Its phase shift that does that, and phase shift mostly happens in iron I think. It sure sounds like one transformer is bad...
Brett N2DTS ----- Original Message ----- From: "John Coleman" <[email protected]> To: "Discussion of AM Radio in the Amateur Service" <[email protected]> Sent: Thursday, May 14, 2009 6:21 PM Subject: [AMRadio] push pull output XFMRs and neg feedback > > This is maybe a little off subject here but I ran across an interesting > thing. It may be of interest for someone building higher quality speech > amplifiers of modulators for AM. > > While constructing two identical amplifier circuits with inverse feed back > for a stereo operation, I discovered that the introduction of the feedback > caused oscillations in one channel, so I checked my circuits to see if I > had wired some part of the push pull drivers or something in the output > different from the other circuit. I had not. But reversing the output > XFMR plate wires did correct the trouble for the most part. Now here is > what is interesting. These are standard old fashion Thordarson push pull > OTs for 6L6s. It has three wires on the primary and three on the > secondary. Primary is green=P1 Red=B+ Brown=P2. Secondary is > Black=ground Brown=8ohm Green=16ohm. I checked the XFMR with the ohmmeter > before ever wiring it in but I did not check its through put phase. But > the XFMR primary has to be wired different (that is the Green and Brown > wires of the primary must be reversed) in order for the feedback to be > inverted as it should be. > > I said this corrected the trouble for the most part but not altogether. > Now here is another phenomenon. I can increase the negative feedback, on > the one amp that has always worked OK, to reduce the gain a lot with no > oscillations. But on the other amp where I had to reverse the wires, I > can only reduce the gain a small amount before the circuit goes into a > high frequency (maybe 18000HZ) oscillation that is barely audible. If I > put the wires back where I thought they should go, I get about a 500HZ > oscillation immediately upon introduction of any feedback. > > The circuit is push pull KT88 grounded cathodes, 425 V plate supply, > regulated 300volt screens, RC coupled grids with separate bias pots for > each grid and set to about 40ma cathode current per tube. Grid leak > resistor is 100K from grid to bias circuits. Diver is a differential 6SN7 > with 47K plate resistors and with a cathode balance pot. > > Phase splitter is also 6SN7 - first plate is direct to grid of second > triode. Cathode resistor and plate resistor for splitter is 22K. > Appropriate B+ decoupling between stages. > > The feedback point is cathode resistor of first stage. Feedback take off > is from 8ohm connection of output XFMR. Appropriate decoupling and > divider between take off point and feedback point. > > This is a very common type of circuit used in many higher quality > amplifiers. This one just doesn't have the ultra linear OTs with the > screen taps and cathode windings. > > I may need to move swap the XFMRs from one channel to the other just to > test or prove if it is the output XFMR that is causing all this. But > before I do I will open the feedback loop and parallel the two channels > inputs. Then do a low frequency test to make sure of my wiring phase all > the way through both amp. I guess I will sync the on the input then go > dual trace to look at the paths in each amplifier. Then start sweeping up > input frequency to see where I get phase shifting that might cause this. > > Has anyone else found NIB / NOS XFMRS that don't match but are supposed > to. May Thordarson just never intended these to be used with feedback and > I just happened to find one that works OK but not the other. > > John Coleman, WA5BXO > > > ______________________________________________________________ > Our Main Website: http://www.amfone.net > AMRadio mailing list > Searchable Archives: http://www.mail-archive.com/[email protected]/ > List Rules (must read!): http://w5ami.net/amradiofaq.html > List Home: http://mailman.qth.net/mailman/listinfo/amradio > Post: [email protected] > To unsubscribe, send an email to [email protected] with > the word unsubscribe in the message body. > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Post: [email protected] To unsubscribe, send an email to [email protected] with the word unsubscribe in the message body. This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
