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PostPosted: 25 Sep 2020, 11:31 
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Hello Everyone,

I have a quick question for those in the know.

Looking at a preamp tube cathode resistor with a 100uF bypass cap across the resistor.
This is pretty standard.

What is the effect to Gain etc if you change the resistor for two resistors equal to the same value as the single resistor in series and put the bypass cap across from middle of the resistors to Gnd.
As opposed to cathode to Ground?
Shown in the picture.

Regards
M. Gregg


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PostPosted: 25 Sep 2020, 14:17 
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Hi, It will provide a shelf for the gain. You can use this idea to boost gain at frequencies above the cut off of the cap/resistor pair. For example if you split a 2k into a pair of 1k and put a 0.01uf across one of them you will get a boost starting at about 16KHZ (1/2PiCR) approximately. The gain will be about 3 db and will give an upslope curve. This can be handy for designs that for some reason are deficient in the very top end. With careful selection of values you can fix it. That is all good....but as always there are some consequences. There will be a phase shift, change in output impedance of the circuit and possibly a change in distortion because of the change in gain at those frequencies. Still normally these things are minor and don't cause issues.

Good listening
Bruce

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PostPosted: 25 Sep 2020, 16:54 
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gofar99 wrote:
Hi, It will provide a shelf for the gain. You can use this idea to boost gain at frequencies above the cut off of the cap/resistor pair. For example if you split a 2k into a pair of 1k and put a 0.01uf across one of them you will get a boost starting at about 16KHZ (1/2PiCR) approximately. The gain will be about 3 db and will give an upslope curve. This can be handy for designs that for some reason are deficient in the very top end. With careful selection of values you can fix it. That is all good....but as always there are some consequences. There will be a phase shift, change in output impedance of the circuit and possibly a change in distortion because of the change in gain at those frequencies. Still normally these things are minor and don't cause issues.

Good listening
Bruce


Hi Bruce,

Thanks for the insight, I have seen this on what I think are some of the old Kondo circuits.
It seems to be over looked by the DIY community.

One example was exactly as you describe a 2K split into two 1K with a 100uF across the bottom 1K.

So would the gain remain the same in the above situation or be lower in the mid-band with the gain increasing as the frequency increases?
Ie the "lost" gain in the mid would come into play with frequency response and show as increased detail in the high frequencies.
Or do you think this was just being used to reduce the gain of the tube in this circuit across the whole frequency range?

Regards
M. Gregg

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PostPosted: 25 Sep 2020, 19:14 
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This is a fairly common means of frequency compensation. At the low end it is generally good for between 3db and 6db improvement in low frequency response. As a class, these are referred to as "Compensated Amplifiers". See this section out of "Theory and Application of Electron Tubes".
Attachment:
Screen Shot 2020-09-25 at 5.05.47 PM.jpg

The application to the cathode biasing resistor as opposed to the plate resistor will also allow for the adjustment of AC gain (via the partial bypass of the cathode degeneration effect) without affecting the DC bias. Selection of the bypass capacitor value is critical for differentiating the two effects (frequency compensation verses AC gain adjustment).


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PostPosted: 26 Sep 2020, 04:31 
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Hi Matt
Quote:
The application to the cathode biasing resistor as opposed to the plate resistor will also allow for the adjustment of AC gain (via the partial bypass of the cathode degeneration effect) without affecting the DC bias. Selection of the bypass capacitor value is critical for differentiating the two effects (frequency compensation verses AC gain adjustment)


Thanks its just prompted me to do some further reading and recap:
Just looking at section 1.18
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf
And
http://www.bartola.co.uk/valves/tag/cat ... eneration/
Interesting reflection on LED biasing but thats not really on topic.

Its the unbypassed section of the cathode resistor I find interesting.

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M. Gregg

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PostPosted: 26 Sep 2020, 13:00 
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Hi Matt and Bruce,

Does this sound correct?
So it would seem in the example referred with the 1K -1K split with mid point connection of the bypass capacitor.
With the value at 100uF it should be fully bypassed across the frequency range (Guess) and that the mid point split allows for some cathode degeneration.

Looking at the link leaving a section un-bypassed can help to linearize the tube? and possibly reduce gain to some extent.

NB I have no connection to the links in the last post its just a reference.

Regards
M. Gregg

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PostPosted: 27 Sep 2020, 07:28 
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Anyone have any thoughts?

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M. Gregg

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PostPosted: 27 Sep 2020, 14:02 
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Hi, A sort of simplified way to look at ones that have a split set of resistors and one is fully bypassed is that the DC and AC gains will be different. The turn over frequency can be chosen depending on what you want to do. I sometimes use something like this in feed back circuits as well when you want only a fraction of the NFB to be applied to the cathode. I usually set the resistors at about 1:10 ratio with only one tenth of the NFB applied to the tube. This allows for much lower values of the feedback resistor and cap. Unfortunately the calculations on this get tedious so I usually guess a set it up and see what happens on the scope and distortion analyzer. The various combinations of bypass resistors and capacitor in combination with the NFB components (series or parallel) can allow you to tailor the the sound quite well and accurately. I have used this to increase stability at above band frequencies (to handle output transformer resonances) in some of my designs. Ideally a well designed device would not need such corrections in the band of choice but peaking is relatively common.

I have seen variations of the partial bypass used in RF circuits for gain peaking as well as in audio.

Good listening
Bruce

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