B15-N Portaflex style preamp

Dr Tony Balls · Post by **Dr Tony Balls** » 17 Dec 2012, 20:44

Hi all. Just sharing my build notes for anyone else. I've put together a single tube preamp based on the old Ampeg B15N. 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 I used a 12SL7 which by all accounts seems to be a 6SL7 designed to run on 12V for the filaments. I used a point to point layout because the whole thing is really too simple to do much else. Pics and correct schematic are below. Sounds great!

Greenmachine · Post by **Greenmachine** » 17 Dec 2012, 23:55

Geez Balls ... you just don't quit do you?

chicago_mike · Post by **chicago_mike** » 18 Dec 2012, 02:02

NICE!!

Where did you pick up the 12sl7?

You can cut off the extra thread on the transformer screw if you want. Most wire strippers , the wide flat ones should have that guage.

Super nice build!

ezugaru · Post by **ezugaru** » 18 Dec 2012, 06:15

I just had an bassgasm....i will built this for sure, much appreciated! Thanks thanks thanks...btw...any chance of soundclips?

P. S. Is that a 1590B enclosure???

Dirk_Hendrik · Post by **Dirk_Hendrik** » 18 Dec 2012, 15:18

Hope you don't mind me saying,
I really hope you come up with a safer and strurdier way to isolate and mount that 4-diode rectifier. Double sided tape is never a proper means of final board mounting as it will, someday, come loose.

Greenmachine · Post by **Greenmachine** » 18 Dec 2012, 15:26

Is this strictly for bass guitar?

Dr Tony Balls · Post by **Dr Tony Balls** » 18 Dec 2012, 15:31

Greenmachine wrote:Geez Balls ... you just don't quit do you?

HA. Well people keep asking me to build them shit! And this kinda stuff I can easily give to the community in terms of schematics and plans when im done with it.

chicago_mike wrote: Where did you pick up the 12sl7?

You can cut off the extra thread on the transformer screw if you want. Most wire strippers , the wide flat ones should have that guage.

Super nice build!

12SL7 came from Tube Depot. I love finding solutions in weird tubes like that because they're all of $6.95 for a NOS one. And yeah i could have trimmed up a lot of the screws...might still, but I guess I got lazy.

ezugaru wrote:I just had an bassgasm....i will built this for sure, much appreciated! Thanks thanks thanks...btw...any chance of soundclips?

P. S. Is that a 1590B enclosure???

LOL no.....thats a 1590D!

Dr Tony Balls · Post by **Dr Tony Balls** » 18 Dec 2012, 15:35

Dirk_Hendrik wrote:Hope you don't mind me saying,
I really hope you come up with a safer and strurdier way to isolate and mount that 4-diode rectifier. Double sided tape is never a proper means of final board mounting as it will, someday, come loose.

I'll probably rivet that little board down but that 3M outdoor stuff is pretty ridiculous. I use it in pedals all the time and it never comes up.

Greenmachine wrote:Is this strictly for bass guitar?

Nah, it sounds great with guitar! The bass control is a little bass heavy but you can roll it off.

ezugaru · Post by **ezugaru** » 18 Dec 2012, 19:54

Dr. Where did you get that transformer? I dont find it on mouser or digikey .... i like it 'cause is small... Thanks!

Dr Tony Balls · Post by **Dr Tony Balls** » 18 Dec 2012, 20:13

I got it from Hoffman, along with some notes on using it, but here it is from digikey: http://www.digikey.com/product-detail/e ... -ND/154789

ezugaru · Post by **ezugaru** » 18 Dec 2012, 20:19

Oh thanks! I was about to ask you if that was the same xD ... since i was looking for 7001PT specifically. Thank you

chicago_mike · Post by **chicago_mike** » 23 Dec 2012, 17:15

I just realized....

You stated people keep asking you to make them shit..and your last name is Balls...

So....you should make..

Dr. Tony's ShitBalls pedals!!

DrNomis · Post by **DrNomis** » 26 Dec 2012, 12:25

Isn't a 12SL7 similar to a 12AX7?, I seem to recall reading somewhere that the 12SL7 has the same mu as a 12AX7 except it's an Octal-Base valve rather than a Noval-Base valve......

If I'm right, I wouldn't mind making one of these.....

ppluis0 · Post by **ppluis0** » 27 Dec 2012, 22:28

Hi Tony,

Congratulations for your implementation of this preamp.

My two cents is to suggest that you lift the couple of 220 ohms resistor and connect them to a voltage divider taken from the last filter condensers in order to have the heaters elevated several volts above ground. As long as you use an isolated AC connector there is no harm to damage the walwart.

This way you reduce the filament hum and also have a discharge path for the HV supply condensers, wich is a safe measure.

Take a read at this explanation written by a forum member: http://www.freewebs.com/valvewizard/heater.html

Cheers,
Jose

delaystomper · Post by **delaystomper** » 26 Jan 2015, 01:27

Hi this a great project thanks for sharing it I'm currently building one that I very similar from the dough Hoffman
Site , it's the tube pedal preamp. I have a question .i want to xke balanced output
Like you did , would you kind enough and post the schematic drawing of that part? I was looking at the pics and it seems like You used a small transformer for the xlr output I would like to be able to connect straight to my PC Card or any interface
Thanks

teemuk · Post by **teemuk** » 29 Jan 2015, 16:44

I would use... (gulp) ...semiconductors:

You can power one dual opamp from the 12V filament supply, but naturally you must implement some rectifying and filtering to turn AC into DC. Make one half of that dual opamp a unity gain, non-inverting buffer, the other half an inverting unity gain amp. Now you have both balanced outputs and a low output Z "buffered" output that isn't too picky on what you connect this preamp to.

e.g.

The schematic is just an illustration of the concept. Do note proper biasing and coupling required by unipolar supply! For extra safety wire back-to-back diodes from each output to Vcc and COM.

That original preamp design has way too excessive output impedance, which also varies in interaction with tone control dials. It expects to see a very high input impedance in order to not color the tone or attenuate the signal too much. Unfortunately, while most -guitar amp- preamp input stages fulfill that clause about high input impedance most power amp input stages (to which you'd be connecting this unit) will not. Even less other devices... In a computer interface expect maybe around 10K input impedance. That's much, much less than the common 1M of guitar preamps. Therefore you practically need to buffer that output.

Pike of the Lake · Post by **Pike of the Lake** » 01 Apr 2015, 18:01

Hi everyone,

My apologies for resurrecting this thread. I'm quite interested in having one of these built for myself. Is anyone on this forum building them or do you know anyone who would do the work?

Many thanks.

Masuto · Post by **Masuto** » 26 Jun 2019, 03:43

Hi guys, all the images are down, any chance for a reup? Also, is this the amp used by Jamerson?

Dr Tony Balls · Post by **Dr Tony Balls** » 26 Jun 2019, 13:33

Sure:

Also, much more recently I made these EF86 boosts. Not the same thing, but its similar. Its a different approach to mounting the tube, and has a more standard approach to the power supply using 120VAC input.