2SK170 should work, but you might have to select them for a particular Idss or adjust some of the resistor values.
You would also have to match the two input devices very well, preferably at more than one gate-to-source voltage, to make sure of both DC and AC symmetry.
J309 might work but you might need to reduce the power supply somewhat, to make sure the 25 V rating isn't exceeded.
Please double check the current ratings before taking any of my suggestions; I admit I haven't done any exact calculations here.
Another one to look at: 2N5459. Again, check voltage and current ratings. But that one may not have high enough transconductance.
Basically you want a high transconductance FET with low noise and sufficient voltage and current handling capability.
Please post or email me the rest of the schematic: I'll look into it further and will do some calculations.
Do you have the schematic for the first version of this line stage? This one appears to be for the Mk II LS-25.
-- mirlo
You would also have to match the two input devices very well, preferably at more than one gate-to-source voltage, to make sure of both DC and AC symmetry.
J309 might work but you might need to reduce the power supply somewhat, to make sure the 25 V rating isn't exceeded.
Please double check the current ratings before taking any of my suggestions; I admit I haven't done any exact calculations here.
Another one to look at: 2N5459. Again, check voltage and current ratings. But that one may not have high enough transconductance.
Basically you want a high transconductance FET with low noise and sufficient voltage and current handling capability.
Please post or email me the rest of the schematic: I'll look into it further and will do some calculations.
Do you have the schematic for the first version of this line stage? This one appears to be for the Mk II LS-25.
-- mirlo
Am I right, is the shematic wrong or does p=I2r not apply in vintage circuits
I'll second that Steve. 
Luke, to calculate the power all you need here is Vsquared divided by R. which works out to be about 0.3W. The supply voltage is 180V (given) the voltage at the grids of the 6H30 is 70V (given) so the difference is 110V, square that and divide by 40K to get the power dissipated in that resistor.
Luke, to calculate the power all you need here is Vsquared divided by R. which works out to be about 0.3W. The supply voltage is 180V (given) the voltage at the grids of the 6H30 is 70V (given) so the difference is 110V, square that and divide by 40K to get the power dissipated in that resistor.
LS25MKII
Hello,
I built a clone of the LS25MKII ARC with the schematic of the post #1.
The working is very correct.
A simple calculation shows that tensions mentioned on this schematic (+ 70V and +20V) are false if HT = +180V.
If +20V on drains of the Fets, one cannot have +70V (Grid of the 6H30) but rather +60V
If +70V on grids of the 6H30, one cannot have +20V on drains of the Fets.
For the LS25MKII, AudioResearch Corporation says:
RATED OUTPUTS: maximum balanced output capability is 40V RMS at less than 0.5% THD at 1kHz).
On my clone, I have approximately only 20V RMS as maximum balanced output capability
Is there a modified version of this schematic that provide in balanced output 40V RMS ???
regards
jlm
Hello,
I built a clone of the LS25MKII ARC with the schematic of the post #1.
The working is very correct.
A simple calculation shows that tensions mentioned on this schematic (+ 70V and +20V) are false if HT = +180V.
If +20V on drains of the Fets, one cannot have +70V (Grid of the 6H30) but rather +60V
If +70V on grids of the 6H30, one cannot have +20V on drains of the Fets.
For the LS25MKII, AudioResearch Corporation says:
RATED OUTPUTS: maximum balanced output capability is 40V RMS at less than 0.5% THD at 1kHz).
On my clone, I have approximately only 20V RMS as maximum balanced output capability
Is there a modified version of this schematic that provide in balanced output 40V RMS ???
regards
jlm
Go to http://www.arcdb.ws/
down the page...
click Linestages...
down the page...
click LS25...
There are circuit diagrams for Mk1 (four tube version) and Mk 2 (FET input version).
Enjoy...
down the page...
click Linestages...
down the page...
click LS25...
There are circuit diagrams for Mk1 (four tube version) and Mk 2 (FET input version).
Enjoy...
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