New build : micro bassman troubleshooting no indicator light but tubes glowing

Lowerleftcoast

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This micro bassman is amazing i used a p10r from jensen and it sounds great. It just lacks some gain despite having a 12ax7 (i use eh 7025) perhaps should i go for jj tubes ?
If you want more gain, there is a 270k/330k voltage divider before V1b. You can clip in resistors parallel to the 270k to test lower resistances (higher gain) in the preamp. (A 1M across the 270k will be equal to about 212k. A 470k across the 270k will be equal to about 170k.)

More gain could be had by reducing the 33k grid stopper at the input as well. You can go as low as 15k and still maintain good RFI filtering. I would probably just adjust the voltage divider rather than change this though.

From my experience, the JJ 12AX7 has a grittier quality than the EH. It may be preferred for high gain type sounds but not always. It shouldn't make much difference in gain.
 

Pat_rocks92

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If you want more gain, there is a 270k/330k voltage divider before V1b. You can clip in resistors parallel to the 270k to test lower resistances (higher gain) in the preamp. (A 1M across the 270k will be equal to about 212k. A 470k across the 270k will be equal to about 170k.)

More gain could be had by reducing the 33k grid stopper at the input as well. You can go as low as 15k and still maintain good RFI filtering. I would probably just adjust the voltage divider rather than change this though.

From my experience, the JJ 12AX7 has a grittier quality than the EH. It may be preferred for high gain type sounds but not always. It shouldn't make much difference in gain.
So what you're saying is that i should replace the 270k resistor ? by what sound the best for me ?

a resistor lower than 270k ?
 

Lowerleftcoast

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So what you're saying is that i should replace the 270k resistor ? by what sound the best for me ?

a resistor lower than 270k ?
The guitar signal is an AC voltage. The tubes increase the voltage. Usually the increase in voltage is more than what a later stage can handle or we may want to adjust the frequencies of the guitar signal so, the voltage is decreased with a voltage divider or a RC filter. We also use (volume) potentiometers so we can adjust the signal voltage of the amplified signal. (A potentiometer is just a variable voltage divider.)

It seems the best sound for *you* is more gain (voltage) than what the designer allowed for in this design. Since the designer used this 270k/330k voltage divider to reduce gain. I have proposed changing that divider to allow for more voltage (gain) to make it to the next stage.
This can be done in a few different ways. The 270k can be smaller or the 330k resistor can be larger. Another option is to replace those two resistors with a potentiometer so you can dial in what sounds best.
In your situation, if you want to dial in that voltage divider you can use a 1M potentiometer as a variable resistor. The center terminal and one of the other terminals can be alligator clipped to the each side of the 270k resistor. When you find what sounds best you can meter the resistance of the pot and calculate the *new* resistance of the (270k) top resistor of the voltage divider. Replace the 270k resistor with the resistance you have found.
 

Pat_rocks92

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The guitar signal is an AC voltage. The tubes increase the voltage. Usually the increase in voltage is more than what a later stage can handle or we may want to adjust the frequencies of the guitar signal so, the voltage is decreased with a voltage divider or a RC filter. We also use (volume) potentiometers so we can adjust the signal voltage of the amplified signal. (A potentiometer is just a variable voltage divider.)

It seems the best sound for *you* is more gain (voltage) than what the designer allowed for in this design. Since the designer used this 270k/330k voltage divider to reduce gain. I have proposed changing that divider to allow for more voltage (gain) to make it to the next stage.
This can be done in a few different ways. The 270k can be smaller or the 330k resistor can be larger. Another option is to replace those two resistors with a potentiometer so you can dial in what sounds best.
In your situation, if you want to dial in that voltage divider you can use a 1M potentiometer as a variable resistor. The center terminal and one of the other terminals can be alligator clipped to the each side of the 270k resistor. When you find what sounds best you can meter the resistance of the pot and calculate the *new* resistance of the (270k) top resistor of the voltage divider. Replace the 270k resistor with the resistance you have found.
Thanks ! your explanations are clear !

I will perhaps do some electronic studies after my degree.

Building amps is a lot of fun despite being a dangerous hobby.

i think i'm going to replace the 270k by a 150k resistor and see what happens. Do you think it will have a big impact ?
 
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