12BH7 and the Simple SE?

[Davec113]

Hi,

I built a Simple SE with a JJ ecc99 driver tube a couple years ago. I used the ecc99 because I have an active tube preamp that I like, so I didn't want as much gain as the 12AT7 would have given me. I changed one resistor (R13) to get ~20 mA of plate current from the CCS, otherwise it's the same as on the schematic.

The datasheet below has data listed with and without shield, are they referring to the internal shield that separates the triode halves? If so, does the shield exist in all 12BH7 tubes? I just want to make sure I'm not wasting $ on a nos 12BH7. Thanks!

http://www.classiccmp.org/rtellason/tubedata/12BH7.pdf

[HollowState]

Dave,
I have many NOS American 12BH7/12BH7As in my personal stock. These include RCA, GE, Sylvania, Tung Sol, CBS and some Euro types like Amperex and IEC Service Master. None of them has an internal shield. In fact, I can't remember ever seeing one that did. Curiously the data sheet you linked to is a "tentative" one. I suspect a shield was never really used, or so few times that I've not seen one. Also, the RCA and GE tube manuals do not mention any shield.

[Ty_Bower]

I've got a small assortment of 12BH7, and I've never seen a shield in any of them either.

There is a curious evolution in these small triodes. As best I can gather first there was the 6J5, which is a single low-mu triode in octal basing. Then there is the 6SN7GT, which looks like two 6J5 in one bottle (still octal). Then came the 6CG7 which appears to be a direct descendant of the 6SN7, except it came in the nine-pin base and has the shield between the triodes. I think they later discovered the shield really wasn't necessary and eliminated it in the 6FQ7. Otherwise, all these guys are nearly the same (at least on paper).

Next up we've got the 6GU7 and its twelve volt brother, the 12BH7. These two are not electrically identical to the first bunch, but certainly close enough that drop-in replacements usually work just fine. These tubes seem to have a strong reputation as superior alternatives to one of the more common low-mu twin triodes. Which brings us to the last, and arguably the least: the oft-maligned 12AU7. Again it's characteristics are not exact matches for either of the first two groups, but close enough. I guess the biggest thing it has going for it is sheer volume of production. There's a boatload of 12AU7 out there.

Assuming you can manipulate the base and wiring, and you've got enough heater current available, I'd think any of these tubes could be made to work in the same circuit. Some should clearly work better than others, but they should all work.

[Tom Bavis]

The data sheet refers to an external shield - most data sheets mention this, as it adds output capacitance (at least when there is no INTERNAL shield, as with 6AU6 and the like). The 12BH7 has all 9 pins used - none left for an internal shield.

[Davec113]

The reason i ask is, I was told the 12at7 (that the Simple SE uses in stock form) has a shield that allows the triodes to operate independently. One triode for left channel, one for right, and the shield would keep one channel from affecting the other. It looks like the pinout for the at7 is the same as the bh7, so I'm guessing that I was misinformed regarding the shield in the at7 and the bh7 should work fine in the Simple SE in place of my current jj ecc99.

Is this the case or am I missing something? Thanks for your patience

[Tom Bavis]

The 12AT7 doesn't have a shield between elements- some that DO are 6AQ8, 6DT8, 12DT8 (all very close to 12AT7 in characteristics), 6BC8, 6BS8, 6DJ8.

[rknize]

Some do and some don't. The Chinese 12AT7s I have seen do.

[Tom Bavis]

I have seen some 12AX7s that appear to have a shield between elements, but always assumed it was a vestige of a predecessor like the Russian or Chinese 6N2 (which DO have a pin to connect the shield).

[nige838]

Originally Posted by Tom Bavis

The 12AT7 doesn't have a shield between elements- some that DO are 6AQ8, 6DT8, 12DT8 (all very close to 12AT7 in characteristics), 6BC8, 6BS8, 6DJ8.

Could I use a 12DT8 in place of a 12AT7 or a 12AX7?

Thanks,
Nicholas G.

[tubelab.com]

The 12DT8 does not have a center tap on the heater. Therfore it will not work in circuits that operate on 6.3 volts, including the Simple SE. If you apply 12 volts between pin 4 and 5 it is similar to the 12AT7 except for the shield connection to pin 9.