> > 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
