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Thread: Why would you do this? (6EU7, 12AX7, Bogen)

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    Why would you do this? (6EU7, 12AX7, Bogen)

    Hi, a friend dropped off a Bogen CHB-35A last week to see if I might be able to make something more useful to him out of it. No problem, it's a simple and interesting project and it's in quite good shape, clean and unadulterated. I figured I'd keep the existing tube compliment; I can work with with it and I like that the tube designations on the chassis will remain correct.

    My question though, is why would a designer choose to use a 6EU7 for the first tube, and then a 12AX7 for the next? Looking at the datasheets for both I can't see any real significant difference besides the pinout. The 6EU7 does have a controlled hum spec which the regular 12AX7 doesn't appear to, although a 7025 meets that same limit. Just curious if anyone can think of a reason.

    Andy

    http://tdsl.duncanamps.com/pdf/bogen_chb35a.pdf

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    Lifetime Member Enzo's Avatar
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    The 6EU7 was designed for low hum and low microphonics. Sounds like a great reason to use it as the first stage to me.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Enzo View Post
    The 6EU7 was designed for low hum and low microphonics. Sounds like a great reason to use it as the first stage to me.
    Right on. And 6EU7 if I'm not mistook is the pre tube in a tube Echoplex which some folks use as a little bit of a guitar booster. Must be good, don't argue with Jimmy Page... The only problem, 6EU7 are a bit scarce. You either have to pry them out of donor amps or old tape machines and the like. EH offer one but it's a bit expensive, $30 or so.

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    No doubt it's a fine tube for the purpose. Just seems like a strange design choice to use two nearly identical but incompatible parts in the same unit. I've never seen these two tubes used together before. Is the 6EU7 lower in microphonics than a 7025 or other 12AX7 variants?

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    Yes, that is one of its attributes.

    They are identical in some specs, yes, and incompatible in each other's socket. But in the product using them, the socket identifies it needs that tubes, and the typical user has no idea the tube innards are similar. All the user knows is something called 6EU7 goes in this socket, and something called 12AX7 goes in the other. In the real world it was never intended that tubes be swapped around. That is something we do here in our basements.

    Look at some PA mixer where most of the ICs are TL072, but the mic inputs use the 5532 instead. The two ICs are compatible in function and even connections, yet one has special performance capabilities that are useful.

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    don't forget the joker g1's Avatar
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    Back in the day, 7025 would have been like a premium 12AX7, with lower noise spec. and more expensive.
    For a time, Fender spec'd both 7025 and 12AX7 in different positions in their amps, so same thing there with more than 1 tube type.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Bloomfield View Post
    No doubt it's a fine tube for the purpose. Just seems like a strange design choice to use two nearly identical but incompatible parts in the same unit. I've never seen these two tubes used together before. Is the 6EU7 lower in microphonics than a 7025 or other 12AX7 variants?
    Some early 60's Gibson/Epiphone amps also used 6EU7 and 12AX7. At this late date it seems daft all right. Perhaps the design engineers were overamped on coffee, thought they could improve the noise figure by a couple dB. For some customers, especially those who like to roll various tubes in preamp positions, I change the socket wiring to 12AX7 style then they can drive themselves crazy trying to decide which is their favorite - hmm... Telefunken? Brimar? Valvo? Tungsram? RCA? GE? Sylvania? EI? EI? YO!

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    Supporting Member mozz's Avatar
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    Out in the garage i have a Bogen ch35 or 50? i converted into a guitar amp. Pretty sure it has 2 6EU7's and a pair of 6l6's. I gutted the wiring but left the filaments wired for the 6eu7's. I thought the pinout was in a much more sensible order than a 12ax7. I also noticed there were 3 filament wires, 2 for the filaments and a third that was grounded on only one end, that wire was twisted along with the other 2, i guessed that reduced the hum even some more.

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    Remember, amps were never made for tube rolling. They didn't build them thinking guys would buy a dozen different 12AX7s and fiddle.

    Don't overlook that low microphonics spec on the 6EU7.

    I don't see why it is so crazy they use more than one kind of tube in a circuit. As g1 points out, Fender used the 7025 for inputs due to its potentially lower noise. They used 12AT7 for the PI, instead of the now common 12AX7 there. That is three different dual triodes in one amp in many cases. In those days, ALL the tube types were readily available, and from more than one company usually. RCA, GE, Sylvania, etc all made most of the tube types. It is only today that amps are all made with only 12AX7s and power tubes. We didn't even get to 12BH7 and 6CG7.

    And if my SVT had 6EU7 instead of 12AX7, I wouldn't have them come in with the 12AU7s in the wrong holes.

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    The low noise/low microphonic characteristic would also be more important for a high gain microphone level input (as is the case here).

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    Quote Originally Posted by Enzo View Post
    Remember, amps were never made for tube rolling. They didn't build them thinking guys would buy a dozen different 12AX7s and fiddle.
    I'm way with you on that one Enzo. But there are some who must faff around endlessly in search of the perfect sound. A dozen? Oh no, fifty at least! Of course we know they'd be better off actually practicing their guitar but perish the thought. When they wear out the socket, amp comes back to me for service. $$ Ka-CHING!

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    Of course, NOW, but not 50 years ago when those crazy designers were at it.


    I work by the hour, so if some yoyo screws up his amp, fine with me.

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    Thanks guys, but I wasn't thinking about the situation today, or tube-rolling or any of that. Or, for that matter 12AT7's or 12AU7's or whatever; those are different tubes doing different things. Nor am I questioning why you'd use a 6EU7 at all. Maybe my question should have been "What advantage is there to using a 6EU7 over a 7025 in that situation?" and I just don't see one. It seems like better design practice to do as Fender did, so that at least if the need arose you could use a 7025 in the 12AX7 socket, or the other way around allowing for some extra noise. Sure, the end user may not have known that, but the factory and any repair shop certainly would have.

    Usually in manufacturing these decisions come down to money, so I thought maybe the 6EU7 might have been cheaper. The only contemporary price list I found (DuMont 1968) with all three tubes shows it to be considerably more expensive than either 12AX7 or 7025. An RCA list might be a better indicator but I couldn't find one. Interesting to look at nevertheless.

    Dave's Homemade Radio. Old Vacuum Tube Retail Price Lists

    So I'm still a bit stumped by it. Not that I'm going to change it now; I have a good RCA 6EU7 for that amp, and old ones aren't terribly expensive or hard to find. In any case this amp will live in a studio not far from me so no worries about breaking down on the road and getting stranded.

    Andy

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    When the amp was designed, using a different tube in the first spot wasn't as big a deal as all that. Tubes WERE what there was. You could go to your local electronics store and buy a 6eu7 just as easily as a 7025. Low noise and microphony have been explained as making VERY good sense for this design. As to the choice?.. It's entirely possible that Bogen had a million of them left over from a previous design and wanted to use them so they just included that tube in the later design. A practice Fender was also fond of. For a unit like that one it wouldn't expect to see service for MANY years compared with a guitar amp. And probably many more before it was THAT tube that needed replacement. In this light the convenience of swapping may not have taken priority over some other convenience, as yet and probably never to be identified.

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    Or could be they had a surplus of smaller sockets to use up.

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    You are overlooking that the 6EU7 ALSO does different things. Already said: a low microphonics tube. That alone is sufficient to warrant its use as an input tube. Or other high gain, low signal application.

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    Bent Member Chuck H's Avatar
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    The 7025 is spec'd the same way I believe. It's not 'just" a selected 12ax7. Or at least it didn't use to be.

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    I like vintage TS data sheets, they're straightforward and easy to understand. So I checked the 6EU7 & 7025. The 6EU7 gives WAY more detail as to applications - basically screams, "I'm a hi-fi and PA tube!" The 7025 just basically says, low noise. Given that you are working with a PA unit or some such, it makes PERFECT sense to me why they'd use one. Or anyone who was looking for a clean clear sound. Which was everyone but Marshall... ame Fender already had a sound established, and Leo probably knew not to screw withe a good thing, or even CBS at first.

    So maybe by the numbers on page 2 or 3 the specs are the same, hut maybe they never read past the description.

    Justin

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    Thanks guys. Justin, those Tung-Sol sheets are nice, more detailed than the RCA ones I usually go to first (mostly out of habit and the book usually being handier than the computer.) The part about improved separation between the triodes makes sense in this application, and looking at the schematic again I see they've gone to the trouble of decoupling the B+ supplies of the triodes from one another. Now it seems like a better choice of tube there; no better in my situation but in the original application, yes.

    Speaking of the schematic, what's that first 12AX7 stage all about? Or the second now that I look again. Both plates and cathodes have 220k load resistors with B+ supplied to each. The first stage has signal taken off the plate feeding the tone stack; the second also has signal coming off the plate into the P.I., and feedback from the speaker connected to the cathode. What's with the B+ supply to the cathodes? Never seen that before. There is also a conjunctive filter like you sometimes see between output tube plates, but it is just between one plate and the centre tap.

    This is turning out to be more interesting than I thought. Not a bad thing.

    Andy

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    Lifetime Member Enzo's Avatar
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    The B+ connection to cathode is a voltage divider. The 220k over 470 ohm on the second 12AX7 triode sets a brute force cathode voltage. Same with the 680 ohm on the first triode. They are not depending on the tube current to set the cathode bias. At least that is how I see it. I calculate about 0.7v for the first, and about 0.5v for the second.

    The 6EU7 is also running on grid leak bias rather than cathode bias. The tube selection may be partly based on that as well as the other factors. Note that the tube is running with only 43 volts on the plate. These are mic preamp stages. You could easily enough convert the 6EU7 circuits to a more conventional front end.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Enzo View Post
    You could easily enough convert the 6EU7 circuits to a more conventional front end.
    I sure would if this is going to be a guitar amp. And for that matter, it's not that tough to rewire the one socket to accept a 12ax7 while you're at it. Problem solved.

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    Lifetime Member Enzo's Avatar
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    Oh yes. I have been lobbying for the designers' choice to use the 6EU7, but I am not suggesting you keep it if you'd rather not. The AUX inputs may have enough gain for guitar, but if you want more, certainly rewire for 12AX7 and make a conventional gain stage there.

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    Thanks Enzo, it hadn't occurred to me that it could be a fixed bias setup; I've never seen that arrangement.

    The plan was always to rewire pretty much the entire thing, so the first tube is going to be two cascaded gain stages. I agree it would be easy to change to a 12AX7 there, but I may just stick with the 6EU7. Like I said earlier, it won't be going far out of my sight and part of me likes the idea of keeping the tube compliment as printed on the chassis, even though I'm well aware that most guitarists will blindly stick 12AX7's in any 9-pin socket they find. This guy won't though; he's careful with gear and he's owned some old Ampegs so he's aware there are other tubes out there.

    Andy

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Bloomfield View Post
    This guy won't though; he's careful with gear and he's owned some old Ampegs so he's aware there are other tubes out there.
    Yeah, he's got this then He might even appreciate (enjoy?) that it's a "special" tube designed just for audio and best suited for first gain stage. Special weshial.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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    To me the beauty of an old Bogen is you get a chassis with existing power supply. The transformers and tube sockets are already mounted. The tube sockets are at least already wired for heaters, and in many cases the stage wiring can be at least partly used. For example the plate loads already go to B+ and the cathodes already have resistors to ground. Of course you may change the values, but likely those remain. Oh you might make a stage a cathode follower or something. I think a lot of the changes to make a guitar amp come between the stages. You have controls on a panel with labels that mostly will be useful. The power amp is pretty much OK as it sits.

    So you can keep and reuse quite a lot of it, leaving more or less just a preamp strip to wire up as you like.

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    I was just looking in there earlier and I was thinking more or less the same; that there were sections that could stay basically as-is like the power amp and the phase inverter and the tone stack too so I think I'll draw up the new circuit and swap out whatever pieces need changing. That seems a whole lot easier than gutting the whole thing all at once, not to mention easier to keep track of. I do enjoy the puzzle of figuring out layouts, but if someone else has blazed that trail I might as well follow it for the most part.

    Thanks folks

    Andy

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    That is why a PA conversion is such a popular project. Especially for a first one. So many approaches. You can try to get away with changing as little as possible. Or you can mostly gut it. In fact, one could strip it back to the transformers and start from scratch.

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    Quote Originally Posted by Enzo View Post
    one could strip it back to the transformers and start from scratch.
    Thats what I did on my Bogen CHB100. I should have just used a new chassis as they stuck both transformers right next to each other, Z mounted, and oriented the same direction, so it hums a little more than I would like, but it is too hard to change it now because it is all built up and works fine, though it was only making about 55 watts RMS with four 7868 power tubes and around 480V B+. I would have thought the power would have been more like 80 watts, but then I am probably not doing the 5% distortion level when I was measuring output power.

    Greg

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    Lifetime Member Enzo's Avatar
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    Just for science, stick a piece of sheet steel in between the transformers and make sure it is grounded to the chassis. Any help on the hum?

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    Quote Originally Posted by Enzo View Post
    Just for science, stick a piece of sheet steel in between the transformers and make sure it is grounded to the chassis. Any help on the hum?
    Yeah I did that carefully with some mu metal a fellow engineer has....amp quieted right up. Even regular steel quiets it up....though not as much as the mu metal, but there isn't room to place it in there unfortunately, and I don't want to change the circuit in the amp to allow the metal there or rotate the transformer, so I'm stuck with the hum. The hum isn't much worse than my 1956 Tweed Deluxe with its incorrect heater wiring (one side grounded) but I guess I'm anal about that sort of thing.

    Greg

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