Welcome, Guest. Please login or register.

September 20, 2020, 01:07:11 PM

Login with username, password and session length

Recent Posts

 

Author Topic: 12ax7  (Read 7449 times)

Diablo_IV

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
12ax7
« on: July 17, 2015, 06:30:35 AM »
I am going to attempt to build my first guitar amp.  I have a transformer whos output is 24.5V RMS.  Would this be enough juice to bias a 12ax7 for the preamp stage?  I think all of the tube preamp schematics ive seen use like 400 volts to power the 12ax7 in the preamp.  Could I use a 12ax7 in a preamp with only 25 volts for a psu?  If it is possible what would be any negatives. 

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: 12ax7
« Reply #1 on: July 17, 2015, 09:02:45 PM »
Short answer: no.

There are a few weird designs with low voltage supply tubes, and some work, sort of, but not the way you expect.

In general they are useful only for distortion and even so, it's different from "real" tube distortion so it's more a gimmick than anything else.

24.5V RMS is enough for a nice 15W chipamp (TDA2030) and a preamp, all SS .

Study this schematic to have an idea, this amp is very good:


You can copy the power amp, which is quite simple, and add any preamp you choose.


Diablo_IV

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: 12ax7
« Reply #2 on: July 18, 2015, 02:35:27 AM »
cool thank you, I think I may use it.  So basically the tda2040 is acting as the power amp in this circuit. It also looks like its acting as a differential op amp. 

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: 12ax7
« Reply #3 on: July 18, 2015, 02:09:41 PM »
Cool  :dbtu:
Yes, TDA chipamps are a beautiful Italian idea, created by a genius, Eng. Bruno Murari.
Quote
The 2002/2003 was designed by Bruno Murari who is well respected for linear IC design. Google "The LM383 Story". The National engineer who designed that chip discusses all the issues in creating such an IC and admits that it is essentially a copy of the TDA2002. The National engineers had respect for Bruno.
There were chipamps before him, of course, but he managed to shrink it inside a 5 pin package, with no external components (save feedback and coupling) and made it work like, as you noticed, "a big Operational Amplifier".

You just build the datasheet example , the simplest version, in fact they suggest a PCB but of course it can be built on perfboard, or even you won't believe it, with NO board, there's a lot of DIY Audio fanatics who hate PCBs or any kind of boards, go figure.   :loco

This is one such amp.
Not the same, it's a 50W LM3886, but the idea is the same: the amp is so simple , just 2 or 3 caps, 3 or 4 resistors, that chipamp is attached to heatsink, legs are bent as needed, and parts are soldered straight to them, go figure:


Here's a stripboard TDA2030 amp:


for the full article:
http://www.paulinthelab.com/2012/05/tda2030-based-amplifier-veroboard.html
His "preamp" really does nothing, going straight from the 22/25k volume pot to the chipamp input is exactly the same, so skip it.
Mainly look at his design to get an idea on how a *schematic*  gets "translated" into a *layout*.

Here's one of my own stripboard designs, in this case a "bridger" , it takes signal input and provides 2 out of phase outputs to combine 2 amps into a double power, double impedance one (as in: 2 x 15W/4 ohms amps into a single 30W 8 ohms one):

Diablo_IV

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: 12ax7
« Reply #4 on: July 18, 2015, 10:14:54 PM »
so is there any need for a preamp with this chip.  IF so what would the goal be.  I also eventually want to install an overdrive channel, something with some extremely high saturation. 

J M Fahey

  • SSGuitar Global Mod
  • Legendary
  • ****
  • Posts: 4142
  • Chip Points: 429
    • View Profile
Re: 12ax7
« Reply #5 on: July 18, 2015, 11:46:10 PM »
Let's gom part by part, as Jack the Ripper said ;)  :

so is there any need for a preamp with this chip.
  IF so what would the goal be.
I understand your confusion, let's clear it :)

1) this amp is sensitive enough that you *can*  drive it straight from a guitar, no preamp needed.

1.5) the veroboard circuit posted adds a "useless" preamp.
Why useless?
because it :
a) has unity gain
b) its input is still a 22k pot ... so what did we earn? : nothing.
c) a useful preamp would allow high input impedance (200k to 1M) , drive a tone control, and add a volume control *after*  it, which would not load your pickups down.

Quote
I also eventually want to install an overdrive channel, something with some extremely high saturation.
And you can.
You can also switch saturation in/out as needed.

In these cases, to start withy something I suggest building an MXR Dist+  distortion pedal and use it as a basic preamp, it has no tone controls but excellent gain and volume controls and can straight drive the TDA2003 .
You can get all sounds from clean (Dist. set to 0) to quite distorted.
FWIW it was the *classic* distortion sound of the 80's British Heavy Metal, go figure,think Judas Priest, Iron Maiden, etc.
Not bad at all;)
And it's designed for 9V batteries, works like a charm fed from the TDA2003 12/16V supply.

So you can make a nice metal machine, 2 knobs, compact and portable, driving a couple 6" speakers and have up to 12W ... a *very*  usable amp.

Not bad for a first project, huh?   :dbtu:


for a stripboard layout:
http://i181.photobucket.com/albums/x108/kalamaca/distortion.gif~original

Diablo_IV

  • Regular
  • **
  • Posts: 13
  • Chip Points: 0
    • View Profile
Re: 12ax7
« Reply #6 on: July 21, 2015, 04:21:52 AM »
awesome thank you.  \m/