adding diode drive to a Fender style amp

[phsyconoodler]

I have a friend who wants a diode clipping circuit added to his fender style amp.I have not done this before and would like to know if it will work.
  He wants the distortion added to the normal channel and I would add a gain pot on the back of the amp.
However,me being rather solid state challenged,I don't know how I would do this.I know it drops the signal down a bit,but it's the early distortion he's after and not necessarily maximum volume.something like what a 2550 marshall clipping circuit sounds like.
  anyone done this before?Any help would be appreciated!

[eleventeen]

Glurk!  That's like adding spinner wheels to a classic Aston Martin.

I'd try steering him towards a pedal-type of deal. Yes, it's another piece of debris, and yes, it has yet another wall-wart, but what he's/you're proposing will not be very controllable, both physically (stuck in the back of the amp) and sonically, not being able to pick the exact point (eg; level) in the signal where clipping begins. That's not even venturing into the discussion area of how harsh and edgy such distortion usually sounds. Or modding the amp. By using the external pedal, he also gets an extra gain control, movable to other amps.

By the time you can hear this type of distortion, it's usually considered pretty danged harsh, very solid statish sounding. My opinion.

I think you could easily be venturing into area where he will not be very happy with the results, which means, at the presentation-of-invoice moment, he's not a happy guy. My tendency would be to avoid the exercise, but that's just me being lazy me. 

[phsyconoodler]

I'm convinced that it will work with some massaging,but I may just add a tube gain stage instead.

[Jack1962]

yea another gain stage would be a much better method , if he is determined to use diode cliping try the clipping circuit in the Peavey Triumph is pretty simple , and it does the job.

                                                    Rock On

[phsyconoodler]

I looked at the Triumph circuit and I don't see any diode clipping in the preamp.
  Can you enlighten me?

[Geezer]

Here it is.........

[phsyconoodler]

Ah yes.
  Rather simple isn't it?I'm looking for more distortion than that simple circuit.Someone told me a diode bridge with 1n4007's would give it a softer clip.Then I read somewhere else about different colored LED's.
  I guess the biggest thing is where to put it in an AB763 circuit?

[Fresh_Start]

Doesn't that one clip only higher frequencies?

How about copying the Marshall 2550 diode clipper?
http://www.schematicheaven.com/marshallamps/25aniv_silverjubilee_25_50w_2550.pdf
I don't understand it well enough to tell you how to insert it into the Normal channel of an AB763, but why not?

Here are some other references:
http://www.muzique.com/lab/zenmos.htm
http://www.muzique.com/lab/tclip.htm

Here's an adjustable clipper that I'm going to try in my next build:
http://www.muzique.com/lab/sat.htm... well, it's one of those anyway.

As an alternative, how about biasing the first Normal preamp stage hotter (Ra 220K & Rk 1.8K for example) then using a MOSFET cathode follower to reduce the insertion loss from the tone stack?  Google for "MOSFET Follies"  That'll whack the second preamp stage a lot harder and might point you in the right direction.

I'm very interested in your questions because I want a diode clipper in my next build and there are so many alternatives!

Cheers,

Chip

[HotBluePlates]

Rather simple isn't it?I'm looking for more distortion than that simple circuit.

Believe it or not, distortion pedals are simply that simple circuit, but with some added circuits to roll off highs/lows and to adjust how big an input signal it takes to clip the circuit.

Hopefully, someone has the relevant sections of TUT 1 that they can scan and e-mail. O'Connor describes the derivation of the circuit in detail and explains the differences between using silicon diodes, germanium diodes and LEDs. But put simply, a silicon diode usually has about 0.7v across itself when forward biased, and LEDs are generally around a volt or 2 (depending on the color). So an LED would need a bigger signal to act on, or would clip less for the same size input signal compared to a silicon diode.

Doesn't that one clip only higher frequencies?

It clips everything that comes out of the coupling cap, but clipping creates a square-wave and mondo higher harmonics that can sound harsh. The cap to ground shunts much of the higher stuff and leaves a smoother distorted sound. The tubescreamer circuit does the same thing but in a different way because the clipping circuit is in the feedback loop of an opamp.

But adjustable clipping is part of the request. How to do that is in the TUT explanation. Because I know the theory of it but have never put a diode clipper in a guitar amp, I can't recall the right/easy way to make it variable, which is why I'm hoping someone with access to the book will be able to give you the info out of it.

[jjasilli]

If the problem is the square wave resulting from a simple diode bounding circuit, KOC offers about a dozen "solutions".  Theory:  Tube diodes offer a softer shaped not-so-square wave; but are costly.  The main difference is that the tube has greater internal impedance.  This can be mimicked easily by adding a series resistor to the SS bounding circuit.  The resistor is added to the tail end at the junction of the parallel SS diodes to ground.  If the resisitor is a pot, the circuit can be made to be variable.  If another pot is place before the diodes, input voltage can also be varied. 

Alternatively, a cap can be added in parallel to the 2 diodes, to soften the edges of the square wave.  See the later TS-9 Tube Screamer circuit.

However, KOC seems to like a string of 2 zeners + tail resistor.  Here, unlike regular diodes, the zeners must be placed in series. 

If the customer likes a particular pedal, then maybe build that bounding circuit into the amp.  In the end, IMHO, this supports eleventeen's notion to just use a pedal  --  or get paid by the hour to design this amp to the customer's satisfaction