[ARC5] Suppressor connection in ARC-5 receivers.

[David Bock]

I isolated the RF gain and the suppressor grid voltage with zeners in the
SX-140. It settled down a lot then. There are two points of oscillation that
takes place. The first is before the diode conducts and the next as it
conducts. You use the second one and not the first. You do that by applying
full voltage (full CW on the control) and back off from the maximum to the
point where the diode no longer conducts for AM and add voltage a bit at a
time to get the best regen action for CW. This is the opposite of what
intuition tells you to do.

Dave W8OHS

-----Original Message-----
From: arc5-bounces at mailman.qth.net
[mailto:arc5-bounces at mailman.qth.net]On Behalf Of Geoff
Sent: Thursday, January 31, 2013 9:30 PM
To: Tim; Ian Wilson
Cc: ARC-5 Mail List
Subject: Re: [ARC5] Suppressor connection in ARC-5 receivers.


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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