300B SET amp input sensitivity, Help. - diyAudio
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Old 15th December 2010, 06:45 PM   #1
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Default 300B SET amp input sensitivity, Help.

I have a 300B SET amp with 150mv input sensitivity and 100k ohm impedance. It has two 100k ohm volume pots.

I need to reduce the input sensitivity of this 300B amp to about 1V. I want to remove the volume pot.

Could someone tell me how I can reduce the input sensitivity to about 1V without the volume pot?
Perhaps I can put a resistor in place of the volume pot?
If so, how can I figure what value resistor I need?
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Old 15th December 2010, 06:55 PM   #2
rongon is offline rongon  United States
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I think in order to answer that question in a meaningful way, the folks here will need to see a schematic.

-=|=-
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Old 15th December 2010, 07:01 PM   #3
el156 is offline el156  Portugal
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Quote:
Originally Posted by manp111 View Post
I have a 300B SET amp with 150mv input sensitivity and 100k ohm impedance. It has two 100k ohm volume pots.

I need to reduce the input sensitivity of this 300B amp to about 1V. I want to remove the volume pot.

Could someone tell me how I can reduce the input sensitivity to about 1V without the volume pot?
Perhaps I can put a resistor in place of the volume pot?
If so, how can I figure what value resistor I need?
Make a voltage divider with 15K resistor to ground and 100K resistor to the input?
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Old 15th December 2010, 08:19 PM   #4
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Quote:
Make a voltage divider with 15K resistor to ground and 100K resistor to the input?
That might work.
How do you know to put 100K ohm resistor to the positive side of the input? Also, why do I need a resistor on the negative side of the input?

Really, I just want to turn down the volume of the amp about 20dB.
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Old 15th December 2010, 08:24 PM   #5
el156 is offline el156  Portugal
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Quote:
Originally Posted by manp111 View Post
That might work.
How do you know to put 100K ohm resistor to the positive side of the input? Also, why do I need a resistor on the negative side of the input?

Really, I just want to turn down the volume of the amp about 20dB.
What you need is a voltage divider to lower the input sensitivity,and the 100K with 15K makes it . Can you post a schematic?
Silvino
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Old 15th December 2010, 08:44 PM   #6
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Here is the schematic.

So the "voltage divider" IS the two resistors?
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File Type: jpg sun audio sv-300be Schematic.jpg (89.3 KB, 121 views)
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Old 15th December 2010, 08:46 PM   #7
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What wattage of the 100k and 15k ohm resistors do I need?

Also, is there a high quality resistor brand that you recommend?
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Old 15th December 2010, 08:51 PM   #8
el156 is offline el156  Portugal
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Quote:
Originally Posted by manp111 View Post
Here is the schematic.

So the "voltage divider" IS the two resistors?
Yes,I would do that! 15K from grid to ground,and 100K from grid to input.
and take the POT away.
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Old 15th December 2010, 08:54 PM   #9
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Yes,I would do that! 15K from grid to ground,and 100K from grid to input.
You are talking about the grid of the input tubes?
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Old 15th December 2010, 09:11 PM   #10
tomchr is offline tomchr  United States
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He's talking about replacing the pot (variable voltage divider) with a fixed voltage divider. From the grid of the input tube insert 15 kOhm to ground. Connect 100 kOhm from the grid of the input tube to the input connector. Remove the pot.

Regular 1/8 or 1/4 W metal film types will work fine.

A better way to approach this would be to redesign the front end to give less gain, but that's a bit beyond the project scope at this point.

~Tom
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