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In Reply to: Re: Here is an old published example posted by mqracing on December 1, 2004 at 12:36:02:
Never said it was a comprehensive or convincing one. I also figured you had already seen it. I had also seen the article from Pearl. Up till then I only knew them from tube coolers and never knew they made OPT's.This whole discussion reminds me of one we had here about a concertina phase splitters balance. I jumped right out there and said that since there was an impedance difference between the signal taken off the plate and the one taken off the cathode it couldn't be truly balanced. One can find numerous respected references that say about the same thing. But in the end they are wrong because they all fail to look at the whole situation and instead tried to look at each signal by itself as if the other one wasn't happening to the same tube at the same time.
I don't think we can get a proper understanding by trying to look at one tube at a time in a push pull circuit where they work together. So to me the point is moot. Frankly I'll just look in a book and see what plate to plate impedance is advised for a given tube and run with it. Usually going with twice what is advised for a SE(tube) configuration ends up being about the same. I guess I am easy and just go with what has worked for countless folks in the past.
Oh just saw this, Lundahl's data sheet states, "In class A each tube sees 1/2 and in class B sees 1/4 of transformers primary impedance". So I guess we might call that a second published source...of sorts. But really who cares:) I can see valid points on both sides. Certainly the majority of the literature says Raa/4. And anyone who wants to say Raa/2 has to also say "only under strict and ideal class A operation". However I do agree that when you pull one tube in the pair the remaining tube simply has to work into a different impedance. I have seen AB1 amps run okay with one dead tube up to a certain volume level. I mean you could certainly hear something wasn't right but they did function.
In the end all that matters to me is the ability to pick a suitable transformer and I don't think that's really all that much of a problem. Triodes do well with quite a wide range. Frankly it has always seemed to me that pentodes would actually do better with a much higher impedance than the norm but I reckon trying to make a 100 watt 20K transformer just isn't feasible.
And hey, again, please accept my apology regarding the post about Damir's moniker. All I can say in my behalf is that I was mistaken and should have looked more fully into the matter before accusing you (or anyone for that matter) of such a thing.
One of these days I need to ask you some questions about grid chokes when fixed bias is used. I assume your forum is okay for that?
Russ
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Follow Ups
- Well, you did ask for a published reference:) - Russ57 10:23:13 12/02/04 (0)
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