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 Post subject: Farnsworth AC-70
PostPosted: Nov Sat 29, 2008 12:13 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 882
Location: Northeast Texas
I have a Farnsworth AC-70 and the output transformer has an open primary. :( The output tube on the radio is a 6V6 (single). I have located an output transfomer from my junk bin. It is from a chassis with a push-pull 6V6 output. The two transformers are physically the same size. The one from the junker reads around 130 ohms on each side of the centertap with my multimeter, while the Riders 11-8 (Farnsworth) shows the original to have 550 ohms DC reisitance.

Can I use this output transformer to replace the original? Will I use it full winding or centertap to one side?

Thanks for your help,


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 Post subject:
PostPosted: Nov Sat 29, 2008 4:03 am 
Silent Key

Joined: Jan Tue 16, 2007 11:48 pm
Posts: 9664
Location: Hueytown, AL
Try it. Most likely will work fine. Use whole winding. Sometimes PP transformers don't work well single ended but this is the exception rather than the rule.


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 Post subject:
PostPosted: Nov Sat 29, 2008 4:22 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
JK- I know we recently discussed this, but one thing I forgot to ask, so I will ask you.

If a person was to use a push-pull transformer in a single ended output configuration and only used half of the primary, then why could you not tie the other half of the primary thru a resistor equal to the load impedance of the one side to ground? I know some audio signal would be wasted, but that way the DC balance in the split primary winding would be maintained so you would not have core saturation problems due to the lack of an air gap.
Curt

_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Nov Sat 29, 2008 6:04 am 
Member

Joined: Jan Mon 01, 2007 7:10 pm
Posts: 96
Location: Grass Valley, CA
Hi Curt,

I think what you want to do is to match the bias current in the output tube through the opposite half winding; you most likely won't get that by simply placing a load resistance equal to the speaker load reflected through the transformer. Matching the bias current should null the magnetic flux at idle. This puts the transformer in the best situation for avoiding saturation (the way it was expected to operate).

A couple of thoughts...

Bias current in the output stage can be the dominant load on the power supply, possibly greater than all other biases added together. You need to make sure that doubling this current won't unduly stress the supply.

Keep in mind that loads presented through a transformer depend the square of the primary / secondary turns ratio. Using the half the primary to load the output tube will present 1/4 the load when using the whole winding. Theoretically you get the best result (best power transfer) when the load line presented on the output tube is optimized for it's operating point, although things work pretty good if you're in the ball park. Get too far from the optimal load line and you get reduction in power output and probably an early onset of distortion. If there are several impedance taps on the speaker winding it might be worth fiddling with the turns ratio to find the closest to recommended load on the output tube.

Dave.


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 Post subject:
PostPosted: Nov Sat 29, 2008 1:39 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34328
Location: Sandpoint, IDAHO 83864
I am not talking impedances here or what is reflected back to the primary by the speaker load on the seconday. What I was getting at was the same thing I, and many others have done for years in building plate modulated low power transmitters. For a modulation transformer, you use a center tapped primary winding and one outside leg feeds the modulated B+ to the modulated tube. The center tap provides the B+ connection. The opposite outside leg goes to the plate of the modulator tube, which runs class A. You tune the final to where you want it, and then adjust the bias on the modulator tube so the plate currents of the modulator tube equals the combined plate and screen currents of the modulated stage.

When the two tubes are equal in currents, the opposing static magnetic fields in each side of the primary winding cancell out, which gives fantastic audio quality, within the frequency response of the transformer used.

What I was simply thinking in this case, is to simply replace the modulator tube with a resistor to draw equal current from the unused primary half that was equal to the current flowing in the half that was used for the cancellation effects.
Curt

_________________
Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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