Output Coupling Caps and Bass response

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novosibir
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Re: Re:

Post by novosibir » Mon Jul 24, 2017 8:34 pm

FourT6and2 wrote:What about .047 PI output caps + a lower value for the bias resistors? Like 180K or 150K?
Why not?
FourT6and2 wrote:I know raising the PI caps increases bass response. But what does lowering the bias feed resistors do?
Cutting bass response, lowering PI gain
FourT6and2 wrote:I've never heard your Dino or British Purist amps.
Why not?
FourT6and2 wrote:How does the sound compare between .047/220K and .1/100K?
Try it, then you know it :D

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Re: Output Coupling Caps and Bass response

Post by FourT6and2 » Mon Jul 24, 2017 8:56 pm

Well, if halving the resistor value = raises -3dB corner freq. 1 octave... and doubling the cap value lowers the corner freq. 1 octave...

Then...

.1uF/100K and 0.047/220K will be about the same...

Except...

.1uF/100K = PI tube distorts a little less?

And the stock value of 0.022/220K and 0.047/100K are about the same, except again... in the second one, the PI tube is staying cleaner?

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Re: Output Coupling Caps and Bass response

Post by novosibir » Tue Jul 25, 2017 2:51 am

FourT6and2 wrote:Well, if halving the resistor value = raises -3dB corner freq. 1 octave... and doubling the cap value lowers the corner freq. 1 octave...

Then...

.1uF/100K and 0.047/220K will be about the same...

Except...

.1uF/100K = PI tube distorts a little less?

And the stock value of 0.022/220K and 0.047/100K are about the same, except again... in the second one, the PI tube is staying cleaner?
Exactly! Keep in mind, that the bias resistor has to be regarded in parallel to the plate resistor, so with a 100K bias R the gain of the PI is somewhat less than with a 220K bias R
The fault almost always is sitting in front of the amp :wink:

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Re: Output Coupling Caps and Bass response

Post by FourT6and2 » Tue Jul 25, 2017 2:54 am

novosibir wrote:
FourT6and2 wrote:Well, if halving the resistor value = raises -3dB corner freq. 1 octave... and doubling the cap value lowers the corner freq. 1 octave...

Then...

.1uF/100K and 0.047/220K will be about the same...

Except...

.1uF/100K = PI tube distorts a little less?

And the stock value of 0.022/220K and 0.047/100K are about the same, except again... in the second one, the PI tube is staying cleaner?
Exactly! Keep in mind, that the bias resistor has to be regarded in parallel to the plate resistor, so with a 100K bias R the gain of the PI is somewhat less than with a 220K bias R
Is there a formula for calculating the difference in gain? How much "less" is "somewhat less"? haha!

If the PI stays a little cleaner, does that mean the the overall output of the amp decreases? Or do the power tubes then overdrive sooner?

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Re: Output Coupling Caps and Bass response

Post by novosibir » Tue Jul 25, 2017 3:24 am

FourT6and2 wrote:Is there a formula for calculating the difference in gain? How much "less" is "somewhat less"? haha!
https://www.ampbooks.com/mobile/amplifi ... alculator/
FourT6and2 wrote:If the PI stays a little cleaner, does that mean the the overall output of the amp decreases?

no
FourT6and2 wrote:Or do the power tubes then overdrive sooner?
they overdrive later
The fault almost always is sitting in front of the amp :wink:

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Re: Output Coupling Caps and Bass response

Post by FourT6and2 » Tue Jul 25, 2017 11:38 am

thanks!

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Re: Output Coupling Caps and Bass response

Post by Roe » Wed Jul 26, 2017 4:41 am

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Re: Output Coupling Caps and Bass response

Post by FourT6and2 » Wed Jul 26, 2017 11:33 am

Roe wrote:try this calculator instead:https://www.ampbooks.com/mobile/amplifi ... -response/
Yeah, I've been using all of the ones that site offers. The bone I have to pick with some of them (like the one you linked to) is you can't plot more than one line on the same graph. And every time it draws a new graph, it changes the scale. So you can't really compare back and forth. Its kind of annoying.

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