re: ST-70 Board with Sansui Iron?
Posted: Thu May 18, 2006 6:07 amHi Christian,
I like your plan and see no problem with it, ie a trioded EL34 or KT88 amp. If you don't have sensitive speakers you might consider going pentode, but this will complicate your power supply and require careful feedback adjustment.
The diytube ST70 driver will be fine for this project. Your biasing will still be limited by an outboard bias supply from your bias tap - just like in the ST70 design. I think your 87V line will be more than enough to bias KT88s or KT90s. Just copy the ST70 bias, and lower the resistance of the first resistor if needed. You could even contemplate using 6B4Gs. With a trioded amp you only need about 6db of negative feedback - enough to remove excessive noise and help dampening some, yet still let the 'triode sound' survive. You can adjust this yourself without anything more than a DMM. Should sound great.
For triode with little feedback omit:
R9,R10,R13,R14,C3,C4,C5,C6
Replace R9 and R10 with ~47K resistors for starters. Disconnect feedback line to observe amount of feedback applied (skip the math and do the real world test). Change as needed. As small bypass cap at C3 & C4 if needed. Consider upping C1 & C2 to .22uF and C7 thru C10 to .1uF, due to less feedback.
Shannon
I like your plan and see no problem with it, ie a trioded EL34 or KT88 amp. If you don't have sensitive speakers you might consider going pentode, but this will complicate your power supply and require careful feedback adjustment.
The diytube ST70 driver will be fine for this project. Your biasing will still be limited by an outboard bias supply from your bias tap - just like in the ST70 design. I think your 87V line will be more than enough to bias KT88s or KT90s. Just copy the ST70 bias, and lower the resistance of the first resistor if needed. You could even contemplate using 6B4Gs. With a trioded amp you only need about 6db of negative feedback - enough to remove excessive noise and help dampening some, yet still let the 'triode sound' survive. You can adjust this yourself without anything more than a DMM. Should sound great.
For triode with little feedback omit:
R9,R10,R13,R14,C3,C4,C5,C6
Replace R9 and R10 with ~47K resistors for starters. Disconnect feedback line to observe amount of feedback applied (skip the math and do the real world test). Change as needed. As small bypass cap at C3 & C4 if needed. Consider upping C1 & C2 to .22uF and C7 thru C10 to .1uF, due to less feedback.
Shannon