SO ~ the PRE-AMP in an amp, why do they sound so different ??? .

ChrisMusic

What is it that gives such widely varying sound characteristics ?

And what do various design choices actually sound like ?

Any help to understand this is greatly appreciated  

FelineGuitars

The preamp section of an amp makes the biggest difference in my opinion 

The power amp certainly makes tonal differences depending on the output power, amount of headroom and kind of valve used, and the size of the transformers (and of course speaker choice too)

When looking at preamps the whole layout and circuit choices as well as component value and type can affect the sound.

Whether the tone controls come before the gain sections - like in Blackface Fenders and Boogies , or whether the tone stack follows the gain - like in Marshalls and tweed fenders massive affects the shaping of the sound.

Values of things like the slope resistor and the the values of the components in the tone stack will determine which frequencies come through more when the controls are moved.

Number of gain stages too (each 12a*7 type valve contains two usable gain stages each referred to as a triode) will affect the nature of the gain structure too. 

There is a lot to it.....

I choose to use a modular amp system where I can swap out different pre-amp sections to be used by a common power amp.

This lets my amp be somewhat of a chameleon whilst still having an all tube signal path

Cirrus

It's such a huge topic that the likes of ICBM, ECC83, PFamps (can't remember your actual name haha!) etc could each write a book I suspect.

I will write a paragraph.

It's all in the circuit design. You make choices that do things like dump some frequencies to ground or emphasise other frequencies. You choose how each valve stage operates to control the gain amount at a range of different frequencies, and the character of that gain - tight, loose, compressed, saggy, stiff etc. You decide how many gain stages you want and where and how you want to control the signal with gain knobs, eq stacks, switchable options like mid boosts, extra gain, loose or tight dynamics... and the whole thing adds up to a certain sound or character.

Clarky

even without different valve choices, there are a pile of different preamp designs and they all have quite different tonal characteristics.. theyoffer different amounts and types of gain.. like a JCM never gets that dirty and is bright / edgey.. the 5150 mk3 red channel is really hi-gain and silk smooth.. the hotter Boogies will get as dirty as the 5150 mk3 but their low end gets dirty too [more so than the 5150] so you end up with looser / flubby lows if you set the bass control too high [actually it's better to use a pedal for "pre-EQ" to prevent strong lows from getting into the Boogie in the first place].. and then there are the cleaner amps like the Hiwatts etc..

but like the preamps, the poweramps are pretty varied too.. some of them stay very clean [so all the drive comes from the preamp], others provide some of the drive [which is quite different to preamp drive.. it's not so silky smooth, it's grittier] likw the cleaner Mashalls, AC30 etc..

although the preamp [as previously pointed out] has the most significant tonal impact, it's the preamp / poweramp combination that dictates the amp's voicing..

that said.. your cab will have quite an impact on your final basic tone too..

it's a "sum of the parts" thing.. guitar / pups -> preamp / poweramp -> cab / speakers.. they all stamp their individual fingerpoint on the tone to varying degrees..

 

ecc83

Biasing can have a big part to play. The standard fitall for 83s is 1k5 but you see big variations.

The world has gone digihappy and convolution but I reckon there is a lot of scope for software control of HARDWARE  parameters?

Dave.

fretmeister

On youtube somewhere there is the head of Egnater talking about the influence of the preamp / tone stack on amp sound and his opinion that he could get a 6L6 amp sounding like a Vox AC30 if just the tone stack was modded properly.

I didn't understand a word of it, but it was interesting!

jpfamps

fretmeister said:

On youtube somewhere there is the head of Egnater talking about the influence of the preamp / tone stack on amp sound and his opinion that he could get a 6L6 amp sounding like a Vox AC30 if just the tone stack was modded properly.

I didn't understand a word of it, but it was interesting!

I would disagree with that statement.

Whilst pre-amp is an important part of the sound of an amp, the power amp configuration of the AC30 is an important component of the sound, as indeed are the loudspeakers.

Drew_TNBD

Coz Magick.

ChrisMusic

Cheers guys, thanks for your replies so far.

@Drew_fx ~ pray tell, what are these Magix of which you speak ?

@fretmeister ~ I'll have a trawl around YouTube and see if I can find that, sounds interesting.  I see Egnater do a version of the rack type preamps from Feline's picture posted above, and I think Pete Thorn demoed it in one of his videos too.

@Clarky, Feline and @Cirrus, thanks to both of you for a good foundation, much appreciated.

@FelineGuitars ~ interesting picture that, I have seen your spectacular preamp collection before.  It is in some part in response to seeing them, that this discussion was born.  Do these preamps actually give a convincing tonal rendition of their original model types through other power amps ?  Do you need anything specific in the spec of the power amp to achieve this chameleon behaviour, or is it fairly flexible.  How convincing is this system ?

@ecc83 and @jpfamps, would you care to flesh out anything about the topologies of different preamp types, and how the choices affect the subsequent tonal performance, please ?

Are there a small number of preamp types with variations on a theme, or is it more widely varied than that ?

Thanks for your help, on behalf of all of us here who are curious and hungry to learn more about amps, and what makes them work and sound the way they do...

FelineGuitars

I think in all fairness if you ned to be 100% on the money and you are in a position to actually push the power stage into overdrive where the different resposes really start to show then as long as you have a good power section (or two) then you will get away with convincing sounds.

For me the system works brilliantly - I mainly use through a handmade MJW 10W single ended EL34 powered power amp although I have been enjoying using a Marshall 20/20 (I have a Marshall 50/50 too but it's a bit loud for home).

It also works really well for when a customer wants to hear their guitar through something similar to the amp they use and I can pull out a module and get a very good approximation for them. And at domestic levels where you can't push the power amp hard , then as long as the power amp and speakers that you have are good and sound good at low levels then you are onto a winner. 

I have just gotten a Two Notes Torpedo to look towards recording some stuf with power amp/cab/speaker /mic impulse responses, and you could use that with a linear power amp and full range speaker system too I guess, but for practise purposes I am happy with the rig I previously described

ecc83

"@ecc83 and @jpfamps, would you care to flesh out anything about the topologies of different preamp types, and how the choices affect the subsequent tonal performance, please ?"

Not possible IMHO because tiny changes in response and/or level can have an effect on tone and "feel" (or so I understand!) . You would do well to study "Designing Tube* pre amps for Guitar and Bass by Merlin Blencowe and I am sure others will suggest other texts.

In just one section on cathode followers Blencowe imbues the DC coupled common cathode-cathode follower stage used in many F.M,V. amps as a tone stack driver with an almost magical status as a source of the tone that many find so appealing in those amplifiers.

It is, as with many "tuneful" parts of guitar amp systems, a "bad" design!

But you cannot design a pre amp in isolation. First decision is, tho' you may not like it, "how much will it sell for and to whom"?

Then you star at the output. What sort of speaker, i.e. middly, bassy, bit neutral, bites yer ears off? Having chosen a speaker pro-tem, does it fit in with question one?  Will they still be making them in 2020? Will the price go through the roof?

This, and many more factors will decide the voicing of the pre amp and of course the genre the amp is aimed at.

MUCH of it is cut and try (well, more...solder, play, tea, fag, tea, solder, *&^%$K! solder, tea, play.........**k!)

*Fear not. "Tubes" was only on the cover for our colonial brothers, inside it is nicely British!

Dave.

Handsome_Chris

@Chrismusic, what are you trying to achieve? Are you looking at building a valve (not tube - I am not a meercan), or just harvesting information?  I remember being in the second camp due to prepping for an interview with a UK amp company.

The good news is that if you have a rudimentary understanding of electronics then there are a lot of schematics on the internet that may assist. I believe that there is lot owed to the Fender Bassman.  If you look at that then you may start seeing that there is a lot of commonality among the valve amps that followed.

I hope this helps.

I now patiently await my flaming from @ICBM and @ECC83 (please say hello to Louisa in HR for me).

ChrisMusic

Hey @Handsome_Chris, good question.  I am a total noob to valve amps, I played and recorded loads of them back in my "past life", but thinking that the technology was on its way out and that the future would be solid state, I didn't really pay the attention to their design that I should have done.  Most of my sonic memories have long faded into the mists of time.  Wishing that I had learnt more, I am now trying to address that.  BTW, modelling was not really a competitor tech wise back then in the dark ages.

I am getting (more than) interested in amps now, and I realise that without playing loads of designs at volume there really is little way to understand how the design and topology choices ultimately dictate the final sonic performance.  I don't think a shop environment is really the right way to test amps out, for me at least.

That leaves buying and testing over a period of time.  Which is where a better understanding would be a great benefit.  There are plenty of good amps which come up for sale on the forum, but to be honest I really don't have a clue about most of them and whether they would fit into that "sound in my head" thing.  It would also be a real benefit if I should get something made especially at any time.  Which ever way I eventually go, it would be great if I could say, well that amp should sound great for me because...

Finally I am hoping that I can gain enough understanding to be a primer to further research, as you alluded to with schematics being available on the net.  I would like to have some grasp of what particular topologies or variations in component values may actually sound like.  I hope I am not grasping at straws.  There is a real depth of knowledge and expertise on this forum, so I am hopeful  

ChrisMusic

ecc83 said:

In just one section on cathode followers Blencowe imbues the DC coupled common cathode-cathode follower stage used in many F.M,V. amps as a tone stack driver with an almost magical status as a source of the tone that many find so appealing in those amplifiers.

It is, as with many "tuneful" parts of guitar amp systems, a "bad" design!

Excellent, that's just the sort of thing to demystify some of this for me.  I can visualise (auralise?) that just fine, I think.

( Although for clarity F.M,V. = ? )

(BTW, so was Drew_fx right about da magix then?)

Thanks for the book recommendation too, hopefully this thread / discussion will be a good primer to that.

And maybe a little easier and more specific than wading straight into a full text.

ecc83 said:

MUCH of it is cut and try (well, more...solder, play, tea, fag, tea, solder, *&^%$K! solder, tea, play.........**k!)

*Fear not. "Tubes" was only on the cover for our colonial brothers, inside it is nicely British!

No worries over either valves or tubes, both analogous, one functional one visual, although one may be more simplistic than the other  

Oh yes @ecc83, now that is a very insightful workflow of any major R&D project, or even a minor one.

As a non-fag-smoker I would probably add going down the pub, super sandwich with home made bread (bread made with Guinness, yummm), and some winter specific real ale, consumed in front of a giant log fire, with bright like minded friends for company.  A great way to cogitate and move things forward, with an occasional lateral leap too.  Drawn from real world experience, happy daze  

ChrisMusic

Thanks for that @FelineGuitars, I had a feeling that would be the situation, broadly speaking.  It is certainly a good solution to a variety of personal styles when it comes to demonstrating your sublime guitars through a real world amp.  It would be interesting to see if the Torpedo is a useful addition in the same context.

I notice that you have modules from Salvation Mods in your Egnator, and Randall do something similar in some of their amps.

•  Do you know how many other systems are out there ?

•  Are they all / some / any compatible with each other ?

•  In your experience, are the different preamp modules all distinctly different sounding, or is it more subtle in some cases ?

•  Is there anything else you can add about your experience of using them ?

Handsome_Chris

I think that if you are trying to buy an amp to get what you want, and you don't know what you need to get what you want, then software modelling may be a starting point (I am now stepping into my fire suit).  It may give you a starting point as to what amps, cabs 'n' fx  you would like to look at.  It may also give you the opportunity to try stuff you hadn't thought of.

@PaulMapp8306, did this for me when I was looking at a drive box, we discovered that it was a tube screamer I was after. This led me down the road to get a Sub Decay Liquid Sunshine, which was handy as it matched my Orange Thunderverb 50.

Going slightly off piste, I've had my AF2 since 2010 and have only just discovered that the JCM800 is my fave amp (sim). So in effect I'm using 21st century tech, to get a 20th century sound: There's progress for you.

As to why pre-amps sound so different, Cliff Chase (FAS/AF2 boss) said something like "A lot of amps are very similar; however, manufacturers put there own slant on tone stacks (ie component values) to give them their character". The advantage with digital modelling is that you can approximate the effect of adjusting stuff.  Our very own Mr @Clarky like to tinker with virtual amps to great affect. One of his beauties is a JCM800, with the master volume after the phase inverter, with I believe is a real world mod: It is a high gain monster. It does need a bit of judicious EQing on the low end or the bass goes a bit flubby.

ecc83

"I now patiently await my flaming from @ICBM and @ECC83 (please say hello to Louisa in HR for me)."

Heh! It is my firm belief that there is at least as much myth, misinformation and outright bllx talked about guitar amps as there is about every other aspect of audio, probably more!

Much of the handwringing and debate about various valves and brands is, I fear totally lost on the punters at the Mucky Duck.

Will say hi for you Chris. I shall probably pop back in next week because I have bought a Core 40 and have some software queries. How does she know you?

Dave.

robinbowes

This is a damned good book too: http://www.ampbooks.com/home/books/bassman/

Another data point... I have an Ampmaker WF4 build. I can totally change the character of the amp (ie. Marshall vs. Fender) by changing just one capacitor.

It's not "magix" but it's sometimes as much an art as a science.

R.

ICBM

ecc83 said:

Much of the handwringing and debate about various valves and brands is, I fear totally lost on the punters at the Mucky Duck.

Oddly, although this is often true, it can sometimes be the other way round - particularly with things that affect the dynamics. When I re-valved my Mesa DC-5 from the factory (and still new, it wasn't that they were worn out) Chinese ones to a full set of old-stock GEs and Sylvanias, it didn't seem to make that much difference at home, but it was so noticeable at band volume that at the first practice, the bassist and drummer stopped and asked what I'd done to it - it was far clearer and more 'present' sounding. It also measured substantially more output power, although not enough to give even 1dB more in true volume. I didn't do any before and after recordings but I would be very surprised if it wasn't audible there too.

I do generally find you need to make that scale of change though - the idea that you can vastly improve the sound of your amp just by changing one preamp valve to a NOS as seems to be often recommended is much more questionable. The differences can be audible, but more easily lost in the general mix.

ecc83 said:

It is my firm belief that there is at least as much myth, misinformation and outright bllx talked about guitar amps as there is about every other aspect of audio, probably more!

I agree with that - although not worse than high-end hi-fi...

jpfamps

FMV stands for "Fender Marshall Vox" and is used to describe the classic treble/mid/bass tone circuit (aka "tone stack") used in Fender and Marshall amps, although NOT Vox (so should really be called FM tonestack, or even just F as Marshall nicked it from Fender).

The circuit seems to have been invented by Fender in the late 50s and is used in the Tweed Bassman, the amp from which all classic Marshalls are derived, and BF/SF Fenders.

This tone control circuit is by far and away the most popular employed in guitar amps, probably because it uses the smallest number of parts to give treble / mid and bass; in fact very few guitar amps from the 60s onwards employ anything else.

Surprisingly (or not?) there is much more innovation in 50s amp design than from around the mid 60s onwards.

Handsome_Chris

Do Carvin and Orange generally use the James tone stack as opposed to the Fender/Marshall tone stack?