Guitar amp design

[SimonC]

Hi all,

I've been asked by a friend to build them a valve guitar amp. I've only done Hifi amps before and most of the reaseach I've done suggests that guitarists like their amps with distortion, under-rated output iron and wobbly power supplies.

Out of principle I want to do an SE, and there are a lot of good comments on the web about the Angela instruments super 6v6. Anyone had first hand experience of this?

Do any you have any favourite circuits and wisdom gained from designing and building this sort of gear that you could share?

Thanks

Simon C

[James D]

The Angela amps are fine and sound good. They don't do real overdrive but crunch up nicely and have good tone and resonable articulation. If you build it use the AskJanFirst 2.5/3K SE OPTs at €42 each. They are miles better than the price suggest and work well in guitar amps as does the 4.2/5K Se OPTS at €24 and €34 each - The more expensive one is a good hifi amp trannie to.

If you are after a more modern sound then look at the projects here - AX84
and here - Ampage

Both sites link to very helpful forums too.

DOn't believe the nonsense you will read and hear on non-specialist guitar amplifier forums and, in particular, take no notice at all about what is said about amplifiers on guitar and guitar playing forums... its 99% nonsense

So just to hit some on the head - its not about distortion, its not about sagging psus and its not about crappy output transformers... and all thses things affect the sound of the amp but if you design for them you will have a noisy, nasty, noisome pile of rubbish...

To understand how a guitar amplifier works you first have to understand how a guitar pickup works and what output signal it produces and at what impedance etc. and how that interacts with the guitar controls and the guitar cable before it ever hits the amplifer input jack. Read this article on pickups.

ANd this one on measuring guitar pickups response

ANd Donald Tillmans articles on pickups and the guitar

SO a good valve guitar amp (There are no good ss guitar amps - period...) is designed around preserving dynamics - both macro and more importantly micro and tone - it is the success with which a guitar does this whilst shaping the final sound that the amp produces that seperates the boys (Marshalls, Mesa Boogies) from the men (Dumbles, Engls, Matchless). So the goal is articulation and tone.

Then, of course their are varying degrees of crunch and overdrive (Note - not distortion unless you are after nu-metal or death-metal sounds in which case you don't need a valve amp at all use a crappy ss guitar amp and play away - normal metal doesn't require distortion just heavy overdrive - if you don't believe me listen to a good vinyl, valve, OB speaker setup playing Metallica, etc.) When you are crunching a circuit you driving the valve out of its linear (or quasi-linear) operation and into the heavily non-linear or curved part of its transfer characteristic - so you are introducing distortion products into the sound at significant levels - typically 10-40% distortion and you control the frequency response of the stage to limit you to 2 through to 9th or so harmonic i.e. you filter off the higher stuff - then you pass it onto the next stage to repeat the process and build up more distortion - this often means throwing away lots of signal to get the level back down into the acceptable input range for the next stage and so on to build up the amp topology. Interstage you can add tone controls to add to the sound shaping and add gain controls between stages to allow for more control over the cruch/overdrive each stage produces...

The output stage needs to be designed to be as good as possible but matched to the psu so that the signal is capable of fully driving the stage to saturation i.e. using up all the current the psu can deliver and the output stage use - it is this that gives valve opts their distinctive sound when driven flat out. To achive this PP stages are often run in deep Class AB2 or even Class B. SE stages are run in Class A and sound very different.

Oh you should read Aiken amps tech info too

I'll post some of my designs later - I realsie I only have them in my notebook not electronically...

James

[Greg.]

Here's a suggestion that Peter might pick up on.

There seems to be alot of knowledge available here from forum members on guitar amps and also a number of members who want to learn, share and discuss this particular aspect of valve amplification.

Would it be worthwhile setting up a seperate section on the forum to facilitate this?

I'm not a guitarist, but with the wealth of knowledge available here, I wonder if this could be an excellent development for the forum. I wouldn't want the emphasis to move away from the current forum theme, but of course, such a section, attracting DIY guitar amp enthusiasts could also be beneficial in introducing them to quality recorded sound reproduction and visa-versa, listeners to good sound might become interested in how that is achieved from the instrumental end.

It's just an idea.

What do you think?

Best wishes,

Greg

[Pedro]

Greg,

thats a superb idea and I fully endorse it. James - you beat me to all key thoughts on guitar amps !

A seperate section would be ideal so we can keep the core HiFi discussion uncluttered whilst responding to the keen interest in instrument amplification there will be from many members. Indeed my suggestion is that we set up a section under heading of "Instrument Amplification" so we include non-guitar amplification as well.

Typically the vast majority of guitar electronics sites out there ( and there are literally thousands ) tend to be US oriented, effect pedal-led , and if they are "tube" related, tend stick to very formulaic approaches ; 12ax7, 6l6, 6v6, el84, and el34s. Usually its PP designs but in fairness there is an increasing interest in simple SE amps.

We have an opportunity to use the leading edge experimentation from the experienced audio enthusiasts to shape some less conventional design and experimentation. A revelation for guitar players !

There are some very good resources out there amongst the dross - i.e. 18watt.com and the Twreck Page which have taken a couple of the best amp designs and generated huge discussion and experimentation in either recreating classics or building on the base design to generate something new and exciting. The Valve experimentation angle though is pretty non-existent.

Simon,

After building several marshall copies, Dumbles and Fenders, my suggestion would be to look into the 18watt.com and Twreck pages first off and read around. the 6v6SE is nice but my faves are the 18 watters and the Dumble-based designs. If you want to go 6V6 and can be persuaded to try PP rather than SE then a superb litlle amp is the Plexi6v6 by Mark Huss ( www.mhuss.com ) which is a great hybrid of an early Marshall and a fender deluxe. Ive built 2 and have been very happy with them for Rock/Blues stuff.

The Dumble-style amps are much more sophisticated tone-wise. Smooth and glassy cleans and much more refined overdrive channel for lead playing. Thats why the original amps (which have mythical status now ) are used by players like Larry Carlton and Robben Ford.

A very simple and quick start is a Fender champ 5E5 clone - I use one as a practice amp.
a few schematics and a good site......
http://tdsl.duncanamps.com/schematics.php

cheers
Pete

[James D]

Just found an early schematic for an amp I built for my son as a play amp. It uses any old ECF valve and a 12SN7 output tube but could be easily tweaked for an EL84 output tube.



I built it to teach him how to set up the gain structures through an amp hence the three gain pots

The first one sets the pentode signal and the Av of the pentode was set to overdrive with 0.5V in and then it goes into the triode which will also overdrive but with 5V or so in. Then it hits the tone circuit so he can play with how much top needs rolling off and mids to boost/cut etc. then it hits the output pentode which can be run triode or pentode and he can play with that and how much signal you can drive into that...

Basically its a learning amp capable of an amazing tonal range and going into overdrive through nice crunch sounds... and showing off the pentode/triode flavours. Its also very true to guitar pickup dynamics and tone so it really shows off your playing technique or not...

It made my sons technique grow and awful lot!!!

James

[Shane]

Who knows, we might even see a WD guitar amp kit...

[Dave the bass]

What about us bassists

I'd love a SE bass amp. I use an SWR Super Redhead at the mo' but would love to build a pre/power amp one day to play through at small jams or practice.

DTB

[James D]

I've designed and built a few bass amps. But valve ones end up big and heavy to carry with huge cabs You know the problem...

SO I tend to use my H&K BassForce XXL for the gigs I do... sad isn't it

What do you fancy for your amp? I can design it for you. My bestest ever bass amp used PP 845s running 1100V on the anodes:-) About 50Watts but a real pain to move... What do you want to use it for? Recording and home use? Rehersal and meduim halls? Kill the staduim? Nowadays it really only the first I guess as you either mic it or DI into the P.A.


James

[Pedro]

This is part heresy but bear with it....

I've just knocked together an Ampeg-style Bass Preamp for our bassist, with the plan to use one of old PA amps as power amp. However in storage the old PA amp has gone awol....

Is a good compromise a great valve pre with a brute force FET amp or something SS so its light ?

Dont shoot me for mentioning the solid state bit ....

Rgds
Pete

[James D]

SS for the power amp part of a bass amp if sensible in many ways... honest