That formula is valid only if the open loop gain is high which does not apply here.
Any change of the value of R44 will change the operating point of the tube. You have to change both R44 and R43 keeping their sum constant.
Any change of the value of R44 will change the operating point of the tube. You have to change both R44 and R43 keeping their sum constant.
You thought that why? You can change any of them, keeping the ratio R42/(R43+R44) the same to preserve the valve operating point.
Marcel's suggestion is a good one.
Marcel's suggestion is a good one.
I contacted the manu. and was told I could just experiment with R44 if I wanted to lower gain, no mention of having to deal with other resistors or operating point. What I didn't know is how to calc dB, or if there was a formula for this circuit, so I had a baseline given a dB value. Marcel's suggestion does make sense, but at this point I might as well "play" with different gains. So the formula is very helpful.
Change it to about 15K or 18K and see if that's more to you liking. (I wouldn't try the whole 10db all at once, just to make sure the circuit is happy with the change... Heck, I'd personally change the resistor to 10K for the first try.) If you want a bigger change, then try 24K.
Also, you do have a volume knob...
Also, you do have a volume knob...
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It actually IS the source and the amp... in that the source is both strong and low impedance, because it's modern, and the amp likely has 20db or more of gain all by itself - therefore a preamp with gain is completely unnecessary in relation to the total system gain, but you still need one for source switching and volume control. This is a common issue.
That said, as you suspect, lowering the gain of the preamp is a great solution. The other thing to consider is a follower amplifier, like F4 or MoFo... where the stock gain of your preamp can be put to very good use.
🙂
That said, as you suspect, lowering the gain of the preamp is a great solution. The other thing to consider is a follower amplifier, like F4 or MoFo... where the stock gain of your preamp can be put to very good use.
🙂
I doubt that they contemplated R44 anywhere near as high as 24K. That puts +38V on the grid, which it won't like at all.
The source voltage of Q2, current through Q2 and anode-cathode voltage of the valve will drop with increasing R44. The grid voltage will only increase a little until the valve runs out of anode-cathode voltage.
Anyway, if you want to reduce the gain more than is possible with my earlier suggestion, replace R42 with a series connection of two resistors of which the sum of the resistances is 47 kohm (mind the power handling) and connect C19 to the tap.
Anyway, if you want to reduce the gain more than is possible with my earlier suggestion, replace R42 with a series connection of two resistors of which the sum of the resistances is 47 kohm (mind the power handling) and connect C19 to the tap.
