Announcement

Collapse
No announcement yet.

Replacing OT, do I need to tweak feedback?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Replacing OT, do I need to tweak feedback?

    I have a couple other threads on this amp, but I figure why not keep overthinking it, right? Amp is an Eden E300T 300W tube amp (same as WTB300V). Original OT was bad, I am trying to put in an SVT CL transformer into it. Looking at the schematics, the Eden amp took the FB point off the 8 ohm tap. The SVT transformer doesn't have an 8 ohm tap, only 4 and 2. It seems like this would affect the feedback ratio, essentially reducing the feedback by a factor of 2. I think it reduces it because a 4 ohm tap will be lower voltage and higher current than an 8 ohm (right?) and the feedback is voltage based, not current, but I am not sure, and therefore I ask.

    Looking at the schematics it looks like the feedback comes back to the same point in the circuit (the cathode of the first PI stage), but they do have a different voltage divider ratio there. Eden also uses a 12AT7 instead of a 12AX7 like the SVT, but not sure if that would also affect it.

    WTB_300V.zip
    93_SVT_CL.zip

    Thanks in advance,
    Greg

  • #2
    NFB is voltage based. The output voltage at the 8 Ohm tap is greater by a factor square root of 2 (=1.414) than the output voltage at the 4 Ohm tap. Or the output voltage at the 4 Ohm tap will be lower by 1/1.414 = 0.707. Consequently the NFB series resistor (R69) should be reduced by a factor 0.707 and C58 should be increased by 1.414.
    These considerations hold if both OTs have the same primary impedance.
    - Own Opinions Only -

    Comment


    • #3
      I had good results by increasing R68 to 150R 1/2W for running with a lower impedance, the anode load is not that important if it is wrong the THD will increase and most listeners like that effect.
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

      Comment


      • #4
        Originally posted by Jon Snell View Post
        I had good results by increasing R68 to 150R 1/2W for running with a lower impedance, the anode load is not that important if it is wrong the THD will increase and most listeners like that effect.
        Jon, just want to clarify, is this with stock OT or replacement?

        Comment


        • #5
          Originally posted by glebert View Post
          Jon, just want to clarify, is this with stock OT or replacement?
          Replacement without an 8 Ohm tap.
          Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
          If you can't fix it, I probably can.

          Comment


          • #6
            Maybe I should add, that my advice in post #2 was meant with regard to restoring original NFB conditions.
            It was not meant to answer the OP's question.
            It should be no problem to use the original NFB divider with the 4 Ohm tap or even increase the series resistor. Both would mean less NFB and will result in increased power amp gain, a little more distortion before clipping, increased output source impedance (= lower damping factor) and maybe somewhat reduced HF response. If it sounds good, fine.

            OTOH, increasing NFB might cause instabilty.
            Last edited by Helmholtz; 04-23-2020, 05:00 PM.
            - Own Opinions Only -

            Comment


            • #7
              Originally posted by Helmholtz View Post
              Maybe I should add, that my advice in post #2 was meant with regard to restoring original NFB conditions.
              It was not meant to answer the OP's question.
              It should be no problem to use the original NFB divider with the 4 Ohm tap or even increase the series resistor. Both would mean less NFB and will result in increased power amp gain, a little more distortion before clipping, increased output source impedance (= lower damping factor) and maybe somewhat reduced HF response. If it sounds good, fine.

              OTOH, increasing NFB might cause instabilty.
              The instability is basically my concern. My thought was to keep loop gain the same, but since I don't know exactly what the impedance ratio was on the original transformer I maybe shouldn't assume them to be equal. I haven't really thought about loop gain since college courses, it was never "my problem".

              Comment


              • #8
                Originally posted by glebert View Post
                The instability is basically my concern. My thought was to keep loop gain the same, but since I don't know exactly what the impedance ratio was on the original transformer I maybe shouldn't assume them to be equal. I haven't really thought about loop gain since college courses, it was never "my problem".
                The influence of impedance ratio on loop gain is relatively small. Even an unlikely difference of 2:1 would only result in a loop gain change of around 40%.
                In case of doubt, do not lower R69.
                Last edited by Helmholtz; 04-23-2020, 07:08 PM.
                - Own Opinions Only -

                Comment

                Working...
                X