EQ/old amp mod question

Started by martinq312, October 14, 2006, 03:55:50 PM

Previous topic - Next topic

martinq312

I just came upon an old reel-to-reel recorder that I decided to transform it into a guitar amp after hearing my guitar through it. It sounds great, with a very nice crunch that is controllable from the guitar volume. However, it's much too bassy at this point, even with the tone control all the way up. Of course, the frequency range the tone control is centered on is not ideal for a guitar amplifier, so I'd like to replace it with a Marshall or Fender style EQ.The tone control is after the second gain stage (between the two 12AX7s). I downloaded Duncan's tone stack calculator, but I'm confused on the Zsrc. This is the source impedence, right? How important is this and how can I find this for the amp that I'm working on?

Also if anyone has any other insight, such as how easy it would be to add a gain control after the first amp. stage (the current "volume" control is right after the preamp stage) in order to have more control over the distortion from the amp itself, or how to incorporate a master volume, I think these would be pretty easy to do but I'm inexperienced at this and would only be guessing at pot values and such.

I've included the schematic so that you can see it. It's quite interesting (wherever it says 7025 it is a 12AX7 tube... I'm not sure what the significance of this is).


MetalGod

well you've got a decent starting block to start with... SE 6L6 output, 5Y3, three 12AX7s.  If you can't get a tone out of that lineup just give up playing  :D

I'd TOTALLY rewire the preamp using a typical tweed Fender front end.  Should sing without too much problem - checkout the schematics at the Fender Amp Field Guide for ideas.

:icon_cool:

martinq312

#2
Thanks for the advice. I didn't know about that Fender Field guide site and it's truly amazing.

PS: If anyone has any ideas on how I would go about taking the stereo (second) preamp and wiring it in as another gain stage that would be appreciated. It looks to be the same as the other preamp section. I've seen an article on doing something similar for old PA amps so I'm sure it can be done but guidance would be very helpful.

R.G.

QuoteI found this really great old amp that sounds incredible with guitar.
I want to completely tear it down and rebuild it with a completely new tone stack and rebuilt preamplifier so it will be better.
R.G.

In response to the questions in the forum - PCB Layout for Musical Effects is available from The Book Patch. Search "PCB Layout" and it ought to appear.

martinq312

I think I understand. It does sound really good and I may not come across anything like it again. Maybe just a little high pass filter can clear up my trouble or maybe I should just try it with a couple different speakers before I go and mess it up.

idlechatterbox

just to sort of play devil's advocate...

If you've got a really cool tone now, with the only problem being that it's too bassy, couldn't you leave it alone for the most part, and tinker with different speakers, cabinet configuration, and maybe splice in a DIY treble booster of some sort?

Not that there's anything wrong with wanting it to sound like a Fender (what's not to love?). But when you say it sounds cool now, I suspect that what you mean is, it sounds cool AND unique. If you've got cool and unique, it seems like you're already there.

Plus, there's the added cool factor of playing through an old-school tape recorder.

"dude... like, where'd you GET that amp... I mean, like, that tape player???"

Just a thought. But good luck either way. Every now and then I pass up an old tape player at a thrift store, so I'm jealous of you finding one that reaches to Tone Heaven.  :icon_biggrin:

wampcat1

If it was me, I would just put a simple hi pass filter before the 6l6 stage. This could be done fairly easily by just wiring a pot in parallel with a capacitor and put it after that .1uf cap.

bw


petemoore

If it was me, I would just put a simple hi pass filter before the 6l6 stage. This could be done fairly easily by just wiring a pot in parallel with a capacitor and put it after that .1uf cap.
  Probably not too hard to trace and find the beginning or middle signal path caps, and read value, then I'd probably 1/2 or less uf value them, if they were .1uf's I'd use .047uf or so. Switchable?...might not be alot harder than installing different value. I would bet theres a nice spot in the SP somewhere you could easily just 'splice' a cap right in...possibly a same value added parallel to an existing SP cap.
  If you've got a really cool tone now, with the only problem being that it's too bassy, couldn't you leave it alone for the most part, and tinker with different speakers, cabinet configuration, and maybe splice in a DIY treble booster of some sort?
  I would say tweeking at a nice safe 9v is something you'll be wanting to do anyway...Jfets, IMO do nice 'pre-circuit' tones.
  I'll introduce <'Risk of Lethal Voltages'> to this thread.
   
Convention creates following, following creates convention.