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In Reply to: RE: The question is why 200 henries? posted by Paul Joppa on May 30, 2016 at 18:17:27
"A recent, specific example is increasing the inductance for a 300B amp from 24 henries to 40 henries, both with a 3000 ohm output transformer. The bass improvement is clearly audible."
Paul, the rule of thumb I use regarding the shunt reactance across an AC circuit is that it should be no less than 5X the operating impedance at the lowest frequency of operation. 10X the operating impedance is better. In the example you cited, with an operating impedance of 3K ohms, XL should ideally be about 30K, although 15K is probably acceptable. The 40H choke in your example fails to meet this requirement, as it presents only about 5K. Making matters worse, it's in parallel with the inductance of the OPT, so the final value is likely to be considerably less. The difficulty in maintaining minimum required inductance is one of many reasons I've never been tempted to build parafeed amplifiers.
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Buy Chinese. Bury freedom.
Follow Ups:
Please see my comment below, about the nearly-resistive load that is possible with parallel feed.
What about a CCS in the plate circuit?A friend just did that with a 45 and loves it.
P.S. I know I posted this before but I don't think you can get enough inductance (either plate choke/parafeed or series feed SE) to play bass properly.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/07/16 06/07/16 06/08/16
The problem with a CCS (aside from my solid-state allergies) is its lack of energy storage. Even ignoring that, I just don't have any reason to go that direction personally. There are many excellent SET OPTs on the market that don't require a plate choke or blocking capacitor.
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Buy Chinese. Bury freedom.
Question, in a series feed SE OPT, how high does the inductive reactance need to be (vs. the reflected impedance) to keep the load line from being elliptical?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"Question, in a series feed SE OPT, how high does the inductive reactance need to be (vs. the reflected impedance) to keep the load line from being elliptical?". A lot higher than is usually available. 40-50henries still has the loadline looking like a smashed banana. JH
A series feed opt with 200h of primary inductance would have to have tiny primary wire to allow enough turns, then the dcr would be out the window and other parasitic losses as well. No way to have your cake and eat it too, hence the opting for different topologies in finals feeding. JH
I have been considering 211 push pull instead of se with hf filaments it wouldn't be a expensive amp to build and run. I am not sure how long the tubes last. I run my main amp 18 hours a day so running costs add up for me. Any one that has posted about a 211 push pull amp over the years(and it seems not many people do) has been positive about them.
Perhaps a Ccore would be the best option to get anywhere near enough inductance. I am thinking 160H would enough So halved across the transformer (is that what happens approximately?) that would work out in the ballpark of 80H which is what the best 211 trans have that I have been shopping around for. 211 is not the only tube I want to try but it is the highest plate resistance of the tubes I want to try. The ones that are higher I want to use in A2. Would 160H put in back into the possibility of fitting into the window of a affordable EI core like one done by heyboer?
From the Lundahl datasheet,
"We define Power Low Frequency Limit, FPL
, as the frequency where LP= R LOAD. (The reactive impedance of
the transformer equals the primary load impedance). At FPL, the output power is reduced to 50%"
I don't think that's right.
If you had a 10k single ended transformer with 80Hy of inductance and you were driving it with a 10k source then at 20Hz (where the reactance of the 80Hy is 10k) the power would be 50%. (10k//10k=5k)
But the 3300 ohm plate resistance of the 211 (for instance) would swamp that equation. So that aside,
What the 10k reactance of the 80Hy of inductance in parallel with the 10k reflected impedance will do is cause an elliptical load line.
With 160Hy the reactance would be 20k at 20Hz so 10k//20k=6.667k and that will still cause an elliptical load line.
And that elliptical load line won't just be for 20Hz.
When low frequencies are present, all the frequencies will be following that elliptical load line and elliptical load lines increase distortion.
TK's 5X or 10X rule would prevent a elliptical load line but I don't see how it can be obtained in a gapped transformer (or plate choke).
Parafeed, using a CCS for the plate load, would work (parafeed output transformer don't have air gaps and have plenty of inductance) but if a plate choke or series feed SE output transformer is used.....using them to play bass seems out of the question.
P.S. I know I didn't answer your question. I don't know how much inductance can be had without causing other problems. Whatever the answer is, I don't think it's enough.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Plan B LL9202 I have posted above a question about the LL920 am verses the silicon core. I am leaning towards going that route first. Probably a higher chance of success.Then I can give up on the parallel feed 211 and have wound a smaller choke for the a2 tubes.
Edits: 06/10/16
"No way to have your cake and eat it too, hence the opting for different topologies in finals feeding"
I agree and that was my point.
Single ended amplifiers don't/can't play bass.
I use SS for my woofers and SET for the mids and tweeters.
I think that's as close as it gets to "having your cake and eating it too".
Thanks.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
"Single ended amplifiers don't/can't play bass."
This I have to disagree with. My 211s work to 20 Hz with no problem. The Magnequest 10K SE OPTs are superb.
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Buy Chinese. Bury freedom.
I didn't know magnesquest made 10k trans. I never had a pair of magnequest trans. But the magnequest preamp trans I have are so much better then any other I have tried.
Edits: 06/11/16
The transformers I'm using are FS-100, designed for 211/845 amplifiers. I run them at 1KV and 70-75 mA in my 211 SETs. I've read that there have been a few different versions of these, including one for parafeed, all with the same model no.
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Buy Chinese. Bury freedom.
Thanks triode I found the blog too and I found the forum. People keep referring to trans that I know nothing about. I want a pair of decent trans.. I am tired of buying shit. I will wait until mike is winding something I can use and afford. How ever long it takes.. I just hope he will wind me a pair. It looks like he is getting busier and busier. I want to try find and research the specs of all the old trans that he winds too. I think they are somewhere on line. I really want a pair of classic push pull output designs with nickel lams or part there of. I will get around too the a big A2 amp eventually. I have enough other amps to finish in the mean time. I will try a a2 3c24 I have trans for that allready. If there any good. I have my doubts. They are soft tones I bought them from japan but they look like chinese to me.
I meant technically speaking. I'm sure subjectively your amps are fine.How much inductance is there in those Magnequest 10K SE OPTs?
Do they meet your 5X/10X rule?
It would take 400Hy to be 5X (50k ohms) at 20Hz.
The load line is going to be an ellipse. I don't think there's any getting around that.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/10/16
I get the sense that you think the reactance of the inductance of a plate choke in a pararfeed circuit is in parallel (shunt) with the reflected impedance at the primary of the parafeed OPT.I believe that is true.
I also get the sense that you don't think the reactance of the inductance of the primary winding of a series feed OPT is in shunt with the reflected impedance.
As far as I know it is.
The only difference is the caveat that you pointed out, the reactance of the plate choke's inductance is also in parallel with the reactance of the parafeed OPT but that would be a small factor. Otherwise both situations are the same.
Bottom line, I don't think a gapped transformer (or plate choke) can have enough inductance to keep the load line from being an ellipse at the lower frequencies.
That is why I say "SET amplifiers can't play bass".
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/10/16
I don't bother with that. I look for transformers that perform well in terms of frequency response and full power (low frequency) distortion in my application . If a transformer can do that, and it sounds good with my specific output stage, I'm done.
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Buy Chinese. Bury freedom.
Edits: 06/08/16
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