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Thread: 6973 in Place of 6V6

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    Member HaroldBrooks's Avatar
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    6973 in Place of 6V6

    Hello to all. Just received two 'Gold Plated' 6973 to 6V6 adapters yesterday, and nice results from the swap !

    I experimented with running the 6973 pair in 2 of my vintage amps, namely a 1949 Vintage Supro supreme, an even older 1941 Supro Model 50. Both amps have already been modified by me to raise gain a bit, lower the lowest frequencies of Bass their character as a blues amps, but Both scream when driven hard up front with a parametric EQ as rock amps.

    Here's my 'Qualitative' assessment of swapping in the 6973 pair : More upper treble, a bit more of the lowest bass, clearer distorted chords, and a type of tone phase shift on some bent notes that is very cool, and was there a lot less using the 6V6 power tubes. So all in all, it's a better setup for 70's - 90's rock, and easily reversible. The change is not huge, but present and audible, as well as the changes to the response.

    I like having this option.

    All comments Welcome !

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    Original 6973's are hard to find. I have a pair i used in a Supro style build and they do have a sound a bit different than regular old 6v6's.

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    Bent Member Chuck H's Avatar
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    The specs read like it's basically a different pinout 7189 tube. Which is basically a souped up el84 tube. And that means most modern OT's for a pair of 6v6 would have a lower than ideal primary impedance. But I couldn't say about those vintage Supros. Many vintage 6v6 amps did use a higher primary impedance than what we consider "normal" today.

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    Quote Originally Posted by Chuck H View Post
    The specs read like it's basically a different pinout 7189 tube. Which is basically a souped up el84 tube. And that means most modern OT's for a pair of 6v6 would have a lower than ideal primary impedance. But I couldn't say about those vintage Supros. Many vintage 6v6 amps did use a higher primary impedance than what we consider "normal" today.
    The 6973 was derived from the 6V6GT according to Vacuum Tube Valley # 15. It is certainly a beam tetrode and not a pentode. It has a higher plate voltage capability than the 6CM6 and the gm is a little bit higher too, but I'd say aside from the higher plate voltage capability and the smaller bottle size it is closer to 6V6 specs than that of the 7189/EL84.

    Greg

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    Bent Member Chuck H's Avatar
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    Yes. interestingly it's described as a beam pentode.?. And the required bias voltage is certainly lower than 6v6 types at similar operating conditions, though higher than 7189 and el84 tubes. The example anode to anode test loads are also more similar to the 7189 and el84 type tubes. I do know that people liked them then and they like them now. Good that they can be had then.

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    Wasn't EH making some modern ones?

    I have a pair of NOS ones that I plan to use in something one of these days, but I haven't tried any new ones. I guess some of the new Supros use them also? Probably give a 6V6ish sound with more highs I'm guessing, but what do I know.

    Greg

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    Member HaroldBrooks's Avatar
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    EH is selling tubes that they call 6973s, but physically their tubes are shorter, fatter, and have obviously different internal components. May be they are the same, but use your own judgment to decide.

    There are still plenty of vintage RCA and G&E 6973 tubes out there, and I scored a pair of RCAs that are pretty dang strong and work perfectly, for right around $40 for the pair on Ebay. The trick is not being too anxious, and wait for a good used set to come up on Ebay. I waited around a month, and then they appeared ! I took a slight risk because the pics were a bit fuzzy and the tubes had no boxes, but they were indeed tested as strong, and they indeed are as per comparison with my originals.

    There are a few 'Substitute' tubes like the 6CZ5 and 6CM6 and the plate voltage maximum on the spec sheet is only 350vdc, but according to some, the rating method for the 6CZ5 is a "Design Center Value" and it's supposed to be more conservative, so running them up to 400v is OK, provided you don't exceed the Plate dissipation, and of course watch for any red plating. You need to Jumper pins 1 and 8 on the subs to ensure you have screen grid voltage at either point and this is easy to do right on the socket bottom.

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    Last edited by HaroldBrooks; 04-02-2019 at 09:46 PM.

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    Bent Member Chuck H's Avatar
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    How do they sound clipped? Reading the spec sheet they seem similar to the 7591 (another "beam pentode") in that the bias voltage for a given current is somewhere higher than el84 types but lower than others. And the ideal anode Z is a little higher too. I've read that 7591's have great clipping character. On par with the best old el34 types. Now I'm wondering about these 6973 tubes.?. I have a couple of amps that run about 365Vp on el84's. The Primary Z is right for the 6973 tubes and it would only require a rewire of the sockets to try them.

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    Hmmm... Disclaimer: I don't have a scope.

    My only experience with 7591s was in my Ampegs: a 63 Reverberocket, a 65 R-Rocket 2, a 66 R-Rocket 2, a 68 Rocket II (coincidentally exactly like a Reverberocket 2 in a different box & on a PCB), & a 72 GU12.

    The only one of those amps I would say had any great clipping character was the GU12. Reason I mentioned no scope was because I don't know where any of that clipping was coming from. But I wish my friend hadn't blown my GU12 before I could get the right speaker; I'd blast that thing on 10 all day if I could. Not so any of the others...

    Justin

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    Bent Member Chuck H's Avatar
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    well, grain of salt and all that. Ampeg guitar amps were typically atypical of how other amps might do some things. I'll look at the schems for the amps you mentioned and see if anything gels...

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    Given the 7591 was conceived & born as a hi-fi tube anyway... I don't think there are many amps that used them that were expected to distort anyway (Gibson?). Except the GU12. They took the AC12 & renoved the negative feedback loop. That's it.
    And that little beastie did period-correct Stones for DAYS.

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    "... If an older Boogie and classic Marshall had a (clearly illegitimate) child and you baked it in an oven set to clown shit crazy." - Chuck H. -
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    Bent Member Chuck H's Avatar
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    Well, being as I've been deep in the realm of el84's for a long while I've recognized how different they are from almost any other tube I build with. And I've often thought that it might be interesting to have a sort of crossover design that was a hybrid with characteristics in between the standard big bottles and el84's. Looking at the specs I sort of thought the 7591 might be such a tube and I've read good things about it.

    The amps you mentioned all use cathodyne type phase inverters. Not known for their refined clipped tone . I didn't get (or wasn't able to see) the primary impedance spec for any of the models. So I still wonder how the 5791's (or the 6973's) might sound in something like a Marshall or Fender circuit.

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    Refined shmefined... Boring! It's a guitar amp, not engine oil!



    Justin

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    Bent Member Chuck H's Avatar
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    To be honest I've rarely come across a production TUBE amp that din't have something to offer when cranked up. Some more, some less. Exceptions might be new uber gainers and some SF Fender models (NOT THE BASSMANS!!! Easy now )

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    Member HaroldBrooks's Avatar
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    Everything before the power section on just about every amp I own changes the tone to a greater degree than just swapping out power tubes, but there are some noticeable differences when using 6973 vs 6V6 or 6L6s... Let me see if I can enumerate what I have found :

    1.) 6973 tubes clip nice, but... they get quite a bit nastier if you bias them hot. Valco biased 6973 tube in their amp using a 250 ohm common cathode resistor for a pair. With a wall voltage of 123acv, this works out to about 16 watts plate dissipation per tube ! quite a bit over the spec sheet 12 watts, to say the least. Increadibly, some of the tubes still don't red plate even given this high bias, but the sound get's nastier in this range. I backed off my tubes to approx. 12 - 13 watts each by using a 350 ohm cathode resistor, and they run cooler, but they sound a tad nicer and clearer IMHO.

    2.) 6973 tubes seem to clip (in the Valco amp circuits) brighter and with more low base (a bit scooped) than 6V6's in the same circuit. Not sure exactly why, but if you dial back the volume control and hit the front end of the amp hard with a boost, the sound is 'Duller', and that's different that amps I own with a 6V6 or 6L6 pair.

    3.) 6973 tubes also clip with some type of low sub-harmonic that is non-musical in it's character (not an even harmonic). It's very, very subtle at first, just barely noticeable... but if you really hit the power section hard (based on the way your amp is setup), you will hear the sub-harmonic discordance more and more, progressively. It's an intermodulation distortion that in the extreme, might be considered undesirable and too gritty, but may also sound really cool ! Think the last chords in Led Zepplin's " What is and what should never be"... Page may have been using a Thunderbolt with 6l6's, but from the Valco amps I own at least, the ones using 6973 tubes sound the closest to his early recordings, regardless of what was actually done at the time.

    The fun starts at 3:31 https://www.youtube.com/watch?v=4JhldliZoLg


    4.) In the balance 6973 tubes do clip differently than 6V6 or 6L6s (to name two others), but the entire amp circuit they are in, combined with the guitar and rig you use will be important to getting the 'Correct' clipped sound for that whole combination, and it could be clear and bright, or nasty and dirty. Not all amps using 6973 tubes sound the same, and that can be easily said for any other output tube as well, but some amps using 6973 tubes sound phenomenally Dirty if the power section is driven too hard, and the tubes are biased hot enough to cook bacon and eggs.

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    Last edited by HaroldBrooks; 04-03-2019 at 05:51 AM.

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    Bent Member Chuck H's Avatar
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    Funny, I never realized how clumsy the stereo mix was for that track before now because this is the first time I've listened to it through headphones. But yeah, there's a noticeable modulation in the high bass/low mids. I guess I just figured it was a dirt box and/or a stationary wah. Page did a lot of things in the studio so I never tried to "emulate" his tone. If your amps are doing this, well, I don't think it sounds unmusical at all. Maybe it's a context thing and it does sound unmusical in the wrong key or with different EQ and gain settings?

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    Member HaroldBrooks's Avatar
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    Perhaps unmusical is an improper choice of words. It's a modulation that corrupts the higher fundamental tones, but yes, it sounds fantastically GREAT and still gives me goose bumps !

    But if I was playing something that required a smooth distortion-(the type that sounds like a smooth buttery cake as opposed to a broken up piece of toasted bread being coughed up), or if I was playing dissonant chords and I didn't want to create too much musical tension, that might be the wrong distortion... But it's a qualitative evaluation, and as everyone knows, beauty is in the eye of the beholder, and for that song, for that time the guitar sounds awesomely beautiful !

    That's why it's a good practice to use switches and pots to change values, so you can ride a spectrum of settings and easily get back to stock if you need to, and never say never !

    Some awfully ratty and small sounding amps sound huge if you get the right mics on them, the right reverb, and the right post amp EQ. It's a complex dish, that one thing is for certain and there seems to be a place for any amp or tone if the time is right.

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    There was some talk here recently about the musicality of the IM distortion being dependent on whether created by 50Hz vs 60Hz mains. Stands to reason that it would be important.
    Sorry I can't remember who here mentioned it, maybe someone from 'across the pond' ?

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    Bent Member Chuck H's Avatar
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    I remember that. And I also don't remember just where it is here. But IIRC it was said that 50Hz was key and 60Hz just didn't do it. Of course there may have been some proprietary hedging on that opinion

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    Quote Originally Posted by Justin Thomas View Post
    Hmmm... Disclaimer: I don't have a scope.

    My only experience with 7591s was in my Ampegs: a 63 Reverberocket, a 65 R-Rocket 2, a 66 R-Rocket 2, a 68 Rocket II (coincidentally exactly like a Reverberocket 2 in a different box & on a PCB), & a 72 GU12.

    The only one of those amps I would say had any great clipping character was the GU12. Reason I mentioned no scope was because I don't know where any of that clipping was coming from. But I wish my friend hadn't blown my GU12 before I could get the right speaker; I'd blast that thing on 10 all day if I could. Not so any of the others...

    Justin
    Everitt Hull, the founder of Ampeg HATED distortion and rock and roll and designed his amps to not distort. So basically most Ampeg guitar amps are designed with too little gain by most guitarists standards. 7591's and 7868's are electrically equivalent at least with old ones, not sure about modern, and they are similar to EL84/6BQ5 in that they have high gain and high sensitivity for a power tube. This means that you can drive them easily with a preamp with low to moderate gain, and a decent phase inverter design that can drive the tubes such as a LTP. The cathodyne doesn't quite cut it as well as the LTP or floating paraphase. Anyway, the 7591/7868's breakup smoothly with a lot of harmonics and touch sensitivity and a more balanced sound compared to a 6L6 which has lots of bass and highs but is sort of mid scooped, or an EL84 which is often lacking in the bass but has really nice highs and mids. The 7591 is closer to an EL34 that way, but since it is a beam tetrode and not a pentode it doesn't compress and clod up like the EL34 can do sometimes. They can distort nicely if that is what you desire but it would take more than just grabbing a preamp design and sticking it in front of a 7591 output section. Dial it in so the power amp distorts first, then the phase inverter next, then the preamp stage before the phase inverter, etc., and the amp will have tons of harmonics and a really nice touch response. I often think the only reasons the 7591 didn't become more popular were that they were at the end of the tube era, and the unique pinout meant people couldn't sub them into the same socket as a 6L6 or a 6V6 without rewiring. Power-wise they are in between both and have a sound that in some ways is better than both.

    Greg

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    Quote Originally Posted by HaroldBrooks View Post
    3.) 6973 tubes also clip with some type of low sub-harmonic that is non-musical in it's character (not an even harmonic). It's very, very subtle at first, just barely noticeable... but if you really hit the power section hard (based on the way your amp is setup), you will hear the sub-harmonic discordance more and more, progressively. It's an intermodulation distortion that in the extreme, might be considered undesirable and too gritty, but may also sound really cool ! Think the last chords in Led Zepplin's " What is and what should never be"... Page may have been using a Thunderbolt with 6l6's, but from the Valco amps I own at least, the ones using 6973 tubes sound the closest to his early recordings, regardless of what was actually done at the time.

    .
    Page used a Supro with 6973's. Many people thought he used a Thunderbolt, and it will give a similar sound, but it was a different model that he used and he finally said in an interview within the last year or two which actual amp it was that he used on the earlier Zeppelin stuff.

    Greg

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by soundmasterg View Post
    The 7591 is closer to an EL34 that way, but since it is a beam tetrode and not a pentode...
    Sylvania, Tung Sol and others are calling that tube a "power pentode" or a "beam pentode". I found no reference from anyone that made that tube designating it a tetrode. Of course what they called it and what it is may just represent sales trends at the time.?. It happens.

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    Quote Originally Posted by Chuck H View Post
    Sylvania, Tung Sol and others are calling that tube a "power pentode" or a "beam pentode". I found no reference from anyone that made that tube designating it a tetrode. Of course what they called it and what it is may just represent sales trends at the time.?. It happens.
    Someone oughta ... dissect a vintage 7591 and report on it. Next time I have one that's packed up, I'll give it the hammer. Unless someone beats me to it. Some tubes, you can see inside the glass & there's the beam forming plates. Or not. Not so easy to see in a 7591.

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    Sorry I can't remember who here mentioned it, maybe someone from 'across the pond' ?
    I remember joking that the 50Hz might make sound Marshalls better over here in Europe.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Leo_Gnardo View Post
    Someone oughta ... dissect a vintage 7591 and report on it. Next time I have one that's packed up, I'll give it the hammer. Unless someone beats me to it. Some tubes, you can see inside the glass & there's the beam forming plates. Or not. Not so easy to see in a 7591.
    Well if it's a "beam pentode" I suppose it'll have beam forming plate. So the question would be whether or not there's a suppressor grid? Which is of course the element that the beam forming plate was intended to eliminate, but maybe.?.

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    Blast & barbecue your infernal potato, I'm about to go open an RCA 7591A just to satisfy y'alls curiosity!

    Jusrin

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    Okay. I took my phone flashlight to a vibtage 7591A, RCA brand. There <IS> a beam plate in there, even though it has round holes that line up with the ones in the plate. So, yeah. Beam Pentode. No suppressor grid.

    Justin

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    Quote Originally Posted by Justin Thomas View Post
    Okay. I took my phone flashlight to a vibtage 7591A, RCA brand. There <IS> a beam plate in there, even though it has round holes that line up with the ones in the plate. So, yeah. Beam Pentode. No suppressor grid.

    Justin
    There is no such thing as a 'beam pentode' even though many manuals termed it as such. It is a beam tetrode since one of the 5 elements of the pentode is replaced with beam forming plates, thus leaving you with a tetrode.

    https://en.wikipedia.org/wiki/Beam_tetrode

    VTV in their last issue talked about the 7591 and said for awhile they were made by RCA with the same exact cathode as the 6BQ5/EL84 in it. The EL84 and EL34 were also made in some places as a true pentode and in others as a beam tetrode. The 6CA7 is similar to the EL34 but is a beam tetrode. Same with the KT77. The 6BQ5 is often an American made beam tetrode that is similar to the EL84 pentode that was often made in Europe. I personally like NOS GE 6BQ5's in my AC30 better than any EL84 type I've used, but they're getting harder to find and expensive.

    EDIT: Here's another link that I just found that is apt to the subject.

    https://www.youtube.com/watch?v=35cdWTQ0GI4

    Greg

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    Bent Member Chuck H's Avatar
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    Another for Justin because he sacrificed a tube. I almost did it because I do have a pair of pulls... But it IS a pair and I've never tested them. If they're good I'd have hated to waste them.

    EDIT: Actually, Justin said he used his phone flashlight.?. But I guess I'm uncertain if he just used it to look into the tube or smash it open

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    Last edited by Chuck H; 04-06-2019 at 03:58 PM.
    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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    Quote Originally Posted by soundmasterg View Post
    There is no such thing as a 'beam pentode' even though many manuals termed it as such. It is a beam tetrode since one of the 5 elements of the pentode is replaced with beam forming plates, thus leaving you with a tetrode.
    Yeah, OK, but who decided that the beam forming plate doesn't count as an electrode? It's connected to a specific potential (same as cathode, just like suppressor grid.) It influences the trajectory of the electrons due to it being at cathode potential. Why is it not considered an electrode?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Tony Bones View Post
    Yeah, OK, but who decided that the beam forming plate doesn't count as an electrode? It's connected to a specific potential (same as cathode, just like suppressor grid.) It influences the trajectory of the electrons due to it being at cathode potential. Why is it not considered an electrode?
    Pretty much agree with this on a semantic level, BUT... In science (and I think most technologies qualify as sciences) the original name for a given aspect (species, genre, genome, compound, etc.) is supposed to be used to the exclusion of absolute accuracy in the interest of provenance, referencing historic data and further study. In that light if the original name given to the design was beam tetrode then it should be used. I don't think those tubes with the moniker "beam pentode" from the manufacturer differ in operation significantly from previous designs called "beam tetrode. That doesn't mean the design ISN'T a beam pentode, it just means it's not called that.

    Just playing devils advocate now.

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    Quote Originally Posted by Chuck H View Post
    Pretty much agree with this on a semantic level, BUT... In science (and I think most technologies qualify as sciences) the original name for a given aspect (species, genre, genome, compound, etc.) is supposed to be used to the exclusion of absolute accuracy in the interest of provenance, referencing historic data and further study. In that light if the original name given to the design was beam tetrode then it should be used. I don't think those tubes with the moniker "beam pentode" from the manufacturer differ in operation significantly from previous designs called "beam tetrode. That doesn't mean the design ISN'T a beam pentode, it just means it's not called that.

    Just playing devils advocate now.
    I think GEC invented them and they originally called them Kinkless Tetrode, hence KT-66...

    Whatever. I may call them Pentodes from time to time if I feel like it, in the same way that I sometimes use an uppercase "K" for kilo. Come at me soundmasterg!

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    Quote Originally Posted by Chuck H View Post
    Another for Justin because he sacrificed a tube. I almost did it because I do have a pair of pulls... But it IS a pair and I've never tested them. If they're good I'd have hated to waste them.

    EDIT: Actually, Justin said he used his phone flashlight.?. But I guess I'm uncertain if he just used it to look into the tube or smash it open
    I DID NOT SMASH A WORKING VINTAGE RCA 7591A!!! Just so we are all clear. As far as whether it's a beam tetrode or pentode, well, I've seen some really funky $#!+€ in old tube manuals. I see 5 active elements in it. I also see tube diagrams where the yube is listed as a beam tetrode. I also see listings for beam triodes too. I shite you not. I've gone through RC30 plenty of gimes & I trust them...

    Whether it's a tetrode or a pentode, it curves like a pent & has 5 elements. As far as GEC & the KTseries, I don't remember which came firstbut as I understand it one was an answer to the other, without having to do a radical retool of the factory.

    Justin

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  34. #34
    Don't forget the joker g1's Avatar
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    Quote Originally Posted by soundmasterg View Post
    The 6CA7 is similar to the EL34 but is a beam tetrode.
    To further muddy the waters on the 6CA7 particularly, it seems there is a MkI and a MkII, the original skinny bottle type being a pentode with an american version numbering. Someone (philips ?) came out with a 'beam type' in the larger 6L6 style bottle. The beam type being preferred, and sometimes erroneously referred to as a 'real' 6CA7 when the skinny pentode is also a valid example.

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    I believe that GEC invented the kinkless tetrodes to get around a Phillips patent on the pentode design, but they initially had trouble with it, so since they were affiliated with RCA in the uSA, they forwarded the design to RCA who then made the 6L6 and the rest is history.

    Names do matter because beam tetrodes have better high frequency response than pentodes as a general rule, so if you were a designer of an amp you have to take that into account with your circuit and your layout. If someone tells you it is a pentode and it really is a beam tetrode and you don't take that into account and design your amp assuming it is a pentode, then you could run into problems. It probably makes more of a difference with beam tetrodes that are used for RF rather than the 6L6 per se, but anyway, that is the gist of what I was able to discover on the difference in the naming. At the end of the day, even though the beam forming plate might be considered another electrode by some, that design with beam forming plates is not the same as a traditional pentode with a suppressor grid and they don't behave the same either. So as I said, names do matter to avoid confusion.

    g1 thanks for the heads up on 6CA7 history. I only know about these in general but it doesn't surprise me that they made both versions. I prefer the EL34 to it in most applications, and also the KT77, but any old stock 6CA7 will be better than what we have today!

    Greg

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