negative feedback loop

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mojoworker, Jan 23, 2012.

  1. mojoworker

    mojoworker Member

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    i have an old gem amp from italy with 2xel84 and 2xecc83. i had work done on it in the past, and the tech took out wire from the output transformer back into the circuit, which really opened up the gain. i am trying to improve a harmony h430 amp with some tone issues and was looking to see if there is a similar feedback loop involved. i am not very experienced technically and was wondering if someone could give me a tip on how to reliably recognise such a loop in the schematic?

    thanks -

    chris
     
  2. dazco

    dazco Member

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    In the schematic a feedback loop will be easiest to recognize by looking for a connection from one of the speaker taps that runs thru a resistor (usually between 100k and 39k) to the unused grid of the PI. So it's taking the signal from a tap (usually the 4 or 8 ohm tap, rarely the 16 ohm) and feeding it back to the PI. If there are no connections from any of the taps going back into the amp circuit then it has no feedback loop.
     
  3. guitarcapo

    guitarcapo Senior Member

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    What exactly are the "tone issues"?

    If you are looking for more gain there are some tricks you can do that are easy.
     
  4. mojoworker

    mojoworker Member

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    my issue with the amp is mainly
    that the reverb/tremolo channel has
    significantly less gain than the other
    input. i was hoping that removing a
    negative feedback loop might be a
    fairly simple way to address that, analog
    to my experience w/ the other amp.
    i would be interested in any other ways
    to improve that situation, if someone
    in the forum has suggestions for the amp.

    thanks -

    chris
     
  5. phsyconoodler

    phsyconoodler Member

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    calgary canada
    If you look at the schematic the reverb channel uses a 6973 pentode to drive the reverb and the other channel has 12AX7's.No negative feedback loop in that amp.

    Make sure all the preamp tubes are ok.It's hard to follow that schematic but it looks like the normal has less losses through that very odd tone control.Those are usually reserved for a pentode preamp.Strange at best..
    Those amps were not noted for their design smarts.But...they sure can sound great.
    The 6973 is NOT an EL84.Similar but not the same pinout.
     
    Last edited: Jan 23, 2012
  6. guitarcapo

    guitarcapo Senior Member

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    What a goofy schematic. The speaker is in the middle.

    In theory any alterations in the nfb would effect both channels equally...if that's the idea:

    I'm looking at V4. What if you put a 25uF 50V cap across that 1500K cathode resistor? Negative facing ground, of course. Or across the 2.2K resistor across pin 3 of V5.....or maybe 3.3 uF across both?

    Individual channel gain?

    ...look at pin2 of V2...why does the reverb channel get a 2.2K resistor feeding into the first gain stage from the inputs...but the normal channel has none? I bet reducing that resistor to a smaller value or maybe even eliminating it entirely would open things up.
     
  7. Jerry Glass

    Jerry Glass Member

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    To increase the reverb channel signal level I'd change out the 27k across the reverb transformer secondary to a 100k or so; you may want to increase the 100k next to it to 150k or 220k to balance out the reverb signal and give it even more push. It will change the voltage divider to deliver more signal to the second stage.

    If you want to boost the whole amp, increase the value of the 68k that references to ground at the input of the PI; this also changes a voltage divider, effectively like turning up the volume a couple of notches. I'd try a 150k or 220k in this spot too.

    I'd go easy on the amount of gain you introduce in this amp; the low plate voltages will cause individual stages to overdrive easily which may make a clean sound impossible if you give too much gain to one area.
     
  8. mojoworker

    mojoworker Member

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    hey guys -

    thanks for your input. i will try some of those ideas out and post the results here -

    chris
     

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