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

[Enzo]

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.

[Chuck H]

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.

[Justin Thomas]

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

[Bloomfield]

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

[Enzo]

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.

[Chuck H]

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.

[Enzo]

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.

[Bloomfield]

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

[Chuck H]

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.

[Enzo]

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.

[Bloomfield]

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

[Enzo]

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.

[soundmasterg]

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

[Enzo]

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?

[soundmasterg]

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