[R-390] Audio Capacitance

[Michael Murphy]

Hi Tom,

The "how" is positive feedback. This is a  path which feeds some of the
output signal from the output stage (developed on the cathode resistor),
back, to the cathode of the audio driver stage. The "why" is less
understood. This feedback method must have been added in the design to
generate a deliberate effect that the designers wanted - like a peak in the
response. The guys on line can help you more than I can. All I know is that
positive feedback if taken too far can cause some nasty effects like
oscillation! Adding a bypass or having the increased resistance is like
turning up the regen control!

Mike Murphy WB2UID

----- Original Message ----- 
From: "Tom Bridgers" <Tarheel6 at msn.com>
To: "Michael Murphy" <mjmurphy45 at comcast.net>
Sent: Tuesday, April 19, 2005 10:06 PM
Subject: Re: [R-390] Audio Capacitance


> Hi Mike
> RE:  R-615
> I had an audio deck that was oscillating.  Put out a high pitched squeal.
> Finally traced it to R-615, which had gone high -- as I recall to about 75
> ohms or so.  Replaced it with a 56 ohm resistor, and it stopped
oscillating.
> I don't understand the theory of why that happens, but it sounds like you
> do.
>
> What happens, if anything, if you short out R-615?
> thanks,
> -tom
>
> ----- Original Message ----- 
> From: "Michael Murphy" <mjmurphy45 at comcast.net>
> To: <Flowertime01 at wmconnect.com>; <r-390 at mailman.qth.net>
> Sent: Tuesday, April 19, 2005 8:17 PM
> Subject: Re: [R-390] Audio Capacitance
>
>
> > Barry,
> >
> > Beefing up those caps is good practice. But, it is a little like
standing
> on
> > your sprinker hose with both feet and taking one foot off.
> >
> > Do the Rippell - C604 and C605 to 0.033 uF and a 10 UF cap for C609.
This
> > should get you somewhere near 100 - 200 Hz for your -1dB point on the
low
> > end and your high end should be fine. Perhaps too fine. My top end was
> > peaking above normal. Removing or reducing the value of C612 (68 pF)
will
> > flatten the high end. In any case you should be going out above 10KHz to
> > the -1dB point. This should get you to 300mW at under 3% distortion. 1
> Watt
> > or so is about the maximum I could get out of the stock 600 Ohm iron for
> 11%
> > distortion with this mod.
> >
> > If you should try to bypass R614, the cathode resistor, with a 100 uF
> > electrolytic in order to increase gain, the positive feedback at R615
will
> > cause trouble, producing a novel circuit - more suited to a code
practice
> > oscillator. The positive feedback produced by R615, the 56 Ohm job, is
yet
> > another mystery circuit of the R390A. I have elected to short this
little
> > bugger out.
> >
> > If you are willing to do a simple rewire to replace the 6AK5 with a
6AQ5,
> > lower R614 to 270 Ohms or so and install a small all-american 5 type
> output
> > transformer, you can easily get to 1 watt at less than 1% distortion and
> > obtain 30 Hz to 20 kHz bandwidth. With a better transformers and more
> > fooling with the circuit, 2 - 3 Watts is possible.
> >
> > Warning - Playing with this circuit is addictive, buy another audio
deck.
> >
> > Mike Murphy WB2UID
> >
> >
> > ----- Original Message ----- 
> > From: <Flowertime01 at wmconnect.com>
> > To: <r-390 at mailman.qth.net>
> > Sent: Tuesday, April 19, 2005 6:28 PM
> > Subject: Re: [R-390] Audio Capacitance
> >
> >
> > > Fellows,
> > >
> > > Yes I did say go from .01 to .1 or .3
> > >
> > > A jump to .03 from .01 just will not give you enough to hear the
> > difference.
> > > As some of the other post pointed out the transformers and other
things
> > are
> > > still
> > > effecting the changes
> > >
> > > Roger KC6TRU
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