Will a 12AX7 sub. for 12DW7/7247?

[blindbrett]

I recently bought a bogen pa amp and it uses both 12ax7's and 7247's in the preamp. My question is can I use a 12ax7 in place of the 7247 or do I need to change some resistors. I am asking simply to learn more about the tubes as I am learning how to repair tube amps and radios. Any help is greatly appreciated. Thanks folks

[Ty_Bower]

No. The 12DW7 is one half 12AX7 plus one half 12AU7. If you put a 12AX7 in its place, one half will be perfect and the other will be all wrong.

Maybe if you could track down a schematic for your Bogen, and determine how the 12DW7 is being used (pre-amp, phase inverter, tone circuit, etc.) it might be possible to figure out a solution.

[vince oldtubes]

12ax7 is hi mu both halves 7247 is 1/2 hi mu 1/2 lo mu


beside a difference of gain from 10 to 50, lo mu's have much higher bias... which means plate resistor around 10k not 100k

[blindbrett]

Thanks for the info. When I first got the amp, there were 12ax7's in two of the 7247 sockets and I though I replaced them with the proper tubes, I wondered if the amp worked with the 12ax7's in their place. I'm new and don't know these things so I ask. Thanks again.

[Ty_Bower]

It's good to ask questions. You can learn a lot about the way tubes work.

Here's a page where I like to go to look for the data sheets on a particular tube:
http://tdsl.duncanamps.com/tubesearch.php

You'll probably want to figure out whether your Bogen is single-ended or push-pull. If it's push-pull, you'll need to understand the different types of phase splitters. There's a bunch of good links in the "On Line Tube Learning for newbies..." thread. I like to read here for an explanation of phase splitters. He also has descriptions of various other tube circuits:
http://www.freewebs.com/valvewizard/index.html

[m6tt]

well...actually, sometimes it does _work_, just not very well. I wouldn't use a 12ax7 you were particularly attached to. Even better might be a medium mu tube like an at7 or an ay7...in the long run you will want to determine (hopefully from a schematic) what that tube was doing and why they needed a dissimilar triode there. You will want to change some resistors if you want the tubes to last and sound good. I'd say just order a $12 soviet 12dw7 off the internet.

[capnjuan]

Hi BlindBrett; noobie here. The 7247 is a disimilar triode; like a 12AX7 as a twin triode but the second 1/2 of the tube is built to dissipate considerably more plate voltage than the first half which, in nearly every other respect, is like a 12AX7. It's possible to use a 7247 in the place of a 12AX7 but not the other way around.

One common application for the 7247 is the driver stage of a guitar amp; the last tube 'feeding' the output tubes. Typical plate resistors for 12AX7s are 100K to 150K ohms; plate resistors for the 7247, particularly the 2nd stage, can be as low as 15K ohms at 2 watts.

if you put a 12AX7 where a 7247 is designed, its useful life will be shorten, performance compromised, and present some risk to your equipment.

Good luck with your project!