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Author Topic: Drive channel is too dirty!  (Read 7318 times)

Bill Moore

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Drive channel is too dirty!
« on: October 30, 2009, 11:28:29 AM »
I ended up with my nephews dead Champ 25, and replaced a few parts, and got it working again. I replaced the speaker with a Mojo heritage clone, and the clean channel is almost Twin clean, really a great sound. The problem is the other channel, even with the gain at zero, it is gritty. Looking at the schematic, there are no less than 7 diodes after the gain control. Has anyone bypassed/removed some of these, or replaced some of them with LED's, or maybe have another fix to get a cleaner sound from this channel? I would like to get it to sound like a single 6V6 amp dimed into a Weber vintage AlNiCo.
I think maybe here at the SS Forum, you guys will have a little more knowledge about the SS preamp, and maybe modified a few. My friend/amp tech, can really make a tube amp sing, but is ambivalent about SS devices. (Although most pedals are not tube!)
Thanks for your help, Bill.

J M Fahey

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Re: Drive channel is too dirty!
« Reply #1 on: October 30, 2009, 11:37:17 AM »
Quote
I would like to get it to sound like a single 6V6 amp dimed into a Weber vintage AlNiCo
Who wouldn't !!
If it could be made, surely they would already have.
Just to know what you are talking about, please scan and post the schematic you have.
Anyway, I think your best bet will be to build a pedal you like and use it on the clean channel.
Juan Manuel Fahey

Bill Moore

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Re: Drive channel is too dirty!
« Reply #2 on: October 30, 2009, 03:00:14 PM »
Thanks for the reply, I'll get my schematic scanned this weekend, and try to post it Monday.
Thanks again, Bill

Enzo

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Re: Drive channel is too dirty!
« Reply #3 on: November 02, 2009, 11:57:47 PM »
Too large to post here, but the whole thing is on Schematic Heaven

http://www.schematicheaven.com/fenderamps/champ_25_se.pdf

I don;t expect a lot of clean from a dirt channel to start with, and frankly, much as I like Fender amps and their clean channel, I have never hear a Fender overdrive channel I thought was worth a damn.


Sure there are 7 diodes in that one stage, but step back and look.  The ENTIRE dirt channel is ONE little dual op amp, and all those diodes are around just one side of it.   How sophisticated a tone can we expect from one op amp stage with diodes?  In my own opinion, you can mess with the diodes, but that will only change that part of the tone contributed by the diodes themselves.  Certainly I am not suggesting you don;t try though.

Does it help to set the gain lower and the volume higher?  As in giving the diode stage less to clip on?  Or does it sound crappy no matter what?

teemuk

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Re: Drive channel is too dirty!
« Reply #4 on: November 03, 2009, 12:25:49 AM »
See those resistive dividers within the clipping gain stage, (R27, R31, R30, R29, R28)? Change the ratio of those dividers and you can shape the clipping harder or softer.

J M Fahey

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Re: Drive channel is too dirty!
« Reply #5 on: November 03, 2009, 07:10:22 AM »
Hi Bill.
I fully endorse what our friends posted above and add: every amp designer has his own pet ideas, which he uses over and over.
That is usually good, but sometimes, if he is stuck with a poor idea, he anyway repeats that everywhere.
One such idea is that "dirty channel" design. I bet they know that simple diode clipping has its limitations, so , like an unsecure cook, they used all spices at once.
Cr2 and Cr7 are the classical back to back diode clipper, used by 99% of designers, and the basis of great classical pedals like TS9, MXR Dist+, Rat, and a 1000 others. Mostly odd harmonics, which are considered by many as "woody" or "muffled".
Cr1, Cr5 and Cr6 form an unsymetrical clipper, which produces an interesting amount of even harmonics, and sounds quite good in my personal opinion, it's the basis of some pedals like the excellent Boss SD1.
Cr3 and Cr4 form a half wave rectifier, almost pure even harmonics, they go overboard and usually give a harsh, buzzy, unnatural type of distortion. It's the (in)famous distortion used in Polytone amps.
As Teemu said, the resistors associated with them work as a passive mixer, and altering their values or ratio will change the type of clipping that predominates.
Read Teemu's excellent book where he explains all type of distortions and "tricks" used to make them better sounding.
GGG or Run off Groove suggest using a rotary switch between different diode configurations, to choose what you like.
After that, you have a mid-killing contour control and very pedal-like tone controls, which should better be labeled "mud" and "buzz" .... but you already know that.
Problem is, the "dirt channel" is in fact a distortion pedal , somewhat similar to a Marshall Shred Master, plugged directly into the power amp.
I guess you are not a Shredder.
The power amp is excellent (set loudness on 10 and leave it there) and I'm sure it provides *very* good crunch, body, sustain, etc. when used at high volume (7 or more) through the clean channel.
Very classic Rock/Blues tone.
The only problem is that we are talking live Club levels here, too loud
 for home use.
Maybe your friend can build an external tube preamp (rack or pedal) to be used straight into the power amp input.
If you want to experiment, short R30 (killing Cr2/7) and R27 (killing Cr3/4), and experiment with the equalization.
The sound with a single coil guitar will be acceptable, with a humbucker one will be somewhat muddy *or* buzzy.
Experiment and post your results.
Juan Manuel Fahey.
PS: in more modern versions, Fenders add a couple of Led clippers, but the general idea is the same.

Bill Moore

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Re: Drive channel is too dirty!
« Reply #6 on: November 03, 2009, 12:17:05 PM »
Thanks guys, I really appreciate all of you taking the time to analyze my problem, and offer advice. I am using the amp at low volumes,(actually in a spare bedroom). My old Airline gives me exactly the blues type breakup I like, I was hoping I could achieve a similar sound with the hybrid by tweaking the drive circuit a little. Like Enzo says, it could be a waste of time, but I intend to try your suggestions, and see what it sounds like.
I only got the little amp working for sentimental reasons, I am neither an accomplished guitarist, nor steel guitarist, but I enjoy playing both. I get as much satisfaction building an amplifier as I do playing through it, but learning to modify the amp to achieve a particular sound, I think is an enviable goal.
I will try your suggestions, and post the results, thanks again for your help.

joecool85

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Re: Drive channel is too dirty!
« Reply #7 on: November 03, 2009, 01:35:26 PM »
Thanks guys, I really appreciate all of you taking the time to analyze my problem, and offer advice. I am using the amp at low volumes,(actually in a spare bedroom). My old Airline gives me exactly the blues type breakup I like, I was hoping I could achieve a similar sound with the hybrid by tweaking the drive circuit a little. Like Enzo says, it could be a waste of time, but I intend to try your suggestions, and see what it sounds like.
I only got the little amp working for sentimental reasons, I am neither an accomplished guitarist, nor steel guitarist, but I enjoy playing both. I get as much satisfaction building an amplifier as I do playing through it, but learning to modify the amp to achieve a particular sound, I think is an enviable goal.
I will try your suggestions, and post the results, thanks again for your help.

I look forward to hearing your results, but you very well may end up needing to do as others suggested and just run a pedal in front of the clean channel.  I've never been real fond of any built in OD, although my Dean Markley K20X does have a pretty decent OD channel.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Brymus

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Re: Drive channel is too dirty!
« Reply #8 on: November 12, 2009, 08:21:08 PM »
See those resistive dividers within the clipping gain stage, (R27, R31, R30, R29, R28)? Change the ratio of those dividers and you can shape the clipping harder or softer.
Thats what I was going to say try changing R28 to a 100-250K
That might smooth it up into some classic rock type gain.
Edit - Or you could try a 100k - 1M pot for R28 and the one next to it 18K with a 20K pot then dial em to see what sounds good and measure the pots when you get the sound you want and replace with fixed resistors.
Also 1N5711 diodes sound alot smoother and less buzzy than the 1n4148s in there now.
« Last Edit: November 12, 2009, 08:27:37 PM by Brymus »

Bill Moore

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Re: Drive channel is too dirty!
« Reply #9 on: November 13, 2009, 07:43:59 PM »
Thanks, I wasn't aware of the difference in diodes. And yes, I intend to try experimenting with some variable resistors, as per teemuk's ideas. If I find a combination I like, I might drill some holes in the rear of the chassis, and install some pots there.
Another thing J M Fahey brought up was my guitar, I don't have a single coil pickup guitar, and maybe the thing will sound great with a Strat! I might take it over to my sister in laws, and have her play through it.
I've got to finish wiring my Twin chassis, and get it off the table before I tear this one down again. But I will report what I've done, and thanks to all for the information.