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Two stupid questions about overdrive

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Re: Two stupid questions about overdrive

Postby SecretSam » Sun May 24, 2020 6:52 pm

Actually ... I sort of remember them from my late teens. Which would have been in the late seventies.

(In between studying for A-levels and avoiding the breathless advances of predatory pop stars and TV producers, we found time to play a bit of guitar back then.)

Of course, my memory is rubbish.
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Re: Two stupid questions about overdrive

Postby Sam Spoons » Sun May 24, 2020 6:58 pm

CS70 wrote:
Sam Spoons wrote:Because they're supposed to sound like overdriven power valves* turning down the master reduces the drive to the power valves and,

The fun thing, I never see many people using actual power valve distortion south of the 2000s, apart maybe some stadium-size acts a-la-Joe Bonamassa?

Almost never these days but back in the day yes (it was the only way once upon a time)

I mean I've tried many times and it's fun but with anything bigger than a few watts you've gotta place it really far to avoid overwhelming everything else (including your ears) and mic it... even my Tiny Terror in 7 watts mode with the 4x12, it was bursting ears.

Yup, I used an Epiphone Valve Junior on an outdoor gig unmiked (we only had vox through the PA) and I was heard, even alongside a live drummer.

Preamp distortion is not so different in principle? I mean you have a tube as gain device and you go over its headroom, so all kind of unexpected things start to happen. The same happens with the power stages - what happens is a bit different for each amp because by definition you're working outside of the performance envelope, but mostly it's simply damn louder (and distorted).

Maybe the trick is just that - louder is always better? :)

I think it's a combination of the power valves distorting (the circuit architecture and valve design is different to a power amp so power valves definitely distort differently to preamp valves), speaker distorting and, yes, louder is often better (them clever buggers Fletcher and Munsen are probably responsible.....).

For me there is a sweet spot, eventually it all compresses too much and sounds lifeless.
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Re: Two stupid questions about overdrive

Postby Folderol » Sun May 24, 2020 7:30 pm

Although individual valves do have specific characteristics, it's more to do with the nature if the stage profile itself.

Pre-amp stages are single ended, a (comparatively) low impedance valve against a very high value resistor. This results in a gradually increasing distortion with amplitude, and mainly even order.

Output stages are (nearly always) push pull and fairly well impedance matched against a transformer. Distortion tends to be fairly low for most of the the signal range, and then climbs rapidly as you get close to the limit. Also it is mostly odd order distortion.
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Re: Two stupid questions about overdrive

Postby Sam Spoons » Sun May 24, 2020 9:47 pm

That's interesting, I had always thought odd order harmonics were less pleasant than even order. Of course, with non-master volume amps it's impossible to tell where the distortion is coming from bu I do know that, usually, small amps cranked sound better than big amps with the master wound back.

Either way my newly acquired Headrush Gigboard sounds much better through my 18 watt combo with the speaker sim off than through a FRFR cab with it on.....
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Re: Two stupid questions about overdrive

Postby ef37a » Mon May 25, 2020 12:54 pm

Holy moly! This is such subjective subject that it is almost pointless to proceed!

Yes, much of what we hear as 'nice' Over Drive is pre amp distortion because, as has been said, once you push a pair of even 12W pentodes into distortion you are well past 110dB SPL from a good guitar speaker. Do it with EL34s/6l6 and it gets very silly.

Not true that single ended pre amps valves mainly produce even harmonics . There are two mechanisms. Pushing the grid to cut off and pushing it into 'grid current' and these produce different types of distortion. Even harmonics are 'pleasant' and we can put up with an awful lot of them and not really notice. Odd harmonics are harsher and give the guitar tone its 'bite'. If you can read the words of Merlin Blencowe in his "Tube Preamps for Guitar and Bass" you will see that there is a wealth of distortion mechanisms that have not, to my knowledge been investigated hardly at all. The small signal pentode for example can be 'abused' and controlled to give a wide variety of tones.

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Re: Two stupid questions about overdrive

Postby Folderol » Mon May 25, 2020 2:35 pm

Interesting. I bow to your expertise.
I was basing this on my own experiments with triode preamps which were relying on the anode characteristics rather than punishing the grid.
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Re: Two stupid questions about overdrive

Postby ef37a » Mon May 25, 2020 3:02 pm

Folderol wrote:Interesting. I bow to your expertise.
I was basing this on my own experiments with triode preamps which were relying on the anode characteristics rather than punishing the grid.

Oh! No expert Will! If you look at most amp pre amp biasing they are almost all just a rip from Mullard's specc sheets. Rk is 1.5k but go lower and the valve can be pushed into grid current distortion. Higher and you push to cut off.

No amplifier I am aware of has tried the simple idea of variable bias to the pre amp valves?

The circuits are SO conservative! Most are just slight variants of the original Fender design even down to mistakes! The DC coupled cathode follower stage that you find in a great many amps, usually driving the tone stack is rather more complex in its operation than is usually thought and is responsible for a great deal of the OD tone.

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