# Reducing gain in 12AU7 Aikido Linestage

#### ChrisM91

Below is the stock Aikido linestage circuit topology. I will be building this design with a B+ of 300V and 12AU7 tubes. With R4 = 330R the first stage gain is 8.9. I would like to get the gain lower, around 6.5 Av.

Would splitting up the first stage plate resistor (R4) and making it a split load work to attenuate the same way as it would a traditional split plate load common cathode design?

This is what I am thinking....see below
I would make the top resistor 80R and the bottom resistor 250R, for a Av reduction of ~25% (Av = 6.7). The total resistance remains the same, 330R

Does this make sense? Would this split load work on the Aikido design?

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#### jan.didden

Paid Member
Your plan assumes that if you would take the signal from the top of R4 would make the gain 1.
Is that the case?

Jan

#### nerdorama

John Broskie proposed a divider circuit that goes between the stages where the gain is too high. I've used it and it works as advertised. Scroll toward the bottom of this page:

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#### ChrisM91

Your plan assumes that if you would take the signal from the top of R4 would make the gain 1.
Is that the case?

Jan
See the second schematic in my original post

R4 is split in to 80R and 250R. The output is taken at the junction of the top resistors.

John Broskie proposed a divider circuit that goes between the stages where the gain is too high. I've used it and it works as advertised. Scroll toward the bottom of this page:
Thanks! I did come across this post in my research.

I did not understand how to select the B+ to grid and grid to ground resistors (200K or 150K in John's example).
What value would one select for 12AU7s and a B+ of 300V?

I also figured my proposed idea would be preferred due to the use of smaller value resistors.

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#### Sorento

Does this make sense? Would this split load work on the Aikido design?
No, it would not reduce gain.
The upper triode's grid picks up the amplified signal from the lower triode's plate.
The upper triode's cathode approximately follows its grid (remember cathode follower gain is a bit less than one).
So both ends of R4 see about the same signal.
When both ends of R4 show the same signal, so does any point in between ...

#### ChrisM91

Thank you Sorento! That was very clear and informative!

Do you mind explaining in the circuit below how the 200k resistors are selected?
How does this value depend on the valve used and the B+?

#### grovergardner

It will work with the 12AU7 and any B+. It just attenuates the signal going to the second stage. I tried that in my Aikido and didn't care for the sonic results. I prefer to use left/right gain pots and a master pot. More expensive and complicated but it sounds better to me and gives you balance control between channels as well, which I need in my room. But there's no harm in trying this idea.

#### ChrisM91

What do you mean you didn't care for the sonic results?

I am going to try a 68K with two 200K resistors in my 12AU7 design to get my desired output gain.

#### grovergardner

I mean I didn't like what it did to the sound of the preamp.