EF86 plate resistance

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pasarski

Well-known member
Joined
Jan 29, 2010
Messages
292
I'm planning on building a mic with a EF86. I'm just little confused about the transformer choice. Internet seem to suggest about 7:1 ratio (U67,MK7,G7) but when I try to calculate it I end up with much higher ratio. I used U67 parameters for this load line (210 B+, 150k Rp, ~-2V bias) and no matter how I try to tweak the tangent I end up with ~35k plate resistance. With the 150k plate resistor in a U67 this equals about 28k output impedance, which needs about 12:1 ratio for ~200 ohm output.

6860070840_74e3df8d76.jpg


What am I missing here (something for sure)?
 
pasarski said:
Internet seem to suggest about 7:1 ratio (U67,MK7,G7)

Don't believe everything you read on the internet. Trust your instinct.

At least some of those mics have "high" output impedance, which is not optimal if you will be using it with a tube preamp that has a high step up input transformer for example. It's bye bye bass.
 
abbey road d enfer said:
Check the U67 schemo. The transformer is loaded by 475 ohms via its feedback secondary. And NFB actually reduces the output impedance.

I thought this must me relevant but the feedback scenario and it's effect on impedance goes over my head I'm afraid.
 
pasarski said:
I thought this must me relevant but the feedback scenario and it's effect on impedance goes over my head I'm afraid.
This is one of the basic consequences of NFB. Gain is reduced, as is output impedance.
Check http://en.wikipedia.org/wiki/Negative_feedback_amplifier
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/feedn.html
and finally Q5 there:
http://ptuece.loremate.com/ane/node/4
 

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