What controls the power amp section?

clarkefan

I'm probably being really stupid, wouldn't be the first time...

Take an old Marshall head.  According to a schematic I was looking at the volume controls come after V1.  Huh?

I've always thought the volume controls controlled the power amp.  The more you turn up the more you more you're turning the power valve section up.   But it seems like what you're actually doing is turning up the preamp.

So all the controls are preamp controls.

So how do you get the power valves to distort?

If I'm being thick please have mercy

ICBM

You get the power valves to distort by turning the whole thing up really loud .

Essentially the 'volume' control is a gain control, and there is an invisible master volume control set to 10 just after the treble control - which is where it would be on a master volume Marshall.

There is no other way to get the power valves to distort even on a MV model - if you turn the preamp volume up and the master volume down, you're hearing preamp distortion.

The only ways to get power amp distortion at controllable volume are with either a power attenuator between the amp and speaker, or with some sort of power scaling.

clarkefan

Wow, thank you @ICBM !  Head blown

So even a MV Marshall is just controlling the level going into the power amp.  I've clearly misunderstood this completely for years...

So, does this mean if an old Marshall had a loop installed and you plugged the guitar straight into the Return the volume would be full on mental loud?

ecc83

There are many variations..To K.I.S.Sir, take a single channel amp. Often have one triode stage followed by a volume pot * feeding a second stage, often a 'combination' gain stage and cathode follower to drive the tone stack. That feeds a Master VC which feed the grid of the PI and THAT drive the power valves ..But! There are amps with 'Post PI VCs' and that means the PI can be driven harder. The gains INSIDE the preamp (pre PI) can be various as can the 'voicings', fixed EQs.

*These are often dubbed 'gain' controls but an electronics bod would think that only applies to a control in a feedback loop.

I can recommend Merlin Blencowe's Tube Preamps for Guitar and Bass (don't worry, he goes to proper English with 'valves' inside) Some maths but you don't really have to get it.

Dave.

Modulus_Amps

you need the pre-amp to drive the power amp properly, if you just plug into a return there may not be enough gain to cause the power section to overdrive properly (depends on the loop though)

ICBM

clarkefan said:

So even a MV Marshall is just controlling the level going into the power amp.  I've clearly misunderstood this completely for years...

So, does this mean if an old Marshall had a loop installed and you plugged the guitar straight into the Return the volume would be full on mental loud?

It depends if the loop is before or after the MV pot - you can do either. The standard Marshall position is after, in which case the volume will be full on mental loud if you plug into the return .

clarkefan

Wow again, thank you

I can't believe I've just fundamentally not understood how these things work for so long!

And thank you @ecc83 , I will look up that book

steamabacus

Dave Hunter's 'The Guitar Amp Handbook' is a good, not-too-technical read which explains a lot of the basics. The first chapters go through some valve amp schematics section by section to explain what each stage is doing. It's written for guitarists, not engineers.

clarkefan

Thanks @steamabacus , I'll check it out

I just had another thought - so even a post - phase inverter master vol is *also* not getting any flavour of power valves being over driven, cos it's still controlling the level that goes into the power valves, albeit also allowing you to push the additional (phase inverter) valve.  Wow.

So we're back to what ICBM said at the top, the only way to push *the power valves* at low volume is with an attenuater/power scaling.

Talk about every day's an education!

DJH83004

steamabacus said:

Dave Hunter's 'The Guitar Amp Handbook' is a good, not-too-technical read which explains a lot of the basics. The first chapters go through some valve amp schematics section by section to explain what each stage is doing. It's written for guitarists, not engineers.

Same name but not me alas  taking Dave's point, Merlin Blencowe's Tube Pre amps for Guitar and Bass is a great read, and also his Valve wizard web site:  http://www.valvewizard.co.uk/ is worth looking at, in fact a lot of the info from his book is on there.
If you can track one down, 'The Ultimate Tone' by Kevin O'Connor is also well worth a read, but you have to be a fan of all things Canadian, particularly traynors   

jpfamps

DJH83004 said:

steamabacus said:

Dave Hunter's 'The Guitar Amp Handbook' is a good, not-too-technical read which explains a lot of the basics. The first chapters go through some valve amp schematics section by section to explain what each stage is doing. It's written for guitarists, not engineers.

Same name but not me alas  taking Dave's point, Merlin Blencowe's Tube Pre amps for Guitar and Bass is a great read, and also his Valve wizard web site:  http://www.valvewizard.co.uk/ is worth looking at, in fact a lot of the info from his book is on there.
If you can track one down, 'The Ultimate Tone' by Kevin O'Connor is also well worth a read, but you have to be a fan of all things Canadian, particularly traynors   

He also insists on showing current flow in the opposite direction to conventional current!

clarkefan

Thanks I actually have The Ultimate Tone, and either pt2 or 3, can't remember  I was going to stick them on eBay, maybe I'll have a quick flick through first

DannyP

jpfamps said:

DJH83004 said:

steamabacus said:

Dave Hunter's 'The Guitar Amp Handbook' is a good, not-too-technical read which explains a lot of the basics. The first chapters go through some valve amp schematics section by section to explain what each stage is doing. It's written for guitarists, not engineers.

Same name but not me alas  taking Dave's point, Merlin Blencowe's Tube Pre amps for Guitar and Bass is a great read, and also his Valve wizard web site:  http://www.valvewizard.co.uk/ is worth looking at, in fact a lot of the info from his book is on there.
If you can track one down, 'The Ultimate Tone' by Kevin O'Connor is also well worth a read, but you have to be a fan of all things Canadian, particularly traynors   

He also insists on showing current flow in the opposite direction to conventional current!

Aaaaalso, Hunter sets out his stall by saying pre-amp gain is crap, it sounds brittle, no-one wants it and power stage overdrive is the holy grail.

I quite like the sound of pre-amp distortion, am I a cloth-eared freak?

John_A

DannyP said:

Aaaaalso, Hunter sets out his stall by saying pre-amp gain is crap, it sounds brittle, no-one wants it and power stage overdrive is the holy grail.

I quite like the sound of pre-amp distortion, am I a cloth-eared freak?

It's Internet myth based on the fact that a lot of amps sound better when turned up a bit, a JCM 800 on 4 on the MV sounds a whole lot better than on 0.1, that doesn't mean you are getting power-amp distortion.  Most modern amps lice dual recs etc rely totally on pre-amp distortion and sound great.

If it sounds good then it is good

ICBM

Also, a lot of non-MV amps do not have pure power amp distortion when played loud, contrary to popular belief. It's almost always a mix of the two after a certain point, and in some cases the preamp actually overdrives first.

DannyP

Plus pretty much all valve pedals like Kingsley, Vox, Blackstar HT etc. are overdriving 12AX7s aren't they? So people must like that sound...

ICBM

DannyP said:

Plus pretty much all valve pedals like Kingsley, Vox, Blackstar HT etc. are overdriving 12AX7s aren't they? So people must like that sound…

Some, eg the Mesa V-Twin, aren't overdriving the valve at all - the overdrive comes from a diode clipping circuit, and the valve just feeds the tone stack and adds "valveness". They still sound very good though.

In fact I much prefer it to the Blackstar HT-Dual which *does* overdrive the valve...

Phil_aka_Pip

jpfamps said:

DJH83004 said:

steamabacus said:

Dave Hunter's 'The Guitar Amp Handbook' is a good, not-too-technical read which explains a lot of the basics. The first chapters go through some valve amp schematics section by section to explain what each stage is doing. It's written for guitarists, not engineers.

Same name but not me alas  taking Dave's point, Merlin Blencowe's Tube Pre amps for Guitar and Bass is a great read, and also his Valve wizard web site:  http://www.valvewizard.co.uk/ is worth looking at, in fact a lot of the info from his book is on there.
If you can track one down, 'The Ultimate Tone' by Kevin O'Connor is also well worth a read, but you have to be a fan of all things Canadian, particularly traynors   

He also insists on showing current flow in the opposite direction to conventional current!

by which, do you mean electron flow, or the silly notion that current can flow in the opposite direction to electrons?

ICBM

Phil_aka_Pip said:

by which, do you mean electron flow, or the silly notion that current can flow in the opposite direction to electrons?

Current does flow in the opposite direction to electrons.

Actually it flows in both directions at once - electrons one way, 'holes' the other. It makes no difference which it is from a macro-electrical point of view, energy is carried round the circuit however you define it. This is why 'conventional current' hasn't been discarded as a concept even though we now know about the existence of electrons.

ecc83

I don't think 'holes' actually move much IC?  Being positively charged atom they are too massive to go fast enough. Then, I  SUPPOSE  when an electron is 'launched' from an atom it must create a recoil force? Rifles don't move far or fast compared to bullets and I think the rifle/slug mass ratio is much smaller than atom/electron?

But then it is all probably not Newtonian, maybe you have to go bat crap crazy quantum!

But we can't give up 'conventional flow' Flemming and his hand and all that swaddling!

Dave.

ICBM

No, 'holes' weigh nothing - the atoms themselves don't move. An electron moves from atom A to atom B, and an equivalent hole moves from atom B to atom A. OK the electron is a physical particle and the hole is 'lack of particle', but both are a valid description of the flow of electrical current.

Before the discovery of the electron as a particle - which I'm pretty sure coincided with the invention of the earliest valves, which were called 'cathode ray tubes' as TV tubes still are! - it wasn't possible to determine what caused the flow of electrical energy, since the two must add up to zero in a circuit.