kdawg

Help with tube preamp tone circuit
« on: September 23, 2008, 04:34:31 PM »
Hello... I have a tube preamp for line level signal that I'd like to change the tone shaping with (This is for hammond organ leslie). It's 12AX7 based and I'm new to Eagle so hopefully the schematic is OK (I drew most of the circuit, but not all and some values may not be 100% right):

http://picasaweb.google.com/funkymusic/Schematics?authkey=fcOVfqxFfv4#5249315456222326770

What I need help with is:
* What value changes can I make to open up the treble more? It seems to cut off too much high end for me even at pot in middle.

* More range on the bass control

* The midrange seems stuffy to me, I wouldn't mind the treble control being able to boost some midrange more too.

I'm just not familiar with the RC networks and how the treble/bass controls in this arrangement affect each other... any tips appreciated, thanks!

-kdawg


kafka

Help with tube preamp tone circuit
« Reply #1 on: September 23, 2008, 09:31:37 PM »
If you want to get an idea of how various tone stacks work, take a look at the Duncan Tone Stack Calculator

CJ

Help with tube preamp tone circuit
« Reply #2 on: September 24, 2008, 02:19:40 AM »
That 0.1 which is in parallel with the resistor to ground is the culprit on the high end.

Play with that first.
Snip a lead, then drop the value to 0.01 and then 0.001 to see if the treble comes up.
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

kdawg

Help with tube preamp tone circuit
« Reply #3 on: September 24, 2008, 02:52:38 AM »
Thanks, I will try that.

I played with the tone stack calculator, pretty neat. I couldn't model exactly, but from what I did do I think I will try changing the 47k to 100k, dropping the 4700pf to 2200pF, and dropping that 0.1 uF that you said CJ to 0.01 uf. I'll see what happens.

As long as it still filters out the brown note, I should be OK. :shock:

-kdawg

PRR

Help with tube preamp tone circuit
« Reply #4 on: September 24, 2008, 04:43:30 PM »
That 0.1u and 330R from output to V2 grid makes no sense. Is that really what is in there?

330K (and an explicit return ont he output winding) makes more sense, but I'm still unsure where it is coming from.

If you snip-out this network, and turn Treble to max, it is really flat from 0.022u+200K= 45Hz to infinity. Try that and be sure this is really your treble problem.

> More range on the bass control

More bass? That's always a problem. Less bass? Do-able, but brain pain.

> how the treble/bass controls in this arrangement affect each other

It is extremely wonky. I suspect there were a dozen guys back in the day who spent all their time playing with tone networks. James did a summary, but clearly there were a lot more variants than explanations.

Ah. Without that 0.1u+"330" output net, it is a re-drawn re-valued Fender BMT. 47K is the "slope" resistor. 18K is a fixed Midrange knob. The re-connection of the bottom of the Bass pot makes little difference.

Long-long ago I worked-out the Fender BMT on paper and sliderule. I do not want to repeat the experience. (I don't think you can still get the right drugs.)

kdawg

Help with tube preamp tone circuit
« Reply #5 on: September 24, 2008, 07:00:32 PM »
My mistake - I flipped things around on that side. Here is the new schematic with the output more detailed:

http://picasaweb.google.com/funkymusic/Schematics?authkey=fcOVfqxFfv4#5249725403395661906

I guess I'd less bass cutoff at the center bass knob position, but if I can fix the mid/treble ranges then that may be enough to make up for it.

-kdawg

PRR

Help with tube preamp tone circuit
« Reply #6 on: September 24, 2008, 08:52:28 PM »
> flipped things around

Now it makes less sense.

Is there a Factory schematic for this unit?


 

Related Topics

  Subject / Started by Replies Last post
5 Replies
2336 Views
Last post June 01, 2008, 03:11:08 PM
by tv
4 Replies
2174 Views
Last post November 02, 2008, 01:14:49 AM
by idylldon
2 Replies
1724 Views
Last post March 06, 2009, 08:59:38 AM
by mrphotodude
10 Replies
7844 Views
Last post July 15, 2010, 09:06:59 PM
by PRR