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stigla 28th December 2002 11:29 PM

How to get good Fuzz from a Guitar-amp project.

I'm currently breadboarding a guitaramplifier, sort of like a fender Combo clone...

My goal with this project is to have a nice amplifier with a good Clean sound, but yet having a nice Crunch / fuzz when I crank it to Overdrive.

So far I have got a very nice clean sound. But alas, the fuzz is lacking! I DO get some fuzz when I crank it, but this is not very dynamic, mostly being slappy and flat and boring. I think maybe I have too little gain to get a good cranked tone...

So, I was wondering, does anyone know how to get good warm fuzz in a tubeamp? My setup is pretty much like a Fender Pro Amp Blackface... 12AX7 --> Tone controls (~22dB loss) --> 12AX7 (2V bias) -->12AT7 phase splitter --> Output stage.

I've read somewhere that it is beneficial to have some sort of LP filters to filter out the high order harmonics to soften the fuzz, but my atempts with this have not been succesfull...

Maybe I would need another 12AX7 stage after the tone controls to add more gain an thus more Overdrive into the following 12AX7? Maybe a pentode?

Thing is, I don't really know how fuzz / distortion / Overdrive is generated in guitar amps, I just presume one drives a tube into clipping.

I've been studying tons of old and new guitar amp schematics, and though there is allot of similarity between many, there is just so many variations from a simple Fender circuit to a complex Mesa Boogie preamp, that my head spinns around and I don't know where to begin to expereiment to make my amp distort really nice.

So, I hope someone has experience on this subject.

I could post a message at, say, AMPAGE, but my experience from previous posts there is that there are not many "tech's" that really are DIY tube gurus that can aswer the above questions, but rather musichans that happens to know a little about tubes... No offence to anyone of course.

mjd_tech 29th December 2002 07:27 PM

Which output tubes are you using? What is the B+ voltage?

stigla 29th December 2002 10:33 PM

I EL34 with a Marshall 50W output transformer.

They have about 450V on the plates.

themadhippy 29th December 2002 11:15 PM

if i rember corectly putting a diode in the feed back loop is how many comerciall units produce the distortion

Lisandro_P 30th December 2002 12:23 AM

Lots of ways... the usual, old-school is simply by cascading gain stages. Usually 2 or 3 will give you a decent overdrive crunch with enough guitar level. You might want to try looking for "classic" tube amp schematics to see how these gain stages are interconeccted (some use voltage dividers, weird cap values to shape the sound, etc), but in a nutshell it's how it works.

As mentioned, diodes in the feedback loop do the trick too, through it's a different sound. Try two diodes in parallel, back to back. Using one regular Si diode and one Ge diode works better as the clipping becomes asymetrical (which is more like tube clipping, yet not completely there :) )

mjd_tech 30th December 2002 04:54 AM

Several possibilities. I am assuming we are not talking about a channel switching setup, just a single channel.

Phase 1:
- Increase the NFB resistor or eliminate altogether.
- Change phase splitter tube to 12AX7.
- Experiment with different values of tail resistor up to 47k. Increasing the tail resistor lowers the headroom of the phase splitter.
- Install a master volume control. use the type with the ganged pot after the phase splitter.
- Reduce the value of the preamp cathode bypass caps to 1-10uf. You want to cut bass response or else the overdriven sound will get muddy. Reduce interstage coupling caps to as small as possible. too small = no bass. too big = blocking distortion. try different caps and listen.

Phase 2:
- change the preamp to resemble the Fender AB763 reverb channel, basically adding one more gain stage. Reduce the value of the 3.3Meg interstage resistor for more gain.
- Wire the EL34 in triode mode.
- Run the EL34 in cathode bias.

I'd start with the phase 1 mods. I like about 6-10 db negative feedback. This should give you more crunch while still retaining the classic Fenderish "gling" clean tone. You can dial in the crunch with the guitar's volume contol.

Triode mode and/or cathode bias gives a softer, more compressed overdriven sound. some like it, some don't.

There are a zillion other preamp configurations, but if you want to preserve the clean tone, its hard to beat the Fender style preamp.

You can try diodes if you are after a buzzier type of sound. I don't like that sound myself. I like Fenderish clean with a bit of AC/DC - Early Led Zeppelin crunch based on picking dynamics and the guitar's volume setting.

tschrama 30th December 2002 08:41 AM


I've been there too. Trying to get some nice overdrive/fuzz from a Fender non-reverb clone. I didn't succeed. I think you really need a Fender reverb schematic. The extra gain stage will do the trick. Personnaly I really like the newer Fender Blues Junior, so I copied that schematic. You could try that too. My absolute favorite is the soldano/Mesa Boogie (very simmilar) type of circuit! Really nice articulate distortion!


A Proco RAT clone will do the trick very nicely too! Cheap and bood sound, but no tubes.

stigla 30th December 2002 11:40 AM

Wow, Thanks for all the input!!

I'll first say a little about my current setup, I've bredboarded.

I have no NFB, as this was the first thing I experimented with, I really liked the sound better without. More punch and slightly clearer, and the Tonestack seemed to "work" better also...

About the tonestack, I'm not using the "fender style" tonestack. I used Tonestack Calculator from Duncanamps and put in the values and saw that the response was very... rollercouster :) This led me to the descision to use a "Orange style" (or James as it is called in the TSC. A little tougher load for the tube to drive, but very presice adjustments of Bass and Treble.

This lead me to another experiment, I tried an active load for the first pre-amp tube driving the tonestack, namly a C4S, a Constant Current Source. I Had some suitable Transistors laying around so I thought I'd try it. I'm not really afraid of mixing Tubes and trans. when it comes to guitaramps...
well, I liked the results very much! More gain and the Tonestack adjustibles responded very much better than with a 100k plate resistor. The CCS makes the triode work at maximum gain, and the impedance driving the tonestack will be much lower.
I set the current at 1.33mA, just as in the schematic. Due to the Constant aspect of my Current Source, I also omitted the Bypass cap in this stage.

One other thing is that of the 0.001uF cap at the phasesplitter input, At first I just used a 0.01 cap, in good belief that it was a 0.001uF. Then I discovered the obious wrong value and replaced, and I suddenly realised how much 1 cap can alternate the sound! got rid of allot of "grumph" and "mud" in the lower regions, even got ridd of some hum!

A master volume was implemented from the begining in the form of a dual 220k log pot instead of the 220k grid resistors of the EL34's. No problems with this so far.

So now I'll try the different tips from you guys and report back later. Think I'll try the simple diodes first, but I really want that Tube crunch! (being a tube-amp and all...:)
I like that good ol' Zeppelin / Stones crunch! (the rythm guitar on "Bitch" in particular!)

I took a look at the AB763 Reverb amp, and the 10pF / 3.3M combo seems to be the trick, I'll definently try that, I've not seen thatone around in many amps, but I remeber seeing it in a VOX amp and wondering what it did. Though the 3.3M will give ALLOT of attunation I think, so I'll experiment...

One question; Is it best to have a preamp tube to distort, or having the Phasesplitter distort?

About getting rid of the 12AT7, is there any particullar reason for this? I really like the idea of having an non 'AX7 in the chain...

tschrama 30th December 2002 11:50 AM


I never liked the sound of a clipping phasespliter, be it split-load or long tialed pair. But it is not standart to make your phasesplitter go into clipping before your output valve do... except if you have a MV after the phaseplitter...

I also like preamp-overdrive better than output overdrive, much much much too loud :eek:


stigla 30th December 2002 12:09 PM

Hmmm, just peeked at the Blus Jr. schematic...

As I understand it, the Overdrive is generated BEFORE the Tonestack, controlled by a Volume pot, and then after the tonestack, there is a tubestage with a mastervolume controll...

What's that FET doing at the cathode of the first stage?

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