I bought a pair of Chinese KT88 SE circuit boards with the intention of using one per channel for a pair of KT120s and one 6SL7 per board as the voltage amplifier. (Gain of about 30-40 required and will be fixed biased)
The pcbs are quite decent and the KT88 sockets are on snap off pcb sections.
However I am struggling to decide the input stage topology.
According to the Tubecad app it seems to be a cross between a self biased totem pole and a current sourced plate follower.
The totem pole gives higher gain, better psrr and lower output impedance but needs a grid resistor on V1b and R2 would need to be reduced to the same value (~ 4.7k) and both cathode resistors being the same.
Any comments or suggestions?
The pcbs are quite decent and the KT88 sockets are on snap off pcb sections.
However I am struggling to decide the input stage topology.
According to the Tubecad app it seems to be a cross between a self biased totem pole and a current sourced plate follower.
The totem pole gives higher gain, better psrr and lower output impedance but needs a grid resistor on V1b and R2 would need to be reduced to the same value (~ 4.7k) and both cathode resistors being the same.
Any comments or suggestions?
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Thanks. I didn't see it on the Valve Wizard website when I was browsing.
Do the values of R4 and R2 look okay?
Do the values of R4 and R2 look okay?
For 6SL7 and assuming a 350-375 volt supply they are fine, R2 is non critical and could be a bit smaller, down to like 180-220k or so since it's bootstrapped and appears larger to the triode. The values for R5, R10, are more critical as they are the cathode bias resistors.
R4 if using 6SN7 seems low and would be better around 33k, as you want it to be 3x or higher than the internal anode resistance of the chosen triode. It could be larger for the 6SL7 too but then you'll maybe need to increase the supply voltage to give it enough headroom.
Medium mu, moderate to high gm triodes are best for mu followers for logistics purposes like this unless you have lots of supply voltage to play with, but then you can get into cathode/heater rating issues too.
R4 if using 6SN7 seems low and would be better around 33k, as you want it to be 3x or higher than the internal anode resistance of the chosen triode. It could be larger for the 6SL7 too but then you'll maybe need to increase the supply voltage to give it enough headroom.
Medium mu, moderate to high gm triodes are best for mu followers for logistics purposes like this unless you have lots of supply voltage to play with, but then you can get into cathode/heater rating issues too.
Don't forget that R4 is bootstrapped by the cathode follower action of the upper triode and will appear to be much larger than the initial resistor values would suggest.
I suspect 10K is fine for the 6SN7 in fact and probably looks like >40K to the plate of the lower triode. I suggest about 47K for the 6SL7.
I suspect 10K is fine for the 6SN7 in fact and probably looks like >40K to the plate of the lower triode. I suggest about 47K for the 6SL7.
Yes, will be using a 6SL7 an the ht will be around 360V so good to know the values are okay. (you can never tell with some Chinese designs!)
Many thanks.
Many thanks.