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

[Bloomfield]

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

[Enzo]

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

[Leo_Gnardo]

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.

[Bloomfield]

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?

[Enzo]

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.

[g1]

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.

[Leo_Gnardo]

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!

[mozz]

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.

[Enzo]

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.

[The Dude]

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

[Leo_Gnardo]

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!

[Enzo]

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.

[Bloomfield]

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

[Chuck H]

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.

[g1]

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