[AMRadio] Gonset G-76

[Brett gazdzinski]

> > The stock audio driver is a 6CM6 beam power tube, and it 
> uses a 9 pin
> socket. This tube is identical to a 6AQ5 except the 6AQ5 is a 
> 7 pin tube as
> is the 6C4. Did you change sockets back when you added that 
> 6C4? Is the 6C4
> really driving the 6DQ6's?

My mistake, it was a 6S4.


> 
> > I think you will find that this is a neutralization issue, 
> and the NC
> setting for best stability seems to vary from band to band. I 
> recall that if
> you set the NC for 10 meters first, the other bands are all 
> good enough,
> with one exception,  40 meters is pretty far off. I'm not 
> sure why. Bypass
> capacitors often go through series resonance somewhere 
> between 5-10 Mhz.
> Above resonance they look like a inductor. You might also 
> look at the 6DQ5
> grid leak resistor, R12 at 27K 1 watt. I wonder if the best class C
> operating conduction angle is being achieved for plate 
> modulation? Since the
> 6DQ5 has a very high pervience, maybe you can increase R12 
> along with the
> drive level so that the "ON" state of the tube is short in 
> duration << 180
> degrees, and the "ON" plate voltage is very low < 100 volts. 
> This might
> boost plate circuit efficiency, and at the same time allow for higher
> positive peaks with better linearity. Only way to tell though 
> is to modulate
> with a triangle wave (speech amp must be pretty flat 
> however), or use a sine
> wave, and set scope for a trapezoid pattern (X:Y).

Next time I have it open, I will try different values of grid leak
resistance.


> 
> 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.
> 
> > This will vary from 6DQ5 to 6DQ5. What I said above applies 
> here too.
> 
> 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.
> 
> > This is pretty complicated stuff, and a approach taken by 
> many engineers
> that design audio amplifiers. Another approach for the folks 
> that rather not
> have their head hurt from such complications is to make the 
> circuitry flat
> within the feedback loop (with loop open) within say 50-5000 
> hz, and adjust
> the coupling capacitors within the feedback loop for minimum 
> phase shift
> down to 50 hertz. Set the NF gain reduction somewhere between 6-20db
> depending on the gain available, and how much NF can be 
> tolerated before
> ultrasonic oscillations kick in from high frequency phase shift in the
> transformers. Sometimes you can add a Bode pole (R-C) to cut 
> the gain down
> beginning at say 10 Khz at the rate of 6 DB / Octave to ward off the
> oscillation, and still have lots of NF over the 50-5000 hz 
> range. Once this
> is done, then you can define the frequency response in the 
> preamp stages
> which are outside the feedback loop. One likely problem with 
> this approach
> is that with lots of gain down to the 10's of hertz, the 
> power supplies may
> not effectively bypass themselves, and the audio will put 
> ripple on the
> power. This can lead to instability in the form of motor 
> boating. Always
> look at the power supplies, and evaluate their performance. 
> Stiffening the
> supplies may be as simple as boosting the bypass capacitor 
> value 5X (caps
> are small & cheap these days), or more elaborate means might 
> be required.

Yes, I could do a lot better here, but it sounds ok, a little 
punchy which is good for a low power rig.


> 
> 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%.
> 
> > Brett you did it! Now you need to worry about speech polarity, and
> negative cycle chopping of the carrier. I added a simple 
> diode -resistor in
> series with the modulated B+ to ground. I recall using two 1n4007's in
> series (cathode ends towards the B+), and then that in series 
> with something
> like a 3.9K 5 watt resistor to ground. This doesn't prevent 
> splatter, or
> over modulation, but rather insures that the modulation transformer is
> loaded (3.9K) when the negative modulation limit is exceeded.
> 

I had that circuit in the G76 at one time, but took it out
for some reason.


> 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.
> 
> > You might be able to work on that IC circuit to lower the 
> gain somewhere
> in the 3-5 Khz region to cut down on the hiss.

The last chip and circuit I used worked well, but since
radio shack does not sell chips anymore, I used what I had around.
I should put an order into mouser...


> 
> 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....
> 
> > Good idea if there are relay contacts available. I did 
> something like this
> on my Gonset G50 where the parallel Heising modulator / RCV 
> audio output
> tubes (2 X 6L6) would literally cook during prolonged RCV 
> time. I increased
> the cathode bias resistor during RCV to about 2X stock value, 
> and then on
> XMIT I bridged that resistor with another to get it down to a 
> little less
> than the stock value.

I used to have it on a set of relay contacts, but moved it,
I had put the stock built in audio setup back in when the 
IC chip blew up, which needs power all the time....


> 
> 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...
> 
> > Maybe we can work you on 20 meters AM sometime?

I can tune up one of the antennas on 20, I have a short 80 meter dipole
and a resonant 40 meter dipole, and a butternut vertical for 80 and 40 
(dummy load).

I was on 40 with the G76 for a bit today, but wound up in a qso
with people up and down the east coast, so switched over to the 812a
rig, a good time was had by all.
Lots of real radio type talk, about homebrew receivers, modulating
a B+W amp, running SSB rigs on AM, etc.


 Brett
 N2DTS