Adjustable Ultra Linear Linestage
This circuit is to fulfill my curiosity of hearing the sonic differences between pentode, ultra-linear and triode gain stage. As with any line stage, this circuit can also be the input part of a power amplifier. I'm considering a single ended amplifier where both the input and output stages have adjustable ultralinear percentage.. I'll show the output stage design on another post.
Anyway, i'm not sure there will be an audible difference between each mode other than gain.. but until i actually build this circuit, i can't really be sure.
The idea is to use a pot to replace R5+R6. Dual 250K linear pot should do for both channel. You then adjust the screen feedback percentage by adjusting the ratio between R5 and R6. Here's how you obtain each mode:
1. Pentode mode is by making R5 = 200k and R6 = 0R. This will keep the screen grid at at fixed voltage regardless of plate swing.
2. Triode mode is by making R5 = 0R and R6=200k. This way the screen grid will follow the swing of the plate.
3. Ultra Linear mode is by making R5 and R6 value somewhere between the values above. Just make sure to keep the total R5+R6 value constant. This is why a potentiometer is suggested. To obtain 50% ultra-linear, we simply make R5=R6=100k.
Here's the comparison between each mode by observing the voltage swing between the plate and screen of the EF86. All graphs are obtained with input signal of 2Vpp.
Pentode Mode, R5 =200k and R6 bypassed
50% Ultra Linear, R5=100k and R6=100k
Triode Mode, R5 bypassed and R6=200k.
Okay.. it's not really triode mode as the screen doesn't exactly follow the plate swing so it's more like 90% ultra linear. This is because cathode followers don't exactly have a gain of 1.
We can observe that the highest gain is obtained by using pentode mode and triode mode yields lowest gain. This will be the most audible differences.
Some notes:
D1 is safety diode to prevent U4 grid from damage during start up. When the cathode is still cold during first starts, U4 grid will be at full B+ potential while it's cathode will be at ground potential. This could damage the grid.
D2 is a voltage shifting zener. Without this, the EF86's screen could be at higher potential than the plate which is something we don't want for long life. Why? Because on each cathode follower (U3 and U4), the cathode voltage is shifted higher than the grid. This will then reflect on U2's screen grid. D2 value should at least be the same as total Vgk on each cathode follower. D2 can also be put under the cathode U3. The one i'm using for the sim is 8.2V zener which makes both plate and screen of the EF86 sit at 140VDC when idle.
I personally like that there is a cathode follower on the output. With this, we can drive the grid of fixed-biased output stage where the grid leak resistor is usually of lower value than if auto-bias is used.
Anyway, i'm not sure there will be an audible difference between each mode other than gain.. but until i actually build this circuit, i can't really be sure.
The idea is to use a pot to replace R5+R6. Dual 250K linear pot should do for both channel. You then adjust the screen feedback percentage by adjusting the ratio between R5 and R6. Here's how you obtain each mode:
1. Pentode mode is by making R5 = 200k and R6 = 0R. This will keep the screen grid at at fixed voltage regardless of plate swing.
2. Triode mode is by making R5 = 0R and R6=200k. This way the screen grid will follow the swing of the plate.
3. Ultra Linear mode is by making R5 and R6 value somewhere between the values above. Just make sure to keep the total R5+R6 value constant. This is why a potentiometer is suggested. To obtain 50% ultra-linear, we simply make R5=R6=100k.
Here's the comparison between each mode by observing the voltage swing between the plate and screen of the EF86. All graphs are obtained with input signal of 2Vpp.
Pentode Mode, R5 =200k and R6 bypassed
50% Ultra Linear, R5=100k and R6=100k
Triode Mode, R5 bypassed and R6=200k.
We can observe that the highest gain is obtained by using pentode mode and triode mode yields lowest gain. This will be the most audible differences.
Some notes:
D1 is safety diode to prevent U4 grid from damage during start up. When the cathode is still cold during first starts, U4 grid will be at full B+ potential while it's cathode will be at ground potential. This could damage the grid.
D2 is a voltage shifting zener. Without this, the EF86's screen could be at higher potential than the plate which is something we don't want for long life. Why? Because on each cathode follower (U3 and U4), the cathode voltage is shifted higher than the grid. This will then reflect on U2's screen grid. D2 value should at least be the same as total Vgk on each cathode follower. D2 can also be put under the cathode U3. The one i'm using for the sim is 8.2V zener which makes both plate and screen of the EF86 sit at 140VDC when idle.
I personally like that there is a cathode follower on the output. With this, we can drive the grid of fixed-biased output stage where the grid leak resistor is usually of lower value than if auto-bias is used.
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