[ARC5] Suppressor connection in ARC-5 receivers.

[Tim]

Looks like at least the Hallicrafters S-120 uses that circuit. The single
IF amp, 12BA6, has its suppressor returned to ground via a rheostat - "BFO
freq" control.  Drives the IFA into oscillation.  Sorta works....
Tim
N6CC

On Thu, Jan 31, 2013 at 9:52 AM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

> Hi Ken,
>
> I would guess that connecting the suppressor to the cathode
> provides a path of low inductance, removing one possible
> configuration of the tube as an oscillator. I think that DC
> considerations would be less important, since the influence
> of DC suppressor voltage is small in most normal operating
> regions.
>
> In most pentodes there is some operating region where there
> is negative gm between the suppressor and screen grids (look
> at the Transitron to see this in action). So if the suppressor is
> not at ground with respect to the cathode, this can set up the tube
> as an oscillator if the conditions are right (external circuitry has
> high enough Q, etc).
>
> I seem to recall that one of the Hallicrafters uses the technique
> of lifting an IF tube suppressor above *AC* ground to add
> regeneration and provide a very low cost BFO function. The diode
> just acts as a DC-controlled resistor (turning on the diode provides
> a low impedance path to ground for the suppressor).
>
> 73, ian K3IMW
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