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In Reply to: RE: conjunctive filtering posted by dcal40@aol.com on August 01, 2016 at 10:58:40
I think that circuit would be sensitive to the specific output transformer, and probably to the speaker as well.
It looks to me like it is there to assure stability in the presence of feedback. With beam tetrode or pentode, the plate impedance is very high, so the voltage gain increases at high frequencies due to the transformer's leakage inductance and/or the inductive component of the speaker impedance
Normally if the tube is triode-wired the plate impedance is low so this is not a bad problem. Also most SET designs have no feedback.
Follow Ups:
Paul, Thanks for that great explanation. In my experience the effect of this output filter appears to have a similar effect as using a zobel network on a cone speaker. It tightens the sound at the expense of some excitement. I have used a 5K resistor in series with a .01mF cap, both in parallel with SE transformer primary. The filter seems to tame ragged frequency response and it seems that the amp can drive the load with less tubby flabby loose low end with a somewhat rolled and "tame" HF. ( does it stabilize output impedence???). I have mixed feelings about the result but overall it is somewhat beneficial with my 6SF5 RC coupled to triode strapped EL34 into a Tomiko 2.5K SE OPT into various speakers, Dayton PS220, Altec 420, infinity overture 1s etc....It seems like a useful tool for 'scrapbox' amps made from less than exotic parts. Input from other enthusiasts welcomed, Thanks, David Calvello
Edits: 08/01/16
I have not seen the measurements for your speakers, but from my limited experience, this is mainly associated with single driver speakers. My also limited experience with zobel networks is that they induce roll-off at the high-end (some highs are lost). Is that related to the taming effect you wrote about?
On a different subject. I have single driver speakers, but they are crossed over to a ribbon tweeter and I don't hear the jaggedness in the treble. An example is Tonian Labs speakers: relatively high sensitivity (95-97dB) and flat impedance curve > 6 Ohms all the way.
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