At 4325h usage of JJ6550 i felt it's time to evaluate the 6550.
These observations might be of value to other users. Don't
hesitate to ask for details and or other type of measurements.
New in my toolkit is a couple of hameg stuff, a 8027 distortion meter,
a 8037 low-distortion sine generator and more.
All measurement is done with one channel driven and 230V mains. 8 ohm resistive load
JJ ecc82 as smalltubes, jj gz34 as rectifier
Status before any changes : bias at 450mV, jj6550 used for 4325h.
At 1V output 0.04% dist
at 17V ( 36w ) 1.36% dist, this is the highest power below clipping
The 6550 seems to have lacked some power in spite keeping bias well
during the 4000h they are used. No power measurment done at start
of period.
Some new JJ tubes tested :
JJ EL34-II :
1V 0.04%
( 1V corresponds to 0.125w power)
18V 450mV bias ; 0.61%
18V 500mV bias (optimal) : 0.67%
( 18V corresponds to 40.5w power over 8 ohm )
JJ EL34 (kit 1) :
1V 0.04% 500 and 600mA
18V 500mA 0.9% 600mA 0.5%
JJ El34 ( kit 2) :
500mV bias
1V 0.05% same with 520mV bias
18V 500mV bias 0,76%
18V 520mV bias ( optimum value for full power ) 0.64%
Conclusion seems to be that at full power higher bias reduces distortion.
At low power bias made no difference , no crossover distortion sighted, in
spite looking at the distortion remains where the 8027 has an output for
the remains. What could be observed was noise hum and higher orders of
distortion components.
As for the 6550 i have original and current reading taken with a maxi-matcher :
tube 1 2 3 4
Bias mA
new 32 35 34 31
now 26.6 26.3 26.0 24.5
Gm ( as per maximatcher )
new 414 430 428 416
now 368 345 355 354
tubes 1 and 2 was left channel ( the measured one)
These observations might be of value to other users. Don't
hesitate to ask for details and or other type of measurements.
New in my toolkit is a couple of hameg stuff, a 8027 distortion meter,
a 8037 low-distortion sine generator and more.
All measurement is done with one channel driven and 230V mains. 8 ohm resistive load
JJ ecc82 as smalltubes, jj gz34 as rectifier
Status before any changes : bias at 450mV, jj6550 used for 4325h.
At 1V output 0.04% dist
at 17V ( 36w ) 1.36% dist, this is the highest power below clipping
The 6550 seems to have lacked some power in spite keeping bias well
during the 4000h they are used. No power measurment done at start
of period.
Some new JJ tubes tested :
JJ EL34-II :
1V 0.04%
( 1V corresponds to 0.125w power)
18V 450mV bias ; 0.61%
18V 500mV bias (optimal) : 0.67%
( 18V corresponds to 40.5w power over 8 ohm )
JJ EL34 (kit 1) :
1V 0.04% 500 and 600mA
18V 500mA 0.9% 600mA 0.5%
JJ El34 ( kit 2) :
500mV bias
1V 0.05% same with 520mV bias
18V 500mV bias 0,76%
18V 520mV bias ( optimum value for full power ) 0.64%
Conclusion seems to be that at full power higher bias reduces distortion.
At low power bias made no difference , no crossover distortion sighted, in
spite looking at the distortion remains where the 8027 has an output for
the remains. What could be observed was noise hum and higher orders of
distortion components.
As for the 6550 i have original and current reading taken with a maxi-matcher :
tube 1 2 3 4
Bias mA
new 32 35 34 31
now 26.6 26.3 26.0 24.5
Gm ( as per maximatcher )
new 414 430 428 416
now 368 345 355 354
tubes 1 and 2 was left channel ( the measured one)
Last edited by peterh on Mon Mar 11, 2019 8:43 am; edited 1 time in total (Reason for editing : Added maximatcher readings)
I just look at the mod and I was mixing up the red lead with the whites lead (Filaments, for the ST-70)).