Silvertone 1472 Early Breakup Fix

Discussion in 'Amp Tech Center' started by K Teacher, Jun 6, 2021.

  1. K Teacher

    K Teacher TDPRI Member

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    ThermionicScott,

    Do you have a Silvertone 1472, similar circuit amp or build a clone and tested it before and after re-biasing the split-load phase inverter to say that what I recommend does not work?
     
  2. mrriggs

    mrriggs Tele-Meister

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    Gotta say, the effect of the "grid" current on the phase inverter was a bit underwhelming. That's, no doubt, due to the big 330k grid stoppers. Basically, when the "grid" is forward biased it puts the 330k grid stopper parallel to the 330k grid leak and 68k load resistor. So the load is (68k||330k||330k) + (68k||330k||330k) = 96k.

    Max peak-to-peak swing at the phase inverter was 62 Volts for the grid-leak-bias, 58 Volts for the cathode-bias with output between resistors, and 59 Volts for cathode-bias with output at cathode.

    Of course, measure the output at the "grid" and it's a whole different story. Voltage after the big grid stopper gets clipped off abruptly once current starts flowing. Max voltage swing at the "grid" with the grid-leak-bias was 28 Volts. Surprisingly, the voltage swing at the "grid" was slightly higher with the cathode bias, both configurations peaked at 29 Volts.

    So there you have it. Conclusive proof that swapping a grid-leak-bias cathodyne phase inverter for a cathode-bias can net you a 0.3dB gain.

    And just cause all threads need more pictures... a shot of the "bread board".

    [​IMG]
     
  3. K Teacher

    K Teacher TDPRI Member

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    mrriggs,

    How about grid-leak-bias cathodyne PI vs. cathode-bias cathodyne PI distortion at max. level?

    Your first post shows 70V out/ no-clipping. It is also interesting to note, that somehow, the circuit changed its own bias to almost the same value I am recommending.

    When you tweaked your experiment to look more like the real 1472 amp, your post shows 65V out with severe clipping.


    A note on speaker change:

    If the signal going to the output tubes is distorted, the amp will produce a distorted sound, no matter what kind of speaker is used.
     
  4. mrriggs

    mrriggs Tele-Meister

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    All the measurements of max peak-to-peak were taken at the level before it clipped. In the series of pics with progressively higher amplitudes, the pic before the severely clipped one is the max clean signal. The severely clipped pics were included to better illustrate where [and how] it is clipping.

    Wish I had a THD meter. I suspect the reason the cathode-bias has a slightly higher peak-to-peak in the final stage is because the grid-leak-bias slightly compresses the signal on the downward swing.

    Now that I think about it, I should have been measuring the peak and not peak-to-peak at the final stage. The downward peak doesn't do anything since the power tube is cutoff long before it gets there.
     
  5. mrriggs

    mrriggs Tele-Meister

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    Here are the peak signals [before clipping] at the power tube grids. Showing only the positive excursions, the bottom of the scale is zero volts, scope set to 2V/div. Both signals from the phase inverter plate and cathode are shown, alternating. The signal from the phase inverter cathode is in the center.

    Grid-leak-bias; plate = 13.4 Vp, cathode = 13.2 Vp
    [​IMG]

    Cathode-bias, output between resistors; plate = 13.6 Vp, cathode = 13.8 Vp
    [​IMG]

    Cathode-bias, output at cathode; plate = 13.2 Vp, cathode = 13.6 Vp
    [​IMG]

    With the improved resolution of these measurements, the cathode-bias with signal from between the resistors is showing 0.25dB gain over the grid-leak-bias.
     
  6. K Teacher

    K Teacher TDPRI Member

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    OK… THIS CIRCUIT YOU TESTED, IS NOT A SILVERTONE 1472 NOR CLONE…

    The only conclusive proof shown here, is that you were able to build and tweak a circuit until the output of the grid-leak bias cathodyne and the cathode bias phase-inverters were the same.

    Based on the info you supplied, the grid-leak bias cathodyne phase-inverter you built, re-biased itself and produced approximately the same bias voltage of the cathode bias phase-inverter and thus, has the same output (difference shown is negligible).

    Well, if there is a large enough grid-stopper resistor in front of a grid-leak bias circuit, then when grid current flows, the voltage drop across this large grid-stopper resistor will produce an additional negative bias voltage to the grid-leak circuit. If the proper grid-stopper resistor value is selected, then the total grid-leak bias voltage can be the same of the cathode bias and consequently, produce the same output.

    Again, this is NOT a Silvertone 1472 circuit.

    But thanks for proving my point… a centered cathode bias voltage (or anything else that produces the same bias) gives the best output performance…


    TO EVERYONE ELSE, REALLY INTERESTED IN RESOLVING THE SILVERTONE 1472 EARLY BREAKUP ISSUE…


    Unfortunately, this thread was diverted, since its beginning, to a theoretical discussion about a made-up circuit that has no useful information on how to address the main objective of this thread, which is to resolve the Silvertone 1472 (or similar circuit design) early breakup issue…

    If you are like me, at this point, you completely lost interest in reading a thread that became a debate between two guys, trying to prove one's point, instead of addressing the issue on the initial post…

    So folks, do your own testing and get your own conclusion… install the cathode bias resistor… worst case scenario, like one argued here, will be no difference whatsoever… on the flip side, you will revolve the issue with ONE resistor!

    BTW, there is a reason why classic guitar tube amps that stood the test of time, dropped grid-leak bias during mid-to-late 50s'…


    DROPPING THE MIC…
     
    Last edited: Jun 13, 2021 at 1:37 PM
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