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6SN7 preamp tube alternative with less gain?

Rewind

Member
2009-01-18 7:42 pm
Oslo
Hi,
I have very sensitive speakers together with a 5 watt Sun Audio clone 2A3 amplifier that uses 6SN7 preamp tubes. I saw an interesting youtube video with a guy explaining why guitarists prefer to use tube amps, and why he should choose a tube amp with just enough wattage so that he can really push the power tubes without too much gain on the preamp tubes and get a meatier sound. For example, if he was only practising on low volume with a 100W Marshall amplifier he would mostly hear the thin sound of the preamp tubes. With a 1W or 5W tube amp he would hear more of the juicy power tubes.

So back to my own setup. I want to reduce the preamp gain without having a gain knob, like on a guitar amp.

I would prefer doing this without modifying the circuit too much, and just change the preamp tubes, but if anyone has any good suggestions on how to do, here it is:
http://www.goodsoundclub.com/Site_Images/Sun-Audio_VT-2A3_3.JPG

I know from building tube booster pedals for guitar that 12ax7 has more gain than a 12au7, and there must be an alternative with less gain for the 6SN7.
 

artosalo

Member
2010-02-16 9:00 am
Remove one or both cathode capacitors of 6SN7. Then you get lower gain without the need to modify the circuit.
The only lower gain alternative I know is 6BL7, but since the front end of your amplifier is DC-coupled, with 6BL7 the need for complete redesign is very likely.
 
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Rewind

Member
2009-01-18 7:42 pm
Oslo
Won't removing the bypass capacitors make a very shrill sound?

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

This calculator will not do 6SN7, but 12AX7. When using a 0.01uF cap I get a treble boost. 0.22uF at least some midrange. At 20uF I get the whole frequency range. But, I don't know what I am doing so correct me if I am wrong.

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/calculator/

ColinA: So I should forget about it? It must have some gain that I can reduce.
 

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While 6SN7 linearity is not an issue, removing the 1st triode's cathode resistor bypass cap. will increase linearity.

If cap. removal does not reduce gain sufficiently, a resistor of 1/2 the volume control's value inserted in the line between the I/P jack and the control will provide 6 dB. of attenuation.

Is the volume control really 1 Mohm? The CMiller of the 6SN7 triode is reasonably small, but I'm very dubious about that much grid to ground resistance. A pentode or cascode rates to be OK with that "tall" a value, but HF rolloff could be an issue with a triode.
 

Rewind

Member
2009-01-18 7:42 pm
Oslo
While 6SN7 linearity is not an issue, removing the 1st triode's cathode resistor bypass cap. will increase linearity.

If cap. removal does not reduce gain sufficiently, a resistor of 1/2 the volume control's value inserted in the line between the I/P jack and the control will provide 6 dB. of attenuation.

Is the volume control really 1 Mohm? The CMiller of the 6SN7 triode is reasonably small, but I'm very dubious about that much grid to ground resistance. A pentode or cascode rates to be OK with that "tall" a value, but HF rolloff could be an issue with a triode.

Just so that we are clear, the 5 watt Sun Audio 2A3 is a hifi amplifier, not for guitar.
Where did you find the 1Mohm resistor? I use an ALPS 100kOhm volume control potentiometer.
Reducing the volume by adding resistance to the volume pot would just give me the same thin preamp sound I was talking about. I want to reduce the gain of the preamp tube, like the in a guitar amp where each gain preamp tube has its own gain knob. I am after the juicy sound of a cranked tube amp, where the power tubes does most of the work, but at lower volume.

You said remove the cap for the first tube, but which is the first?

I do find I get some HF roll off compared to the solid state Firstwatt F5, but I still prefer the tube amp.
 

Koonw

Member
2013-04-09 9:37 pm
Thank you for simulating my amp. More than I could ask for. Please explain more what you did.

R5 is made a variable of 5-10k pot in series with C5, as the resistance is increased the gain will drop. The 1st stage has a gain of 20, the 2nd about the same, if R5 is very large or opened, the 2nd stage will have zero gain (X1). You can vary C5 to roll off below 100hz. You can then overdrive the 1st stage.
 
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Rewind

Member
2009-01-18 7:42 pm
Oslo
R5 is made a variable of 5-10k pot in series with C5, as the resistance is increased the gain will drop. The 1st stage has a gain of 20, the 2nd about the same, if R5 is very large or opened, the 2nd stage will have zero gain (X1). You can vary C5 to roll off below 100hz. You can then overdrive the 1st stage.

So you think to reduce gain I should not remove the cap, but instead add a resistor (F5) between the cap and the ground?
 
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Personally I would start with SY's solution, reducing gain in the first stage is easy and you might get a reduction of 3 - 4dB.

Changing the impedance the cathode in the second stage sees in order to reduce the gain could result in much higher plate resistance in the driver stage which in conjunction with the 2A3's miller capacitance will result in earlier HF roll off. It might or might not sound OK.
 

Koonw

Member
2013-04-09 9:37 pm
Personally I would start with SY's solution, reducing gain in the first stage is easy and you might get a reduction of 3 - 4dB.

Changing the impedance the cathode in the second stage sees in order to reduce the gain could result in much higher plate resistance in the driver stage which in conjunction with the 2A3's miller capacitance will result in earlier HF roll off. It might or might not sound OK.

Yes you're correct. So don't use too high resistor. With 10K res., the output impedance of driver is about 21K, while the input impedance of power stage is 27k including grid resistor. There is no problem so far with my sim.
 

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