It sounds like you may have a shorted turn in one of them, NOS notwithstanding. It has happened to me, and it forms a tuned circuit resulting in a phase shift as it is an LC network now.
73 de W7TFO On Thu, May 14, 2009 at 3:21 PM, John Coleman <[email protected]> wrote: > > 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 > -- 2nd Parentheses 2:13 "And the slow of wit shall see satire and know it not." ______________________________________________________________ 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
