Yes, all the HI FI tube amps seem to have a balance control for the output tubes, some have jacks to measure the cathode current.
Very interesting about the cathode feedback. I am not sure why it would work better than the same amount of feedback from the secondary of the output transformer to a low level stage. Feedback in just one stage can run higher, and have more effect, since its not cutting the gain in the low level stages as much. If the low level stages are very clean and quiet, its a good idea. If the low level stages are not clean and quiet, it may give mixed results? I think you also get less phase shifts when only doing one stage, instead of the overall amp. Maybe each stage can have its own feedback loop, each adjusted to give the best results, feedback vs gain, noise, distortion, etc. Feedback from the secondary of the mod transformer to a low level stage can be very hard to do, with all the shifts and so on. In my 32V3 transmitters, I had to limit the high frequency feedback a lot, or I got vhf/supersonic oscillations. I would likely be much better off running a separate feedback loop around the mod iron and mod tubes. On a side note, adjusting the feedback values can be used as a tone control, the more feedback at whatever frequency reduces the output of that frequency. Brett N2DTS > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of patrick jankowiak > Sent: Thursday, December 04, 2003 9:06 PM > To: amradio@mailman.qth.net > Subject: [AMRadio] Push-Pull Parallel Tubes In Modulator > > > One thing of importance in hi-fi amps is separate bias adjustment for > each tube. I know we are discussing modulators, but also I think that > the best possible fidelity is important. So I'd like to offer some > comments about turning PA amps into hi-fi amps from my own experience. > I regret these are all pentodes (beam power tubes), so I cannot offer > much on triodes, although I have toyed with the idea of push-pull > 8025's zero bias class B with 300VDC on the plates and a 6-8K p-p > load. No sockets needed with these either, but I digress. > > In regards to AB1 operation, Another handy circuit enhancement is a > small (1-10 ohm) resistor in series with each cathode to ground (not > so easy with directly heated tubes). Such a resistor lets you measure > not only the resting current of each tube, but also the cathode > current when driven hard. At near-clipping, you can check the current > of each tube in the quad, and match them up, so each side of the > OutPut Transformer has the same power input. > > I wrote a short article about these and other mods to an old RCA > theater amp using PPP 807's. That old amp was originally class-A, > tube-hungry, and sounded more like a cheap tin can than a hi-fi amp, > but after the mods, it was pretty much an easy-listener, and the > necessity for matching tubes and high bias currents was eliminated. > Values are also shown for parasitic resistors, etc. > > I think the best improvement I have ever seen in a tube amp for > linearity in the face of odd loads and funny waveforms, is the cathode > feedback system where the cathodes each return through a winding on > the output transformer, where the winding bucks the individual cathode > current with a voltage equal but opposite to about half the grid drive > voltage. For whatever reason, this mod seemed to make the amp behave > as though its internal source impedance was very low. The output > seemed to regulate better. It works best when the output tubes can > physically deliver twice the normal current required. (more after this > URL) The best winding for this on 20-150W amps has been the 25VCT line > winding, or the 0-4-16 ohm taps (the '4' tap now becomes GND and your > 8-ohm speakers float). > > http://www.montagar.com/~patj/mi12188a.htm > > That said, the 807's can't deliver twice the current in that RCA amp. > So I got a great sounding amp with 40 watts instead of a mediocre but > loud amp with 70 watts. > > The following circuit shows a 50W amp modified the same way, but with > much larger tubes substituted for the originals, and after the mods, > that amp provided the same output to strange loads as stock, but with > much better linearity and fidelity. Also of note is the output iron > which was rather large on that amp. The larger iron might have made > better use of the extra current. In both cases of using a cathode > winding on the OPT, more voltage is needed from the driver to overcome > the error voltage. Also, the overall feedback loop (the RC network > which surrounds most of the amp) will need some changing, as less > overall feedback is needed once the cathode feedback is implemented. I > measured the gain of the amp stock, and when done, set the overall > feedback network's resistor to a value giving the same overall gain. > The network's capacitor was set to reduce ringing on square wave test > signals or to flatten or adjust high frequency response as needed. > > http://www.montagar.com/~patj/aph1050.htm > > About the need for extra current, The extra current takes place during > times when there might otherwise be amplitude distortion due to > irregular input waveforms and transients. i.e. the output tube's grid > drive says "go from 0 to 40V in 1 microsecond", but the OPT says "no > way I'm reproducing that!". Well, since the OPT lags due to its > inductance, there's no immediate bucking of the cathode of the tube > and the output stage power gain is not reduced until the OPT current > actually rises. Here's where the extra surge of current comes in, for > those short time intervals. This might all sound very strange, but you > can hear the difference in quality and you can see the action on a > scope, watching the cathode current vs. plate voltage during a > repetitive irregular waveform. (how this can be applied to a high > power modulator with no special OPT windings I have no idea yet) I > suppose in theory, the overall feedback circuit should do this but it > is not effective. This mode corrects things around the output > transformer only. In the 50 watt amp last mentioned, when you put a > 20Hz pulse with a duty cycle of 10-20% in, you can really hear the > transformer 'thunking', a testament to the correction going on there. > > One comment about the parasitic supperssors, it was at the peaks of > the audio cycle where I noticed most of the oscillation in my amps, > and the suppressors on plate, screen, ad control grid nailed it in > some cases, and in other cases, the value of the cap in the overall > feedback loop nailed it. Oscillation during these peaks can ruin an > OPT, and usually cause a large increase in tube current. > > Patrick > _______________________________________________ > AMRadio mailing list > AMRadio@mailman.qth.net > http://mailman.qth.net/mailman/listinfo/amradio
