[AMRadio] Modulator question


Jim Candela jcandela at prodigy.net
Sun Jan 31 00:15:08 EST 2016


Paul,
   In addition to Rob's questions, there are a lot of things to consider. You mentioned that the screens are tapped off at 1/2 the B+. The screen current will rise non linearly with grid drive excitation, and if you are using a voltage divider for the screens, then the screen voltage will dip noticeably when cranking up the power. Perhaps try bypassing the screen tap with a bypass capacitor, like 50 mfd or so. With low duty cycle speech instead of a sine wave, that simple change can help.
   How are you biasing those tubes? Is this class Ab1 or Ab2? With Ab2 there will be grid current on voice peaks.

   Another factor is plate to plate load impedance. If the load is too low, then the tube will labor to produce power. Sometimes if the transformer turns ratio is fixed, then adding two more tubes will work better; i.e. a quad of EL-34's in push pull parallel.
  Does the plate voltage droop much as you crank up the power?
JimWd5JKO 

    On Saturday, January 30, 2016 9:39 PM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
 

 By "triangular" you mean something like a sawtooth?  Do you hear
distortion or are you only viewing it?  Sometimes the scope pick up is
the problem and the circuit under test will turn out to be okay.
However, that's probably more likely with RF.

73

Rob
K5UJ

On Sat, Jan 30, 2016 at 7:31 PM, Paul Baldock <paul at paulbaldock.com> wrote:
> I'm building a modulator using two EL34s.
>
> I am testing it in to a matched resistive load. The output between 30% and
> 100% of max (50Watts) gets quite distorted. Driving with a sine wave the
> grids look good, but the output starts to get more triangular the harder you
> push it.  FYI the plate voltage is about 660V and the screen tapped at
> around half of that.
>
> Any thoughts from you audio buffs?
>
> - Paul  KW7Y
>
>
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