Power supply dropping resistors - tone changes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by tmac, May 16, 2006.


  1. tmac

    tmac Goldmember Gold Supporting Member

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    Rebuilding a SF Fender Super Reverb and right now has the standard 1K and 4.7K resistors for the preamp and PI plate B+:
    If I wanted to drop the voltage from 270V to ... say down around 230V (it was suggested to me to try using 4.7K in place of the 1K, and 10K for 4.7K) what tonal changes would occur? (Deluxe Reverb is 10K and 10K but is filtered differently) I've heard lower B+ will brown the sound and lower headroom a bit but I also read where it will brighten the tone. Seems like it would soften the sound some? I've read where some techs will drop the V a bit. Thanks for any info!:dude
     
  2. Rich M

    Rich M Member

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    That's not true. While they are not in the signal path, they will affect the bias points of the pre tubes, which are. I'm not sure how they will affect the sound but it is easy enough to try. If you have an extra hole, you could try making it switchable with a voltage-compatable switch so A-B testing would be easier.
     
  3. tmac

    tmac Goldmember Gold Supporting Member

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    Hmmm, the switch is a good idea...any other opinions on what tonal changes dropping the voltage will give? Seems like the BF Fender's have preamp voltage all over the place from Princeton Reverb on up the line and they all sound pretty similar so there's probably not much of a tonal difference other than maybe distorting quicker.
     
  4. Shea

    Shea Member

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    Some say that with higher plate voltages, including higher plate voltages in the preamp, the tone gets a little brighter and distortion gets a bit harder. Lower plate voltages supposedly result in warmer tone and smoother distortion. I'm not swearing to this, it's just what I've read.

    Shea
     
  5. donnyjaguar

    donnyjaguar Member

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    One of the Traynor guys was experimenting with the resistor that drives the first voltage amplifier. In earlier Traynors it was a 10k and in later ones it was removed altogether. Anyway, the earlier ones are known to sound better and he discovered that adding this resistor to the new amps (by that I mean 1970s here) it sweetened it up and made is sound very much like the 1960s versions. I haven't tried it on my Traynor (1978) but I'm certainly going to when I get a chance. It will reduce the gain a little as it shifts the operating point of the tube and although its more subtle than profound it can make a difference that's noticeable. For those not familiar with Traynor, the schematics are curiously familar with early Fenders. :)

    There's nothing wrong with running low plate voltages in an amplifier. If you look at the Williamson I recall there's about 67V on the plate of the first voltage amplifier. For those not familiar with the Williamson it is a well respected design with the hi-fi crowd.

    Cheers!
    DJ
     
  6. glasman

    glasman Guest


    I just did this experiment on my kids amp. The preamp rail was at 380V and I knocked it down to about 320v (brought the plates down to about 200 voilts at idle) and it definately improved the clean and distortion sounds from the amp. Less harsh, very smooth and warm. Top end is more chimy now.

    Gary
     
  7. doctorx

    doctorx Member

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    I posted a dissertation on modifying preamp circuitry on my web page, check it out.

    I don't know where I got it, I've had it for years and if anyone knows who wrote it let me know and I will give credit where credit is due.

    It might help answer your questions.

    Donny, which Traynor are you talking about? I have a couple of YBA1-A's and a guitar mate. Where in the amp circuit is/is not this resistor? I'd like to try that myself if it would work in my amps.

    thanks,

    regis
     
  8. hasserl

    hasserl Member

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  9. donnyjaguar

    donnyjaguar Member

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    Hey DrX,

    Look in the power supply section. There's a 10k to drop the voltage down to the next electrolytic. That in turn feeds the first voltage amplifier stage. Old has 10k, new has 0k.

    If your GuitarMate is older it may already have this resistor in there. In my experience there are a lot of different Traynor schematics. I've seen at least 8 different ones for the YBA1 amplifier. I have a 1978 YBA1 too and I've never seen the schematic published.
     

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