[AMRadio] FS: restored Gonset G-76


Jim Candela jcandela at prodigy.net
Thu Feb 9 09:43:32 EST 2006


Brett,

    You might poll other G76 owners about the audio
issue. My two cents seems to concentrate on what you
said where the receive audio was bad (before the IC
mod), and the transmit audio is low unless you drop
the carrier way back, and even so you seem to indicate
distortion is apparent on the modulation envelope.
That 6CM6 audio driver / output is likely the culprit.
I would look hard at the DC operating point of that
tube, and also consider the possibility of a turn to
turn short in that output / interstage transformer. If
you scope out that area, I think you will find the
6DQ6's faithfully reproduce whatever they are driven
with, and I bet the distortion is seen on the drive to
the modulator grid2 (grid one at ground).

   It has been about 10 years since I went through my
G-76's, but I recall the driver distortion as fixable,
and the 6DQ6's were really nice as is by Gonset. Full
modulation at about 65-70 watts out with a nice scope
pattern is what I remember. Maybe my recollection is
putting this in a better light, but if so, not by
much.

   If you go for the EL-34 approach, consider the two
additional supplies needed, (abt 300v G2, and -25v
g1), the higher class AB1 idle current (about 15-20
watts heat per tube), and the lower efficiency of AB1
versus class B (more heat, and power supply draw). It
would be nice and clean though, and I guess that is
why your tempted to go that route. If you go for it,
keep us informed as the group will want to know.

Regards,
Jim
WD5JKO
 

--- Brett gazdzinski
<brett.gazdzinski at verizonbusiness.com> wrote:

> I dug out the G76 last night, and gave it a workout.
> I need to take care of the audio, the receive is not
> that bad
> with the IC chip I have in there, but to much hiss.
> 
> I had it on 40 meters, and with the dummy load
> selected, I was
> picking up sw broadcast stations at s9, the thing is
> sensitive,
> likely more sensitive on the lower bands than it
> needs to be...
> 
> The TX audio sounds like crap with the D104, it had
> problems 
> making much over 50% positive, with plenty of
> distortion.
> I tried some other 6DQ6 modulator tubes with no
> improvement.
> Phasing made no difference, but reducing the RF
> power made 
> things better, up to 90% at about 40 watts out.
> Plenty of grid drive, 685 volts on the high
> voltage...
> 
> Its restricted, ok for a small signal, but the
> waveform
> on the scope looks all screwed up.
> 
> So I am thinking about gutting the audio, and
> putting 
> in something like a pair of 6CA7's in AB1, driven by
> a phase splitter circuit.
> 
> I do not like the postage stamp size driver/audio
> output
> transformer, what with the 6DQ6's needing driving
> power.
> 
> It should be a fun project to get the thing working
> a little
> bit better then it does now.
> 
> Brett
> N2DTS
> 
> 
> > 
> > 
> > 
> > Dan says:
> > 
> > I have not seen any mods on the Internet for
> G76'ez...
> > Anyone know of any?
> > 
> > Reply from Jim WD5JKO:
> > 
> > No not too much that I've seen. I have two G-76's
> > myself, and the quality of the power supply is a
> > factor in the quality of the transmitted signal. I
> > have a home made (not by me) AC power supply that
> > weighs a ton, and has a 3 tube regulated low B+. I
> am
> > sure this helps.
> > 
> > As Bret mentioned, the rcv audio output tube is
> also
> > the audio driver for the xmit 6DQ6's. Having good
> > fresh tubes here goes a long way towards better
> > quality. Since the output tubes are acting as
> hi-mu
> > triodes, the idle current could be too low if the
> high
> > B+ is also too low. I forget how I did it, but I
> have
> > a plug in meter that measures modulator cathode
> > current. I also fiddled with that audio output
> stage
> > by beefing up the idle current on xmit (maybe I
> did
> > that on my G-50..), and heavily AC bypassed the
> > cathode resistor, and added a little bit of
> inverse
> > voltage feedback. Both receiver, and transmit
> quality
> > stock seemed pretty decent, and was much better
> after
> > the above said modifications.
> > 
> > For receive, some say the IF band with is kind of
> > broad. I recall cutting out some resistors that
> were
> > across the IF coils. Without the resistors, the
> "Q"
> > was higher, and therefore the bandwidth was more
> > narrow.
> > 
> > Six meters is a joke on the G-76. Don't try it
> unless
> > you want to burn up your 6DQ5.
> > 
> > If you are interested, I might be able to open up
> one
> > of my G-76's so that I can document whatever I
> did.
> > 
> > Regards,
> > Jim
> > WD5JKO
> > 
> > --- dw73454 at alltel.net wrote:
> > 
> > > 
> > > THIS TIME TO THE LIST ON PURPOSE...heheheheh...
> > > 
> > > 
> > > Brett sez:
> > > 
> > > > The receive audio sucks. 
> > > > And the TX audio sucks (surprise!).
> > > 
> > > Hmmmmmm....I have heard G76s on 10 meter AM that
> 
> > > sound great! I wonder what they did to improve
> the
> > > TX audio?
> > > I have not seen any mods on the internet for
> > > G76'ez...
> > > Anyone know of any?
> > > 
> > > Dan 
> > > 
> > > 
> > >
> >
>
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