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Trying to understand tube amp theory and schematics

Discussion in 'Shock Brother's DIY Amps' started by samato, Aug 16, 2010.

  1. samato

    samato Tele-Afflicted

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    Encouraged by and not wanting to interrupt the discussion rainedave and gaddis are having in this tread, I have some of my own questions so I thought I'd start a thread.

    In "The Guitar Amp Handbook", Dave Hunter says on page 19:

    "As with our preamp tube, the output tube's operating level (bias) is determined in this circuit by a resistor tied between the cathode (pin 8) and ground. This 470-ohm resistor decides how "hot" the tube will run in relation to the high-voltage DC supply fed to it. The bias resistor is run partially bypassed by a 25uF/25V electrolytic capacitor that further affects the frequency response of this tube, in this case giving it a bigger, deeper sound than it would have with the resistor alone. Note once again that no signal is passing through this capacitor, but that it's helping to voice the output stage by playing a role in determining how the tube operates."

    I understand when he says "signal" he is referring to the sound-carrying path and he calls the power-transferring path emanating from the power transformer and rectifier "voltage".

    The part in bold is what I don't understand. I thought the role of that 25uF/25V bypass cap was to allow "signal" to avoid the 470-ohm resistor while forcing "voltage" to go through the resistor. Can anyone help me figure out what I'm missing here?

    Oh, this is all referring to the Fender Princeton 5f2 circuit. Schematic and Layout attached below.
     

    Attached Files:

    Last edited: Aug 16, 2010
  2. Ronsonic

    Ronsonic Tele-Afflicted

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    You are righter than he is. Yes, there would be signal, in phase with the input at the cathode and it would affect the operation of the tube. Bypassing it to ground with the cap prevents that and you have more gain.
     
  3. samato

    samato Tele-Afflicted

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    I guess I shouldn't really be concerned with the signal coming from the cathode as what really matters is the signal coming from the plate (which goes to the speaker jack)?
     
  4. pontmercy

    pontmercy Tele-Holic

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    a guy around here sells some books/dvds on tube amps. I hear they are actually very good. recycledsound.net
     
  5. samato

    samato Tele-Afflicted

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    Thanks but I won't be buying any more books for a little while. It's not that I don't want them but the two I have are very good and there is an unlimited amount of information available online. I might check the local libraries to see if they have anything though. I need the money to start gathering parts.

    The books I have are "The Guitar Amp Handbook" by Dave Hunter and "Vacuum Tube Guitar and Bass Amplifier Theory" by Tino Zottola.
     
  6. mgwhit

    mgwhit Tele-Holic

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    I think Hunter means that your output signal isn't passing through the cathode bypass cap, like it would through, say, a coupling cap. But Ronsonic is totally correct that there is an in-phase duplicate of your input signal present at the cathode and its associated component path to ground.
     
  7. samato

    samato Tele-Afflicted

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    That makes sense. I'm just trying to wrap my head around what effect this has or how it determines the voicing of this stage but I have no problem with just accepting things without complete understanding for now if it helps me get a better idea of the big picture.
     
  8. limbe

    limbe Tele-Afflicted

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    The Guitar Amp Handbook and the Vacuum Tube Guitar and Bass Amplifier Theory books are among the best books on the subject I have seen.
     
  9. mgwhit

    mgwhit Tele-Holic

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    There are two principles at work here, which you probably already understand (or at least know) in isolation:

    1. Capacitors block DC, but can pass AC (dependent upon the frequency of the AC and the capacitance of the capacitor).

    2. The gain of a triode is inversely related to the amount of resistance on the cathode.

    So when you bypass a cathode resistor with a capacitor, you are creating two separate cathode resistance paths: one with very little resistance for AC signal above a certain frequency (i.e. through the bypass cap), and one with the full resistance of Rk for AC below the certain frequency. Since the frequency of the AC at the cathode is the same as the AC on the grid (input), this means that certain higher AC frequencies will be amplified more than lower frequencies.

    (Apologies to any EE's in the audience if that is gibberish, but it works for me.)

    Check out the section in Hunter where he walks you through the design of the 2 Stroke and discusses the boost switch.
     
  10. rainedave

    rainedave Tele-Holic

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    Here's how I understand it. If the voltage across the cathode resistor increases, it has the effect of making the grid-to-cathode voltage more negative. That will decrease (block) the number of electrons that flow from the cathode to plate. That equals a decrease in gain.

    The bypass cap prevents the input signal from increasing the voltage across the cathode resistor by shorting it to ground (bypassing it). Therefore, gain is not decreased.

    But, the value of the cap also determines which frequencies of the input signal are shorted to ground and which frequencies it blocks. The frequencies that do not pass through the bypass cap to ground create a negative feedback. This has the effect of attenuating (decreasing) the gain of those frequencies. Often times, from what I've read, the bypass cap is chosen with a value that attenuates low frequencies. Since the bypass cap attenuates low frequencies and doesn't effect high frequencies it is called a "Bright" cap.

    Read this page:

    http://www.pentodepress.com/home/amplifier-calculators/cathode-capacitor/

    That is an oversimplified explanation and if it's wrong, someone please correct me.
     
    Last edited: Aug 17, 2010
  11. samato

    samato Tele-Afflicted

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    I kind of get it but it's hurting my brain right now so I'm gonna move on and come back to it later!

    Thanks for all the input. More questions to come...
     
  12. Ronsonic

    Ronsonic Tele-Afflicted

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    It's sort of a hard one to explain simply. rainedave did a pretty good job.

    It is common to think of an unbypassed cathode resistor as providing negative feedback.

    Without the cap, as the signal goes positive on the grid, the cathode goes positive - when the signal goes negative, so does the cathode. Since the tube amplifies the difference between the grid and cathode this reduces the gain of the tube stage.
     
  13. koen

    koen Friend of Leo's

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    This for the power tube, right?

    A great book BTW, I browse through it occasionally, and learn something every time.
     
  14. markw51

    markw51 Tele-Afflicted

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    The cathode bypass cap simply bypasses audio frequencies around the cathode resistor. A tube will still work without a bypass cap but it won't have as much gain because then the audio has to pass thru a resistor. Audio frequencies see the bypass cap as a near zero resistance.

    A bright cap is put across 2 terminals of a volume control. A bypass cap is not a bright cap.
     
  15. rainedave

    rainedave Tele-Holic

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    Thanks for clearing that up. The beauty of forums is that misinformation (mine, in this case) can be corrected.:oops:

    But, from everything I've read, cathode bypass caps do effect the frequency range of the signal, and are often used to attenuate certain frequencies.
     
  16. markw51

    markw51 Tele-Afflicted

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    Yes that is true. Any capacitor in the signal path can be used to shape the sound. A smaller cap will attenuate lower frequencies.
     
  17. samato

    samato Tele-Afflicted

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    Thanks guys, this stuff is starting to make a lot more sense to me.

    --------------------------------------------------
    markw51 - I see you are in Rockville, MD. I'm originally from Silver Spring and have lived all over Montgomery County over the years. I'm actually going up there over Labor Day weekend for a friend's wedding. Do you shop for music stuff at Chuck Levin's? Is it still as good a store as it used to be?
     
  18. tubeswell

    tubeswell Friend of Leo's

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    In simple terms - without a bypass cap, the cathode voltage would swing up and down slightly, 'in sympathy' with the (much greater and stronger) plate voltage (when the tube is under signal conditions). This is known as 'cathode current feedback'. This prevents the tube from having as much gain as it otherwise could have if you were able to hold the cathode voltage constant. (This is because the 'gain' of the tube is obtained from the difference between the plate voltage and the cathode voltage - and if the cathode is swinging up and down slightly in sympathy with the plate, then the cathode-to-plate voltage is slightly less than what it otherwise might be).

    When you put a bypass cap in parallel with the cathode resistor, the cap charges up on the cathode voltage 'upswing' and releases this charge on the cathode voltage 'downswing'. This has the effect of stabilising the cathode voltage, thereby effectively holding the cathode voltage constant, when the plate is swinging under signal conditions (provided that the cap is big enough to hold all frequencies of voltage change, that are present at the cathode, constant.

    If you make the bypass cap smaller, it will only hold the higher frequencies' voltages constant*, therefore those higher frequencies will get amplified more (because for those higher frequencies the cathode-to-plate voltage is greater than what it is for the other (lower) frequencies for which the cathode voltage is still swinging in sympathy with the plate).

    * because the cap's size (capacitance) determines what range of frequencies it will be effective for - smaller value caps are only effective at higher frequencies, whereas larger value caps are effective over the full frequency bandwidth. Hence a relatively large cap (like a 25uF cap) will typically provide gain over the full range of freqs you get in a 6V6 output stage. whereas if you decrease the value of the bypass cap, you will prevent the lower frequencies from having as much gain as the higher frequencies.

    Read this article a few times (esp the part about cathode bypass caps)

    http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf
     
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