[sdiy] Space Expander 7247 substitute

[Roy J. Tellason]

On Sunday 05 June 2005 03:18 pm, anthony wrote:
> I was wondering if anyone would have any good ideas about what might work
> in place of the 7247 in a Fisher Space Expander reverb amp. I have several
> 12AX7's and 12AU7's and some 12AT7's. I thought I'd start with the AU and
> then the AT. I doubt the AX would work as well and I've heard of people
> replacing them in some guitar amps with AU's to get a more mellow tone.
> That seems like a good analogy.
> I have a bunch of other twin triodes liek 6BQ7's etc. too.

My handy tube substitution book (!) shows 12DW7 to be a good one to replace 
that one with.

Regarding that other stuff,  they're not the same in spite of having similar 
pinouts.  I know off the top of my head that 12AX7 is a "hi-mu" triode while 
the 12AT7 and 12AU7 are "medium-mu" triodes,  having a lot less gain.  I'd 
need to find a tube manual to get other differences.  There were some other 
ones in that series,  too,  all the way out to 12AZ7 but they were a whole 
lot less common.