was unsure because the original schematic indicated DC.

    But is everything else an appropriate translation of the original? No errors or left out parts? Just making sure, I'm about to order the components.
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I haven't looked it over thoroughly enough to answer your second question. RE, your first question: You can power the filament from AC, DC and pulsating DC but only filtered DC is acceptable for the plate supply. Unless you like the sound of Ummmmmmmmmm!

    Chris
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I looked it over and all seems correct except the missing filter cap.

    Chris
     
  17. Solidus

    Solidus

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    Jun 19, 2011
    ...and I'm assuming because other pins are also coupled to the power filament line, only filtered DC can be used.

    What would I use for the filter cap?
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Yes, that's why. If you want to keep the filter cap small you can float the filament above ground by tying it across the transformer secondary. This will reduce the draw on the filter cap because it only needs to filter the plate supply.

    Chris
     
  19. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Why not power it from a 9VDC wallwart rated >=1A?
     
  20. Solidus

    Solidus

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    Jun 19, 2011
    Because I wanted to have a fully-enclosed power supply. I am going to wire a PC power-supply-style jack to the back so the cable is detachable.

    Unless that is, I could disassemble it and wire it to the back of the jack.

    Would 12VDC >1A work too? I probably have dozens laying around if so.

    I'm about to start cutting and machining the panels. What I don't have in electronics newbie hell I make up for in mechanical aptitude :)
     
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