[Collins] Re: 32S-3 1st Audio Amp

[E C Moxon]

It appears that early 32S-3s had no cathode resistor in the 1st audio stage.

Adding it is the topic of Service Bulletin No. 2, September 21, 1967, and
the change "will be covered in the fifth edition of the 32S-3 Transmitter
Instruction Book." The bulletin cites overload that may occur with "a RTTY
tone keyer or from audio oscillators during tests. No distortion should be
encountered when using a microphone." It is indeed absent in the 3rd
edition, 9/64, and is there in the 7th edition, 5/69.

I added it to SN 13360 a couple of years ago.

Here's some info taken from the notes I put on the schematic.

With mic input much above 100mVp-p, plate voltage distortion was apparent
just by eyeballing the scope trace, due to low plate voltage (50V). With the
resistor, plate voltage rose to 90V, and max input signal for that stage was
a healthy 4Vp-p, although 200mVp-p clipped the 2nd audio amp.

The gain of the first stage was only slightly affected by the resistor
(~1dB). The main effect was lower plate current (higher plate voltage) due
to the self-biasing, putting the stage comfortably up into its linear
region.

Since the gain is relatively insensitive to the cathode impedance, the
cathode bypass in this triode stage can't do much to the audio response even
if its reactance is small compared to the equivalent resistance at the
cathode.

A capacitor with short leads on the cathode of the high-gain 1st stage may
be desirable from the standpoint of stray pickup in the rf environment found
in the transmitter, or for dealing with stability or undesirable interstage
coupling at non-audio frequencies - even with the plain wire supplying the
cathode current.

73 to all,
Ed, K1GGI

> From: "HPMeyer" <hmeyer@hvc.rr.com>
> To: "CCA" <collins@mailman.qth.net>
> Date: Fri, 14 Feb 2003 05:15:48 -0500
> Subject: [Collins] 32S-3 1st Audio Amp
>
> Am working on a 32S-3 that was purchased in June of 1964.
> Have three different schematic versions with variations on the
> 1st Audio Amp.  The unit itself has a 0.1 ufd input cap with a=20
> 100 Kohm grid resistor and no cathode resistor.  One of the=20
> schematic variants has an un-bypassed 680 cathode resistor=20
> and the other has a 680 ohm bypassed with a 0.1 ufd cap. =20
> I understand the theory of cathode bypass resistors and caps,
> but would like to know the reasoning for the different circuits=20
> and which is the best for overall performance.  Started looking
> at this because my Mic gain setting (using an EV664) is about=20
> in the 8 o'clock position and it just seemed like there may be a=20
> problem with too much audio gain somewhere.
> Thanks...Herm Meyer...KC2DFJ