[AMRadio] push pull output XFMRs and neg feedback

Thu May 14 22:10:03 EDT 2009

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" <jc at pctechref.com>
To: "Discussion of AM Radio in the Amateur Service" 
<amradio at mailman.qth.net>
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
>
>
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