# 300B SET amp input sensitivity, Help.

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#### rongon

I think in order to answer that question in a meaningful way, the folks here will need to see a schematic.

-=|=-

#### el156

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

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

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

Here is the schematic.

So the "voltage divider" IS the two resistors?

#### Attachments

• sun audio sv-300be Schematic.jpg
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#### HP8903B

What wattage of the 100k and 15k ohm resistors do I need?

Also, is there a high quality resistor brand that you recommend?

#### el156

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

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?

#### tomchr

Paid Member
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

#### el156

You are talking about the grid of the input tubes?
Yes ,the grid of the input tubes !

#### HP8903B

Thanks Tom for the clarification.

I found a better solution: instead of lowering the input sensitivity of the 300B amp I think I can increase the input sensitivity of my 845 amp.

I found a resistor from the input tube grid to the positive input and a resistor from grid to ground in the 845 amp. It seems like I could remove these two resistors to increase the input sensitivity. What do you guys think of this? This would be a much easier solution.

#### HP8903B

The 845 amp also has two volume pot.

#### HP8903B

Yes ,the grid of the input tubes !

Thank you very much for the information. This forum and its members are wonderful.

#### HP8903B

Can I just remove the resistor from grid to ground? Or do I need a lower value resistor?

#### tomchr

Paid Member
I found a resistor from the input tube grid to the positive input and a resistor from grid to ground in the 845 amp. It seems like I could remove these two resistors to increase the input sensitivity. What do you guys think of this? This would be a much easier solution.

Schematic?

On most tube amps you'll find a resistor in the range of 470 kOhm to a few MOhm connected from the grid of each tube to ground. That resistor is needed for bias stability and does not appreciably affect the overall gain of the amp. Leave it!

Sometimes you'll also see a resistor of a few kOhm in series with the grid. Those are called grid stoppers and are used to avoid oscillations in the tube. They are needed.

~Tom

#### HP8903B

Got it. Thank you.

#### HP8903B

I don't have a schematic for this amp.

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