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Fender champion 600 cathode bypass vs coupling caps

Discussion in 'Amp Tech Center' started by ballynally2, Aug 5, 2018.

  1. ballynally2

    ballynally2 Tele-Meister

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    I have been previously inclined to take altering coupling caps over changing cathode bypass caps when trying to reduce the famous 'bass flab', especially in Fender amps. To me it's a more straightforward mod and doesn't immediately impact the initial gain structure at the onset/input. I feel that by significantly lowering the early cathode bypass caps you are already messing with the gain structure (and EQ/onset of distortion/ NFB) in a way that would have to be corrected further down the chain. And to have an impact you really have to go down to 3uF or less instead of the standard 22uF cathode bypass cap. One could change cathode resistor values to even out the mix but i am rather reluctant. I feel that with coupling caps between gain stages (after the plate) there's less to do to make up the loss
    Could someone please convince me otherwise? I might be biased..correction: i AM biased!
    Real techs please step up to the plate :)D, ok i'll stop now)..
     
  2. dougstrum

    dougstrum Tele-Holic

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    Oct 6, 2015
    blu ridge mtn cabin
    Not sure about champ 600 specifically. I like using 10/25 for bypass caps. Don't think you would notice loss of gain except on the lower
    frequencies, which you are trying to control.
     
  3. JD0x0

    JD0x0 Poster Extraordinaire

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    Feb 22, 2009
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    I use 4.7uF bypass caps in Fenders, pretty much gives a full bypass while taking out the 'garbage' bass frequencies.

    Calculate your low pass for your coupling caps. My 'high gain' Champ I went with 3nF on the first triode and 10nF on the second triode (but that's with a larger grid leak resistor, which is why you need to calculate for each state)

    https://www.ampbooks.com/mobile/amplifier-calculators/coupling-capacitor/calculator/


    Also, people often overlook grid stopper resistors. Adding one to preamp gain stages, and increasing the ones in front of the power tubes can go a long way in preventing 'blocking distortion' which is a major source of flub.
     
    asnarski likes this.
  4. wanderin kind

    wanderin kind Tele-Meister

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    you can play with the primary inductance of the output transformer also.

    more inductance usually means more bass, or less distorted bass,

    you can also limit power to the speakers by playing with the core size of the output transformer, the smaller the core, the less max power than can be delivered,

    for example some folks like to upgrade the Bandmaster amps to a bigger OPT, but a speaker upgrade should go along with this, as it might have been Fender's idea to limit speaker blowouts by limiting power via the OPT.
     
  5. ballynally2

    ballynally2 Tele-Meister

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    Thanks for your reply. I find that with using cathode bypass caps above 3uF there's no audible difference in bass response (more than 2dB and looking at the graph) with guitars (81hz low E), so i don't really see the point.I see more negatives than positives at this stage. With 2.2/ 1.5/ .68 you start to actually hear the difference.
    It starts to have a real impact. The reason why i prefer changing coupling caps instead is that i won't have to deal with balancing out the gain/eq/feedback/onset of distortion in other ways. Unless i want to, of course:). Fender/ Traynor etc still use fairly big cathode bypass caps and mostly .022uF coupling caps. Going to .0022uF looks like a good idea. I think it's more straight forward.
    But you make a good point about grid resistors and i will take a closer look. How much of an impact has the one in front of the power valves compared to cathode bypass and coupling caps?
     
    Last edited: Aug 7, 2018
  6. ballynally2

    ballynally2 Tele-Meister

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    Yes, limiting speaker blow outs by using smaller OT has been Fender's trick in the 1960s. My 63 Vibroverb RI officially has 40w power but it's closer to 28w due to the small OT. But i like the way the amp clips when pushed so a bigger OT wouldnt be my cup of tea.
     
  7. ballynally2

    ballynally2 Tele-Meister

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    The thing is that i consider bypassing the Champion 600 tone stack. That will up the gain that i will have to reduce in other places as i don't want to change the onset of distortion too much. Right now it's kind of a black face circuit with fixed tone resistors so can't lower the bass. As i'm used to changing coupling caps first i might change my mind and go to the cathode bypass caps with this amp if i decide to use the tone stack bypass.
     
  8. If you are going to remove the tone stack for a more 5F1 tone, here's an easy mod from the MyLesPaul forum. It works for me but I can't tell you how it addresses the flub as I have done other mods too.

    lift r-20 and r-21 at their junction
    lift c-1 at the junction of r-18
    Now install a capacitor between where you lifted c-1 and you lifted r-21
    ( I used .1mf @ 400v ,because if I recall ,that's what a proper tweed had in there,but .022, .047 work just as well )
    Insulate or bend capacitor lead well so it touches nothing it shouldn't
    Remove c-3, dont ask questions, just do it.

    Champ600.jpg
     
  9. ballynally2

    ballynally2 Tele-Meister

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    that's funny.i just replied to a post of yours in the champ 600 owners thread.I always ask questions.
    As i do like the sound of black/silver face champs for edge of breakup sound i probably wont go tweed.
    As im using a 10" i get sufficient bass.Ive lowered coupling cap value from.022 to 0022 but it didnt quite cut it.Back to stock.
    So i decided to change the first cathode bypass cap (C3) fr 22 to 2.2 uF. Much better bass response.Kept the 2nd stock.rca 5751 pre, jj6v6.emi blue alnico 10 (but might change to GAsc64 10" as the alnico has a tad too much sizzle..
     
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  10. wanderin kind

    wanderin kind Tele-Meister

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    somewhere i saw a amp circuit that had a pot on the bypass cap, i think it was a Supro. kind of an interesting frequency/ volume control, eh?
     
  11. ballynally2

    ballynally2 Tele-Meister

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    I'm a big fan of the Supro amps and the recent ones are very good. In fact, if i was looking for a small (ish) amp right now i would pass by Fender et al and go for Supro.
    I really like the way the clean to overdrive works with these.

    Someone said this, and i agree:'' My own experience with the new reissues is that they did a great job at delivering a nice mid-gain overdrive sound with lots of clarity and articulation at manageable volumes. Not as farty as a cranked Princeton or small tweed, but not as bright as a mid-powered blackface combo. Really cool little amps with a Tele or Led Paul IMO.''
     
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