Take a look at a schematic for me, please.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JimRayden, Jul 8, 2005.

  1. JimRayden

    JimRayden Member

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    I traced this russian tube thingy a while ago and made the necessary mods to turn it into a guitar amp. If anyone has the time or interest to test this schematic out in a computer simulation, I would be more than interested of the results. Like how much headroom it has, how it saturates, etc.

    I've heard it myself and it isn't too good when dimed. I want someone to test it out and tell me if it has any potential or should I rip the innards out and rebuild it. If it has hope, what mods are necessary, etc.

    Oh yes, the B+ is something 340-350 and the tubes are EL84 and 12AX7
    http://www.hot.ee/playout/babybear/

    Thank you.

    ------------
    Jimbo
     
  2. JimRayden

    JimRayden Member

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    Anyone? :\


    ------------
    Jimbo
     
  3. rooster

    rooster Member

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    Exactly what do you mean by "not too good?"

    rooster.
     
  4. JimRayden

    JimRayden Member

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    Well I haven't fired it up in a while but as I recall the overdrive was really buzzy. The cleans were perfect and the sweet spot was OK but high gain buzzed out.

    -----------
    Jimbo
     
  5. Blue Strat

    Blue Strat Member

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    What are the plate and cathode voltages on all tubes?

    The 20K and 47K resistors in the power supply chain (up top) will influence the preamp headroom. Decreasing these values will increase preamp tube plate voltage and headroom somewhat.

    Adding a cap in parallel with the EL84 cathode resistor will help keep low end from mushing out.

    You can experiment with plate and cathode resistor values (up or down 20% to start) and see how these affect tone. If you notice trends you like, you can continue in the same direction.
     
  6. ekp

    ekp Member

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    My guess is that the output stage is just about as opposite the classic tube guitar amplifier as you can get. The classic tube output stage is push-pull pentode Class AB. Although this output stage uses a pentode, it is connected like the ultra linear. The screen actually is negative feedback and that changes the relationship between the amp and the speaker. This feedback can also help produce buzz.

    The ultra linear connection makes the pentode have triode-like plate characteristics. And triodes are rarely used for guitar amp output stages.

    Being single-ended, it is inherently Class A and consequently leaves a desire for sag.

    You did not mention the transformer, but it might be saturating as well.

    If you are interested in a classic tone, try another path.
     
  7. JimRayden

    JimRayden Member

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    Yeah, I just recieved a wreck that seems to be push-pull. It seems to be a total disaster, so I won't even try to trace it. Just get the iron and get outta there. :p I'll let you know when I'm done.


    ----------
    Jimbo
     
  8. JimRayden

    JimRayden Member

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    http://ftp.taaviaudio.com/homes/JimRayden/babybear2.gif

    OK, here are the voltages.

    What puzzles me is the -15.5V on the input of the power tube... Supposed to be like that?

    I don't know much but... The triode plate voltages seem to be WAY too low...

    Mmkay, I'm looking for a decent headroom that turns into sweet overdrive when turned up (who isn't :p). What would get me closer to that? Or should I just mod the circuit into a Blackface?

    Also advise me how to turn this into a non-linear amp (if that gets in the way).

    ---------
    Jimbo
     
  9. ekp

    ekp Member

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    Jim,

    The -15.5 V has two possible sources. The less likely is negative grid current. The more likely is that this circuit is oscillating at a super sonic frequency and the power tube input capacitor is getting charged up by power tube grid conduction.

    Try pulling the triode out of its socket and then check the pentode voltages to kill the oscillation via that capacitor. Or, get/borrow an oscilloscope.

    The triode voltages seem low, but then the plate resistors are high. This limits headroom, particularly of the first stage. I would use something like 100K. The gain will not change much - only down about 1.5 dB or just abit more than barely noticable.

    If you would like to email direct use eric@pritchardamps.com

    Have a great day, Eric
     
  10. ekp

    ekp Member

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    Jim,

    If the amp is oscillating, then its behavior is all messed up and it does not sound right..... While the reverse implication is not always true, it is something to be checked out...

    Good Luck, Eric
     
  11. PacoCasanovas

    PacoCasanovas Member

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    ok....just off the box:

    change the cathode-cap from 20µF into 470nF

    its also a possible mod to add a cap (220nF) paralell to the 2.7k resistor....

    some other toughts:

    first stage gain = amp factor x (plate resistor / plate resistor + plate resistance)

    for all pro's normally you'll have to involve all the load-resistances too...but lets keep it simple for now.

    plate resistance = 1/transconductance

    so if you decrease the plate resistor you have more plate-voltage, thats correct. But note: the plate resistance will change too, get lower...means:

    with higher plate resistors = more gain! But also less distortion....mmh sounds strange hmm? let me explain this: the output-signal's zero-point is now on the plate and not between plate and ground! That means, the plate-resistor value is important to one signal-wave.....if plate resistor is small, there you'll have more clipping......so what this all means....you have to find something between clipping and gain-factor..

    let me explain my mods:

    a cap on paralell to the cathode resistor will lead all freqs up from a certain point (cut-off) to ground, means the lower the value of the cap = less bass , cause all freqs below the cutoff were not boosted that much the rest freqs!
    If your tone is too fuzzy, you may got too much bass....roll it off by changing the caps....or if you want less gain just cut the caps...

    btw...add a 100µF/63V cap (better if you can find a no-polarized cap) to ground paralell with the cathode resistor from the EL84

    some cool tipps: if you have too much highs, add some caps (250pF to 2,2nF) paralell to the plate resistors (note, must be parts made for higher DC & AC voltages)

    add a resistor of 220k between the cw-point of the pot and the 10nF cap...

    or add a tone-stack..... its always fun to check out stuff...

    have fun by trying some stuff and take care (plate voltages)



    all the best & greetz
    Paco
     

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