Portaflex B15N style preamp DIY

Hi guys,

Inspired via a search for a bass DI (e.g., R3DDI), had an idea about building an Ampe* B15 pre.

Came across this page:

http://www.freestompboxes.org/viewtopic.php?t=21081#p212078

From the OP:

"It uses a 12VAC input from a wall wart and then a transformer to step up the voltages for the plates. This is a pretty standard design choice with 12A*7 tubes because the filaments can run straight off of the wall wart voltage. However the B15N used 6SL7 octal preamp tubes. To deal with that issue a 12SL7 was used which by all accounts seems to be a 6SL7 designed to run on 12V for the filaments."

What do you guys think? Anyone build something similar?

My purpose is to use as a DI primarily for recording.

Cheers,
Chris

> What do you guys think?

Think about what? 12SL7 being a 6SL7? It is the same as 6V and 12V car tail-lights-- they do the same thing, you use the one that matches your car battery.

Offhand I only know one exception to this. 25/35/50L6 is NOT a 50V 6L6. RCA misused the "L6" here.

Hi Paul,

Sorry, I guess I should have searched around on the forum better:

http://groupdiy.com/index.php?topic=56536.0

Cheers,
Chris

PRR said:

> What do you guys think?

Think about what?

Click to expand...

About using it as a DI for recording?

Output impedance is probably higher than the guitar so you'll need to DI your DI box.

we built a transformer for a similar application,

cap input to a Neve core with a 4:1 ratio instead of 2:1like the line xfmr,

sounds pretty good, if you figure that thesecisabout 2K then z ratio is 16:1 so the tube plate will see about 2K*16=32K,

that is at 20 Hz so 40 Hz is 64K, etc,

that tone stack probably has less loss than a fender stack so there might be more output than the Alembic 12ax7,

waxscum said:

Output impedance is probably higher than the guitar so you'll need to DI your DI box.

Click to expand...

Cool, confirmed my suspicion on that. Thanks!

> using it as a DI for recording?

Gain of 40 * 40 * 1/10, 160, so half-volt pickup comes out at 80V. (!)

You can turn-down, but only so far.

I'd be inclined to try terminating the 220K output resistor with a 2.2K resistor, get part-Volt out at reasonable impedance suitable for Line In on console/interface.

have you tried the Gyraf G9 mic pre as DI box? a friend of mine say that it is the ultimate bass DI. 

Hi Paul,

Thanks.

Need to read up on this, want to learn. Found this so far...

Cheers,
Chris

PRR said:

I'd be inclined to try terminating the 220K output resistor with a 2.2K resistor

Click to expand...

Hmm... I have not. Something to look into. Thanks!

12afael said:

have you tried the Gyraf G9 mic pre as DI box? a friend of mine say that it is the ultimate bass DI.

Click to expand...

whay not build a 6N1P bass DI?

or stick a 12SN7 in there instead of a L7 for reduced gain, run it thru a big 4:1 SE transformer?

You mean like a R3DDI?

CJ said:

whay not build a 6N1P bass DI?

Click to expand...

Well, that's another option I did not know about!  ;D

CJ said:

or stick a 12SN7 in there instead of a L7 for reduced gain, run it thru a big 4:1 SE transformer?

Click to expand...

In the original schematic, it would probably be better to put EQ just after first tube stage, before volume. Then use tube2 as cathode follower output....?

Would require some recalculations, but component values seems a bit shaky already?

Jakob E.