Preamp gain stages: how they sound and why

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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Marioverkill wrote:I kinda tought its content was related to harmonics.I couldn't decipher it because I had no idea about the Fourier Transform concept.I will study some more.
Don't care too much about FFT. I've posted those files because it's useful to better understand how a certain waveform is in certain points of the circuit, and what's the harmonic content of that waveform. So you can see if a particular stage adds mainly odd order harmonics (that is not bad as many people think), even order harmonics, but also if it adds low order harmonics or high order harmonics.

EG: I like a bit more 3rd harmonic in my vintage-tasty hi-gain lead channels, while in modern ultra-gain I prefer a bit less of it. Spice simulations are pretty good in low order harmonics' analysis.
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

LeftyStrat wrote:This is great info, thanks Roberto. I'm particularly intrigued with how the cathode follower distorts.
Take a look at the harmonic content in sweep Rk files. Lowering Rk on a cathode follower you'll dampen high order harmonics, so an extragain lead that sounds harsh due to some problems in early stages, can be smoothed by a 68k on the CF. Of course, you'll affect attack and dynamics too, so it's more like a "i don't know what to do" trick. :wink:
Cliff Schecht
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Re: Preamp gain stages: how they sound and why

Post by Cliff Schecht »

roberto wrote:
Marioverkill wrote:I kinda tought its content was related to harmonics.I couldn't decipher it because I had no idea about the Fourier Transform concept.I will study some more.
Don't care too much about FFT. I've posted those files because it's useful to better understand how a certain waveform is in certain points of the circuit, and what's the harmonic content of that waveform. So you can see if a particular stage adds mainly odd order harmonics (that is not bad as many people think), even order harmonics, but also if it adds low order harmonics or high order harmonics.

EG: I like a bit more 3rd harmonic in my vintage-tasty hi-gain lead channels, while in modern ultra-gain I prefer a bit less of it. Spice simulations are pretty good in low order harmonics' analysis.
Yeah my tube SPICE models in whatever software I've tried so far don't like being heavily distorted. They're very simple models, maybe taking into account some band-limiting (Miller caps) and such but lacking modeling of the non-linearities in much any form IME.

In NI Multisim I am able to crash their software every time I clip a tube too much and in Altium it seems a bit better but still misses the softness that the tubes 3/2nd law introduces.
Cliff Schecht - Circuit P.I.
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Reeltarded
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Re: Preamp gain stages: how they sound and why

Post by Reeltarded »

roberto wrote:
Reeltarded wrote:Yes! Thank you for the research, and the sharing!
This is the kind of thing that makes the wheels in my head spin on the ice lol
This is the reason of sharing: more weels are spinning, more ideas for everyone.
Notions are the food of invention!
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Marioverkill
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Re: Preamp gain stages: how they sound and why

Post by Marioverkill »

Thanks to the admins for making a sticky !!

Roberto:If you(and anybody else)could refer to the pertinent graphs when talking about mods(like different bypass caps,etc)that would be great.

Mario
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Cliff Schecht wrote:Yeah my tube SPICE models in whatever software I've tried so far don't like being heavily distorted.
Yeah, they work good as an approximation of what really happens. Low order harmonics fit quite well, and you can clearly see how the harmonic content will be with different polarizations. Don't believe in high order harmonic FFT analysis. Also, it's necessary to set the right time before you start to scope, the right step time, and so on. Maybe we can share some informations about it too.


Thank you moderators for the stiky.
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Marioverkill wrote:Roberto:If you(and anybody else)could refer to the pertinent graphs when talking about mods
What do you mean exactly? Rk sweep on CFs?
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Re: Preamp gain stages: how they sound and why

Post by Marioverkill »

Anything in general.Say you are taking part of thread where somebody is talking about increasing the bypass cap of a 100k Rp,1.8K Rk,from 1u to 1.5u,it would be nice if people would refer back to say the Gain Stages Frequency Analysis graphs.Just using your graphs in general in the discussions if applicable.

Carry on,don't pay too much attention to my ramblings.

Mario
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Structo
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Re: Preamp gain stages: how they sound and why

Post by Structo »

I was looking at the Zintolo’s Gain Stages Frequency Analysis with great interest.

Although I'm not real sure of the nomenclature.

For example, on page 13 it says 220k 1k8//1u:

Does that mean, 200K plate, 1K8 cathode, 1uF bypass?

Then on the coupling graphs.

Coupling Cap: 220k 1k8//1u 22n:

220K plate, 1K8 cathode, 1uF bypass, 22n coupling cap?


I'd like to see one like the second stage of a Dumble is

220K grid stopper, 150K plate, 2K2 cathode, 4.7uF bypass, .05uF coupler.

Not sure if you can model it but the 220K grid stopper is bypassed by a 500pf cap.

Thanks, Tom
Tom

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Reeltarded
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Re: Preamp gain stages: how they sound and why

Post by Reeltarded »

I do believe so, Mr. Tom.

I have seen it like that with dual bypass separated by a single/slash, and the cathode//bypass with 2, like that.

Plate Cathode//Bypass 1/Bypass 2.

It's tablature for amp freaks. :)
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martin manning
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Re: Preamp gain stages: how they sound and why

Post by martin manning »

I think what is needed is a more complete description of the baseline schematic and what thing or things are being varied. It's a good piece of work but unfortunately without that clear description there will continue to be questions and confusion as to what exactly is being demonstrated.
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Marioverkill wrote:increasing the bypass cap of a 100k Rp,1.8K Rk,from 1u to 1.5u,it would be nice if people would refer back to say the Gain Stages Frequency Analysis graphs.
In this case, you have to look at the 100k 1k8//1µF graph and divide the shelving frequency by 1.5 (that is 1µ5 / 1µ).
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Structo wrote:I was looking at the Zintolo’s Gain Stages Frequency Analysis with great interest. Although I'm not real sure of the nomenclature.
You are right. 220k 1k8//1µ 22n means 220k Rp, 1k8 Rk, 1µ Ck, 22nF Co (coupling cap to next stage, with 1M load to ground).

// means "parallel with" in matemathics.

Sometimes I use also 10k//(2k7+220n) this means that I have a 220n cap in series with a 2k7 resistor, and then in parallel with that 10k (think about the 3rd OD stage of a CAE).
Structo wrote:I'd like to see one like the second stage of a Dumble is
220K grid stopper, 150K plate, 2K2 cathode, 4.7uF bypass, .05uF coupler.
Today I'm not at home, this evening I'll try that.
Last edited by roberto on Sun Mar 11, 2012 5:06 am, edited 1 time in total.
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Reeltarded wrote:It's tablature for amp freaks. :)
:lol:
martin manning wrote:I think what is needed is a more complete description of the baseline schematic and what thing or things are being varied.
When you see a 50-110k means that I've changed Rk on the cathode follower from 50k to 110k in 20k steps.

When you see 1-10V this means I've changed input sine voltage rms from 1 to 10V.

Ask me everything it's not clear.
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Re: Preamp gain stages: how they sound and why

Post by Marioverkill »

Thanks again Roberto.

Mario
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