300B SET amp input sensitivity, Help.
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 15th December 2010, 06:45 PM #1 HP8903B diyAudio Member   Join Date: Jun 2010 300B SET amp input sensitivity, Help. . Last edited by HP8903B; 12th August 2016 at 11:06 PM.
 15th December 2010, 06:55 PM #2 rongon   diyAudio Member     Join Date: Jul 2009 Location: Across the river from Rip's big old tree... I think in order to answer that question in a meaningful way, the folks here will need to see a schematic. -=|=-
el156
diyAudio Member

Join Date: Apr 2010
Location: ALBUFEIRA ;PORTUGAL
Quote:
 Originally Posted by manp111 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?

HP8903B
diyAudio Member

Join Date: Jun 2010
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.

el156
diyAudio Member

Join Date: Apr 2010
Location: ALBUFEIRA ;PORTUGAL
Quote:
 Originally Posted by manp111 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

HP8903B
diyAudio Member

Join Date: Jun 2010
Here is the schematic.

So the "voltage divider" IS the two resistors?
Attached Images
 sun audio sv-300be Schematic.jpg (89.3 KB, 162 views)

 15th December 2010, 08:46 PM #7 HP8903B diyAudio Member   Join Date: Jun 2010 What wattage of the 100k and 15k ohm resistors do I need? Also, is there a high quality resistor brand that you recommend?
el156
diyAudio Member

Join Date: Apr 2010
Location: ALBUFEIRA ;PORTUGAL
Quote:
 Originally Posted by manp111 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.

HP8903B
diyAudio Member

Join Date: Jun 2010
Quote:
 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?

 15th December 2010, 09:11 PM #10 tomchr   diyAudio Member     Join Date: Feb 2009 Location: Calgary 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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