Hi Don: I have never read any info regarding this but my experience has been that there is a little better frequency response when a class "A" grid XFMR is loaded slighted with a resistor but not necessarily the stated "Z" that the manufacturer may say in specifications. I generally end up with a much higher resistance than that indicated by the Z ratio. I believe the idea is to prevent ringing and de-resonate the XFMR slightly. The load resistor will reduce the Q of the XFMR coil circuit. Series resistors from the secondary to the control grids also provide a de-resonating effect as well. This is especially true if the tubes being driven are triodes. There will be certain amount of 90 degree feed back from the plate of the tube being driven back to its control grid where the driver XFMR may resonate with it. This is the same reason that we use neutralization circuits on RF amplifiers. There were even HIFI amplifiers designed in the past using all XFMR coupling in push pull circuits and cross neutralization capacitors where used to neutralize the effects of the above mentioned feed back and peaked frequency response.
Consider however that the primary of the XFMR is connected to a device producing the current change, which will have a back loading effect of its own. In tube circuitry this would be the plate resistance of the tube. Not to be confused with the recommended load impedance. In triode drivers (12AU7 for example) plate resistance is typically much lower than with pentode drivers (6AU6 for example). The triode will have less gain as a rule but it will create back load for the XFMR to help prevent resonate peaks and ringing. The pentode can do the same thing with a little proper inverse feed back but again, this will lower the gain. But be careful not to build an oscillator accidentally. John, WA5BXO PS: I had sent this earlier but with the wrong account so I apologize if come through twice. I am experimenting with GMAIL 73 ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/amradio@mailman.qth.net/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:AMRadio@mailman.qth.net To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
