I have a Frankenstein version of the venerable Eico ST70, modified with no preamp section so it only operates as a basic power amplifier.
Gain is silly high - especially with a Audio Research SP-8 preamp. Short of rebuilding with a H-K Citation V circuit, what input tube would you use to replace the 12AX7?
See here for the 12AX7/6SN7/5881 circuit
http://tronola.com/html/st-70_mods.html
Gain is silly high - especially with a Audio Research SP-8 preamp. Short of rebuilding with a H-K Citation V circuit, what input tube would you use to replace the 12AX7?
See here for the 12AX7/6SN7/5881 circuit
http://tronola.com/html/st-70_mods.html
In that power amp section, there are several parts that act as a system together to set the gain of the amplifier and its overall performance.
In that power amp section, the 12AX7 voltage amp stage has a 200k plate load resistor. That's a high value, meant to get a lot of gain from that stage.
R85 sets the amount of negative feedback around the power amplifier section. You could reduce the value of that resistor to increase the amount of negative feedback, thus reducing overall gain. However, you may run into stability problems as the NFB loop causes oscillation due to the reactance of the output transformer or possible limitations of the power supply.
C35 introduces a high frequency shelf filter to compensate the NFB loop against oscillation. If you change the value of R85, you'll probably also have to change the value of C35.
If you change the input tube type from 12AX7 to a type with lower gain (like 6CG7, 6N1P, 12AT7, 12AU7, etc.) you'll need to change the values of R53 and R55 to work with the new tube type.
If you change the tube type from 12AX7 to something with lower gain, you'll also be reducing the level of NFB around the power amp section, so once again you'll need to change the values of R35 and C35 to the correct ones for the level of NFB desired.
Because this power amplifier section uses global negative feedback, you'll want to make sure your modifications include optimization of the negative feedback for the new devices you choose.
Or you could just swap stuff in and out and see what you like best. It depends on what your goals are.
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In that power amp section, the 12AX7 voltage amp stage has a 200k plate load resistor. That's a high value, meant to get a lot of gain from that stage.
R85 sets the amount of negative feedback around the power amplifier section. You could reduce the value of that resistor to increase the amount of negative feedback, thus reducing overall gain. However, you may run into stability problems as the NFB loop causes oscillation due to the reactance of the output transformer or possible limitations of the power supply.
C35 introduces a high frequency shelf filter to compensate the NFB loop against oscillation. If you change the value of R85, you'll probably also have to change the value of C35.
If you change the input tube type from 12AX7 to a type with lower gain (like 6CG7, 6N1P, 12AT7, 12AU7, etc.) you'll need to change the values of R53 and R55 to work with the new tube type.
If you change the tube type from 12AX7 to something with lower gain, you'll also be reducing the level of NFB around the power amp section, so once again you'll need to change the values of R35 and C35 to the correct ones for the level of NFB desired.
Because this power amplifier section uses global negative feedback, you'll want to make sure your modifications include optimization of the negative feedback for the new devices you choose.
Or you could just swap stuff in and out and see what you like best. It depends on what your goals are.
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