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 »

You are right. I will check original links and repost them.
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sepulchre
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Re: Preamp gain stages: how they sound and why

Post by sepulchre »

This is some fine info here, Roberto. We all benefit from your homework.

Thanks!
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jazbo8
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Re: Preamp gain stages: how they sound and why

Post by jazbo8 »

roberto wrote:You are right. I will check original links and repost them.
Thanks! I was also wondering which SPICE models did you use for the simulation? Is the control grid current based on a simple diode model, or something different?
southernRoller7
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Re: Preamp gain stages: how they sound and why

Post by southernRoller7 »

Awesome post man!
These go to eleven.
vibratoking
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Re: Preamp gain stages: how they sound and why

Post by vibratoking »

This is good stuff. Thanks for posting your work. As mentioned, without schematics the reader is really left to guess what configuration you really simulated. Schematics really define what you did.
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Roberto wrote:Thank you everybody for the compliments.


I finally got some time to find the right links, so I repost them:

GRAPHICS that show how much wide is the difference between bypassed and unbypassed gain (so the bright effect) of different stages. Cathode cap is 1u where not indicated. Note how 220k plate resistor has a smaller difference between bypassed and unbypassed gain comparing to a 100k plate resistor gain stage.

Here you can find some different gain stages. The first value is the plate resistor, the second one is the cathode resistor:
SIGNAL 100k 1k5
FFT 100k 1k5

SIGNAL 100k 10k
FFT 100k 10k


Here the response of some cathode followers. The first value is the voltage at which the two stages are supplyed, the second value is the plate resistor of the stage connected to the cathode follower, the third is the cathode resistor of the stage connected to the cathode follower, the last value is the cathode resistor of the cathode follower:

SIGNAL CATHODE FOLLOWER 400V 100k 820R 100k
FFT CATHODE FOLLOWER 400V 100k 820R 100k

SIGNAL CATHODE FOLLOWER 400V 220k 820R 47k
FFT CATHODE FOLLOWER 400V 220k 820R 47k

SIGNAL CATHODE FOLLOWER 400V 100k 1k8 100k
FFT CATHODE FOLLOWER 400V 100k 1k8 100k

SIGNAL CATHODE FOLLOWER AC30 TOP BOOST
FFT CATHODE FOLLOWER AC30 TOP BOOST


Here the response of some cathode followers variating the cathode resistor of the cathode follower. The first value is the voltage at which the two stages are supplyed, the second value is the plate resistor of the stage connected to the cathode follower, the third is the cathode resistor of the stage connected to the cathode follower, the last value is the cathode resistor range of the cathode follower:

SIGNAL CATHODE FOLLOWER 280V 100k 820R 50-110k
FFT CATHODE FOLLOWER 280V 100k 820R 50-110k

SIGNAL CATHODE FOLLOWER 380V 220k 1k8 50-110k 1Vrms
FFT CATHODE FOLLOWER 380V 220k 1k8 50-110k 1Vrms


Now everything should be fixed :wink:

If you have any question, feel free to ask!
**MODERATOR EDIT** the links above are no longer working
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V2
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Re: Preamp gain stages: how they sound and why

Post by V2 »

I might not understand what is being shown in the 'graphics' file. Shouldn't the unbypassed cathode gain stages have flatter gain across the frequency spectrum (and be different from the cathode-bypassed gain stages)?
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

You have to open that file and look at it page by page, not graphics by graphics.

I have to find the original word file on the HD of... maybe two netbook ago.

From page 1 to 8 you can see how different the gain is across the frequencies with a 1uF (IIRC) cathode bypass cap.

On the other pages always check the two graphs on the same page to see the differencies.

Hope it helps.
EtherealWidow
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Re: Preamp gain stages: how they sound and why

Post by EtherealWidow »

Hey! I have a couple of things that are perplexing me on the first page graphs.

I'm assuming that in every graph example on the first page, Rk is either fully bypassed or not bypassed at all. How is the value of Rk affecting the frequency then? I thought its effect was only on bias and therefore headroom. Not frequency response.

Also, why does 820R have such a HUGE bass rolloff? When you go up to 1.5k it evens it out a lot more, and then the colder-biased it is the bass starts getting rolled off again?
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

They are shelving filters, so you have to consider two effects:
- the difference of gain between the bypassed (mid-high frequencies) and unbuypassed (low ones): higher when Rk is higher, lower when Rp is higher.
- the frequency at which the cap "starts" to bypass.
EtherealWidow
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Re: Preamp gain stages: how they sound and why

Post by EtherealWidow »

So the cathode is partially bypassed you're saying? What value cap, if I may ask?
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

It's not partially bypassed, but has a bypassed gain and an unbypassed gain, and all the values in between. It's 1 uF.
EtherealWidow
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Re: Preamp gain stages: how they sound and why

Post by EtherealWidow »

Whoops. Just realized I misused the term "partially bypassed".

http://www.diyaudio.com/forums/tubes-va ... -bias.html

Alright. The fact that the cap is 1uF makes sense now that there is a difference in frequency response. And you know what, I just saw one of the above posts where you said that the cap is 1uF. Sorry about that. Thanks for all your hard work, man.

If I'm viewing that original graph right, there is a big bass rolloff @ 820R for Rk, then it evens out at about 1.5k, and then starts to roll off again at higher Rk values. I'm wondering why that is? I can't imagine why it would be that way and if I use the calculator on ampbooks.com it would say otherwise. Hope I'm not coming off as hostile, just trying to figure this out.
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roberto
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Re: Preamp gain stages: how they sound and why

Post by roberto »

Why do you say that the behaviour changes at 1k5? I don't see that.
The gap increases more and more (as expected) increasing Rk.
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