[ARC5] Suppressor connection in ARC-5 receivers.

[Geoff]

The SX-140 also uses that circuit for the 6BA6 1650 KHz IF amp. It worked so 
bad I was almost tempted to toss it in the trash since it was given to me 
but curiosity prevailed and I merrily hacked away until it became a nice 
double conversion radio with filtering, product detector and more.


----- Original Message ----- 
From: "Tim" <timsamm at gmail.com>
To: "Ian Wilson" <ianmwilson73 at gmail.com>
Cc: "ARC-5 Mail List" <arc5 at mailman.qth.net>
Sent: Thursday, January 31, 2013 3:26 PM
Subject: Re: [ARC5] Suppressor connection in ARC-5 receivers.


> 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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