[AMRadio] Gonset G-76


Brett gazdzinski brett.gazdzinski at verizonbusiness.com
Sat Feb 11 19:35:39 EST 2006


I did some work on it today.
First off, I replaced the tube I was using for the audio driver,
I had a 6c4 (triode) in there, some past attempt to clean things up,
and strangely enough, only one pin needed to be moved.
That gave me a little more audio drive.
I also increased the RF grid drive by bypassing a 4700 ohm resistor
in the B+ line to the rf driver tube. I found my G76 does an odd thing,
if you go to the tune position and peak the grid drive, when you go to 
the high power mode, the peak shifts.
What I did was go to drive level 2 and then tune the grid drive to get
minimum plate current on the output tube, measuring the grid voltage
had that position the highest, then go back to the maximum grid drive 
position on the drive level switch.

I don't know if its something with my G76 but if the grid drive
is low at all, the positive modulation falls off badly.

Next was to add some neg feedback from the modulated B+ to
the driver tube grid, bypassing it for highs
to prevent phase shift positive feedback (supersonic).
I tried different amounts and parts values and got the rig 
sounding not bad, before the feedback, the highs were extreme, 
with very little lows at all.
The amount of feedback acts like a tone control, little 
is very bright, more feedback rolls that off and makes things 
sound good for a D104.

The end result was about 50 to 60 watts output, positive 
modulation is up about 80% at 100% neg, if I exceed 100% neg
modulation, the positive will go up to 110%.

I checked out the receive section, I had removed the resistors
across the IF cans (narrows the IF bandwidth) and the 5pf caps
bridging from primary to secondary (also narrows the bandwidth).
besides the noisy audio IC chip, the receiver works well.
 
I need to go back and change the B+ source to the audio stages,
and take if off the relay so the tubes are not on all the time,
with a stock rig they do the receive audio, but they 
don't on mine, no reason to leave them on....

I also went through my 6DQ5 and 6DQ6 tube collection, testing
them out, some were bad, some were good...

I did not get a chance to get it on 40 today, I had to go out,
but maybe tomorrow I can work someone with it.
 

I got on 80 meters tonight, a very rare thing for me, and
the 80 meter antenna (if you can call it that) was acting 
crazy from the snow. I got to get rid of that thing and
put up something that works.
Back to the loading coil setup I guess, but in a different spot
not directly over the house...

Fun with radio...

Brett
N2DTS

 
> Brett,
> 
> 
>    You are correct about the rolled off low frequency 
> response on a G-76.
> That said, these rigs "stock" seem to have more lows than say a Heath
> Apache. One caution however is that the stock iron cannot 
> take a lot off
> bass, so any bass to midrange boosting attempt needs to take that into
> consideration. Since a "flat" transmitter response of say 80 
> hz to 4000 hz
> will sound bassy and muffled in the average receiver with a 5-6 Kc IF
> bandwidth (@ - 6db),   you might add consider adding 
> additional low end to
> that G-76, but do so at somewhere between 3-10 db down from 
> the 1 Khz level.
> At 6 db down, the iron only has to handle 1/4th the power, so 
> low frequency
> core saturation is likely eliminated, and you can still 
> broadcast down to 80
> hz. Maybe bridge that 470 pf with a series R-C instead of 
> just making a
> bigger C, and you could manipulate the hi-freq roll off with 
> some R & C
> additions instead of just that .001 to ground.
> 
>     It would be interesting to take that G-76, and add a 
> voltage divider to
> the modulated B+ so that you can put the scope in X:Y mode to 
> look at the
> Trapezoid pattern. I use this mode all the time with my 20a + linear
> combination, and the Trapezoid is VERY useful towards setting 
> up the linear
> for a minimum of distortion. With plate modulation, the 
> Trapezoid is less
> necessary, but still useful when setting up, or evaluating a 
> transmitter. A
> plate modulated class C amplifier usually modulates upward better with
> increasing drive....to a point.
> 
> Jim
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On Behalf Of Brett gazdzinski
> Sent: Thursday, February 09, 2006 9:22 PM
> To: 'Discussion of AM Radio'
> Subject: RE: [AMRadio] FS: restored Gonset G-76
> 
> 
> I guess I have different expectations then most here.
> I would say its not possible to have the G76 sound good
> on receive or transmit, what with the design and parts
> they used.
> You have a 470pf (yes, that pico)coupling cap between the 1st and
> second stage microphone amplifier, and they inject the
> receive audio in there also, and that uses another 470pf (yes pico)
> coupling cap, with a .001 to ground.
> 
> How can anyone get nice lows with a 470pf coupling cap, or highs
> with a .001 to ground. I would think a good telephone would have
> a better frequency response.
> 
> We wont even get into the modulator grid driver/audio output
> transformer, which makes the dx100 driver transformer look massive.
> As an audio output transformer, it looks like it would be good for
> about 1/4 watt output.
> 
> The mike input amp has a 470K resistor to ground, no way you
> are getting any lows at all there....needs to be like 5 meg
> for lows with a D104.
> 
> I did some playing around with it tonight, and found two problems,
> a bad 12ax7 mike amp, and also found the modulation gets better with
> much more grid drive.
> 
> I have some changes to do in the mike amp stage, but it sounds
> back to its old self as long as the output power is below 60 watts.
> 
> The audio waveform looks ok up to 100% mod at 60 watts, any more
> and it starts looking funky, I did not look into the driver tube
> and its circuit yet, or the clamp tube screen circuit in the 
> RF output.
> 
> Brett
> N2DTS
> 
> 
> 
> >
> > Hi Guys,
> >
> > I have been following this thread and I also agree with Charlie.
> > My G76 gets great audio reports as it is. I am using a 
> D-104 and have
> > very good audio reports. I am currently listening to a group on the
> > East Coast using a a 5" Motorola Micor speaker and it sounds great.
> > Power out is around 70 - 75 watts into a sloper. I use it to
> > check into
> > the Midwest Classic Radio net on Saturday morning on 3885, the DX-60
> > net on Sunday morning on 3880, on Tuesday the Southeast AM
> > Radio net on
> > 3885, and the First Wednesday CCA AM net on 3880. Believe me if it
> > sounded BAD I would get chastized on the CCA net since I am the
> > coordinator of the event. (defense if needed) My 32V-3 is 
> under going
> > major surgery and some day when I am retired it will get 
> finished. We
> > encourage the use of anything on the CCA net that runs AM and I am
> > living proof that we abide by that.
> >
> > Anyone out there using a G-76 mobile?  I have the mobile supply and
> > would like to give it a try.
> >
> > 73 Larry WA9VRH
> >
> >
> >
> > On Thursday, February 9, 2006, at 06:44  PM, 
> k0ng at inebraska.com wrote:
> >
> > >
> > > Jim:  I agree with all of your comments as my G76 works 
> well as is.
> > >       Not that anything cant be improved but....if it 
> aint broke ..
> > >       Maybe in this case, there is a problem ??
> > >
> > > Regards to all,  Charlie,   K0NG .
> > >
> > >
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