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Valve (tube) amplifier schematic help

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

  1. Solidus

    Solidus

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    Jun 19, 2011
    I'm in the process of trying to build this, and I'm about to start obtaining components, but had a few questions regarding some of the specifics of the schematic.

    Does the polarized capacitor marked C3 @ 1uF have to be polarized, or could it be replaced with a non-polarized one of the same capacitance?

    If it has to be and I can't find a polarized cap, would a non-polarized cap and a inline diode function equivalently?

    Also, in regards to the tube (I will be running a 12AT7 tube in the device) is 450mA too much current? If so, how would I go about reducing the current? Since I will be running the tube heater filament directly off a transformer designed to output at 12V / 450mA, does the output need to be rectified?

    More details and information about the application as it becomes necessary.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Sure, a non-polarised cap will work in that position.

    Don't put a diode in series, that would be Baaaad, Mkay.

    It looks to me that since the heater is 12.6V centre tapped that you can use a 12 volt supply for the heater. It will draw whatever it draws (that's the 450mA, right?). Check the datasheet.



    It looks to me like 12VAC would be fine.
     
  3. Solidus

    Solidus

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    Jun 19, 2011
    How would it be bad? (I'm just wondering, I don't have much experience with this stuff)

    Also, the datasheet says it runs nominally at 12.6V /150mA or 6.3V / 300 mA centre tap.

    Are these things fairly "hardy" - that is, can they accept over-specifications? Obviously, it most likely leads to reduced lifespan, but as it's not something that would be continuously on, as long as it doesn't blow like a light bulb it should be fine.

    If 450mA is too much over, how would I go about reducing that current level?
     
  4. Solidus

    Solidus

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    Jun 19, 2011
    Also, I know this may be one of the most electronics-newbie-ish things you've ever heard, but can you help me with the issue of grounding?

    I'm used to the rudimentary circuit notion that I picked up in elementary physics that one wire goes in, one comes out to complete the loop, so this notion of one wire leading into a circuit and other wires mysteriously going to "ground" is confusing the hell out of me to be honest haha.

    I have some idea, that for most things it functions as the negative wire just like on a battery or what not, but when I get into circuits like this it really messes with me.

    So is the correct interpretation that all of the electrons have a common return path and are in the same "pool" of circuit, if you will? That is, I take every wire that is marked ground and twist them all together to complete circuit?
     
  5. duke37

    duke37

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    Jan 9, 2011
    You can provide 12.6V 0.15A or 6.3V 0.3A (AC or DC) as you say. The recommended tolerance if I remember correctly is +/- 7%. If you set the voltage correctly, the current will be right.

    The anode current will be very low in your circuit and so a low heater voltage may give enough emission without damage.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Your preamp would sound so bad that you would come hunting me with a pitchfork.

    Believe me. You can just place a non-polarised capacitor there.

    Polarised capacitors are only "special" in the way that someone with a disability is "special". i.e. if you don't have that kind of "special" it's generally considered a good thing.

    The fact that you *can* use a polarised capacitor there doesn't mean you must.

    Voltage is what you can have too much of. The current is determined by the load. 450 mA is what it can supply, not what it must.

    In fact, having a supply rated for a higher current than you need is a good thing because it won't be running flat out.

    It's like a car that can do 200km/h, that doesn't mean it will do that all the time, just that it can.
     
  7. BobK

    BobK

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    Or that the car will do 200 km/h uphill against the wind!

    Bob
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    This is possibly the most politically incorrect statement ever put to print. It's also one of the funniest and most honest things ever put to print! Hell, no one can deny it drives the point home! ;) Steve, if this question is ever asked again don't be surprised if you find your writings being plagiarized. :D

    Now, on to Solidus's posts. After reading them, and considering that you will be playing with plate voltages. ... May I suggest that you wear rubber soled boots and keep one hand in your pocket?

    Chris

    EDIT: Disregard the last statements. I don't see a lot a gain with 9V but with 9V plate supply I think you'll be safe.
     
    Last edited: Aug 22, 2012
  9. Solidus

    Solidus

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    Jun 19, 2011
    Oh okay. So it's like a water hose - voltage is pressure, amperage is flow - having a lot of water moving doesn't hurt, only if it's at a high pressure. Do I have the right idea?

    And my project is to integrate the tube distortion module (this) in front of an amplifier with some speakers along with a bypass to make a guitar amplifier. Hope that provides some context.
     
  10. Solidus

    Solidus

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    Jun 19, 2011
    I intend to power this thing on the first time with a 10-foot stick if that's what you intend :p
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Not exactly. A 12V car battery can deliver enough current to turn a 1/2 inch drive socket wrench white hot and weld it to the chassis.
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I couldn't resist spicing your circuit. Probably the deep affection and memories that tubes bring to mind. My spice model didn't allow for a 9V filament, so the results are with a 12V filament voltage.

    Anyway with a 2KHz 100mV signal input it output 6VPP. This measurement was made directly to the plate of V1B with the output network omitted.
    I imagine that the output will be less than half that with the output network (VR2, VR3,C3,C4) connected.
    Keep in mind that it's a HiZ output, so your load must be too.

    Chris
     

    Attached Files:

    Last edited: Aug 22, 2012
  13. Solidus

    Solidus

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    Jun 19, 2011
    You 100% lost me with that except for the graph showing the correlation...explain for the noob? :)
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    High-Z output means that you can't use it to drive a speaker. :) (actually it means it can't supply very much current, and therefore power -- perhaps only a couple of milliwatts)

    The output of this needs to go into another amplifier. And that amplifier needs to have a high input impedance (so it doesn't try to draw current from the output of your preamp) and also be able to handle the large signal from this.
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Spice is a simulation software. It enables the user to animate or simulate a schematic as though it was a physical circuit.
    The simulation results were impressive with an unloaded voltage gain of 60. The 12AU7 still rocks! The design will do what you want it to do. ;)

    What device will this will drive?
     
    Last edited: Aug 23, 2012
  16. Solidus

    Solidus

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    Jun 19, 2011
    Here is my plan. I was intending to make a DIY guitar amplifier with a tube distortion effect / overdrive module. From what I've read on the subject, while it's primarily used as an effect pedal in that sense, the 12AU7 can be swapped with the 12AT7 for a more subtle effect, and conversely vice-versa.

    Seeing as I had a few tubes and stuff of that nature laying around and my roommate in a month's time will be teaching me to play guitar, I figured why not try my hand at this.

    Chain this tube pre-amp in-line in front of a solid-state 50W speaker amp kit that I'm going to buy and assemble. That will directly drive a coupled pair of Bose 2.75" drivers that I robbed (not in the literal sense) from a mini-iPod stereo. That way, it gets around the impedance issue. A bypass (SPDT toggle) will allow the tube overdrive module to be bypassed in favor of the dry, non-effected signal, and another bypass will allow the effected signal to be out-routed to a more powerful amplifier or combo if that is the wish. Ins and outs are all 1/4" specification TRS jacks.
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    This amp will provide the biggest bang when the input Z to the power amp is >=100K. That doesn't mean that it won't drive a 10K input though. It will but not without distortion. A low input Z like 600 Ohms would be totally out of the question though.
     
    Last edited: Aug 23, 2012
  18. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I think (not certain) that the "valve distortion" ("tube distortion" for Americans) sound is only really achieved when you drive a valve output stage into clipping. Normally these output stages have one or more pairs of power pentodes (or beam tetrodes) driving both sides of a centre-tapped output transformer, aka push-pull. The distinctive sound is due to a combination of the characteristics of the output valves when overdriven, and the output transformer. I suspect you won't get any of that "special" sound by overdriving a small-signal triode running at a few dozen microamps with an anode voltage of 9V! Anyone else have an opinion on this?
     
  19. Solidus

    Solidus

    349
    4
    Jun 19, 2011
    Well, I wasn't necessarily planning on trying to overdrive the circuit as much as warm it to modify the sound.

    Kris, you are correct - the valve distortion factor is mainly because the original idea was you were pushing the amplifier past its limits - and there are things at work with valves that don't quite transfer over to the solid-state realm such that the clipping, overdriven sound is not harsh and biting.

    Take it for what little knowledge I have applicable, I'm an amateur electronic music producer and hard clipping on a mixer comes across very harsh to the point that it's noticeable. That's the reason the top of the VU meters is always red.

    There's something about valves that softens the hard-edge clips of the sound when the input signal is too gainy. Mainly, this circuit is designed to distort the sound though and give it a darker feel. Different valves in the 12A-7 series do this to different extents, and 12AT7 is more subtle than 12AU7. From what I've read, running 12AX7 valves in this circuit isn't a good idea for whatever reason though.
     
  20. Solidus

    Solidus

    349
    4
    Jun 19, 2011
    Chris,

    Is that a factor which can be controlled using the Gain pot on the circuit? If not, how would I limit it to optimize the amplifier?

    Also, out of curiosity, what part of South Florida are you from, if you don't mind? I used to live down there in my childhood.
     
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