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tale of 7591a

11/27/2000 2:43 PM
Ken Gilbert tale of 7591a
this is a list-serv post by JC Morrison, with some interesting history on the 7591, for all you geeks that are interested in this sort of thing.  
begin forwarded post  
At 06:38 PM 11/26/00 -0800, Ed Coleman wrote:  
>I think Lord Valve had a couple of samples a while back, his view was he  
>wouldn't comment 'til he had a chance to crank 'em up , spill beer on 'em  
>and treat 'em like a typical guitarist might (did I offend anyone?) , I  
>don't have his address,its somewhere on the web , a quick search will find  
>'im . Ed  
all respect to ed and fat willy (aka "lord valve"), but man... why does anyone have to have approval before checking these things out? or anything else for that matter? i have to say that my last peek at what is going on in the joenet has me hell-bent on putting cathode followers in everything. there is nothing quite like dumb blanket statements like "cathode followers suck" to get me pumped up on em. hell, if so many people are up in arms, there just HAS to be something great going on there. either that, or all those experts who've eightysixed em out of hand, might still have a little work to do? no couldn't be... i forgot! everything's been all figured out. just use a transformer! or was it an op-amp?  
this is a perfect segue into a little tale about 7591A's/7868's: if it is not generally known, who among you know why they were ever made in the first place, and why they disappeared so quickly even after RCA did such a massive marketing campaign to cram em down the early american consumer electronics manufacturers grundgy throats (whether they wanted them or not)? huh?  
it all began with philips and the patent for the audio power pentode. some of you may be aware that no audio power pentodes could be made in this country without paying philips a royalty... and thus us americaninos were forced to live with the beam power tetrode, because shit if we gonna pay them a dime. we won the god damned war for crying out loud! 6L6, etc. etc. etc.... well, for the most part, us cowboys in the new country were more than happy to do without the suppressor grid in our output stages. but something happened in the very beginning of the sixties that sent tremors far and wide through the halls of RCA. it was called the EL-34. and all of a sudden, that pesky upstart goniff mashugganah, sol marantz, actually let sid smith give up on the good ole US of A and buy euro! jeez louise. and if that wasn't injury on top of insult, that jerk fisher, and that creep hafler, figured sid might know what he was doing and they started making amps with those damned skinny little fairy tubes too! it just ain't right! so the machinery down in darkest jersey cranked itself up into a fever pitch. "we got to get us an american vacuum tube that'll compete with those friggin snobs over in england, holland and (gasp!) germany", they said, and "whats more, if they (american consumer electronics manufacturers) don't buy em, we won't sell em anything else that they want, so there!" so, they mixed a little sweep tube scifi in with some octal audio packaging and, VOILA', 7591. it was prone to high frequency oscillation so they rotated the plate 15 some degrees and presto chango, 7591A. marantz was too small and too idealistic to buy the amazing new EL-34 like beam power tetrode made by RCA right here in the goddamned USA but fisher was talked into it, as were others. the higher transconductance, lower impedance and lower distortion EL-34 (and EL-84) made it easier to make simple circuits that sounded and performed exceptionally. this was now also the case with the 7591A, which sat sort of in between the EL-34 and EL-84 for power. all should have been fine and dandy, except that MO valve (secretly together with tungsol) got foolishly ambitious and sick and tired of losing at hardball with the bastards in eindhoven. they managed to sell the idea of massive amounts of raw audio power available only with KT-88's (6550's). and suddenly, there were too many options. it was the beginning of the wildest marketing race in the history of modern industrialized business. from about 1962 to 68. very similar to the car business then, or computer hardware trade now. some pitched power, some convenience, some the latest tech (transistors). even marantz was doubling up on EL-34's to stay in the arms race. everyone had to have something "new" every year. the 7591A got lost in the shuffle. it just wasn't remarkable or even better than an EL-34. this wasn't something that only flopped for RCA. it happened to almost everyone else in some way or another. for a short minute, it looked like philips might actually end up with egg on their face just like everyone else! no way. kings of the cheap but good, philips ended up buying or putting all of their european competion out of business, including telefunken. they eventually bought amperex, sylvania and magnavox, who got radically over extended, and after the japanese destroyed the american television business (and RCA with it), philips wiped out anything left standing in the USA. only sony stands in its way to total world consumer electronic domination today. i'm getting carried away here a bit, and i've generalized a fair amount as well, but the point is that the 7591A was never anything more than a desperate local replacement for a EL-34. thats a fact. it always drove me nuts how sacred that tube is held in some circles. don't they know? its just a tube! and i still think that i'd rather have an EL-34! in any case, the most obvious replacement for a 7591A has always been an EL-34, except that it doesn't fit under the cover of a fisher 400. but try telling that to any of the folks that have amps that used them... ok, thats my venting for the day. don't forget that the 6CA7 is NOT A PENTODE! (its a beam tetrode!) and NOT an EL-34.  
i hope everyone had a happy turkey-day!  
end forwarded post  
11/27/2000 3:45 PM
Stephen Conner

"don't forget that the 6CA7 is NOT A PENTODE! (its a beam tetrode!) and NOT an EL-34"
I've got a pair of Japanese tubes labeled '6CA7' that are plainly identical to EL-34s. You can clearly see all three grids (and there is an obvious lack of beam plates) So I don't think it's as simple as that.  
Steve C.
11/27/2000 10:01 PM

Hi guys.  
I have a set of US tubes labeled "6CA7 / EL34", and they are beam tetrodes! They look much more like a 5881 than a European EL34!  
I know there are both US and European tubes with double lables, but obviusly there are differences!  
Also, in the "Valve Data Book" to my AVO MKIV tester, there are different settings for 6CA7 and EL34.  
On the other hand, in the GE's "Essential Characteristics", is says that the 6CA7 is a "Power Amplifier Pentode", while a 5881 or a 7591 are a "Beam Power Amplifier".  
Regards, "Edgar"

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