I just thought of a couple more things we need to check KArl
Take a voltage reading on pin 9 of the output tubes and take a resistance reading from Pin 3 to ground the output tubes...Do the resistance check with it unplugged of course but we need to find out why these tubes aren't pulling current.
Take a voltage reading on pin 9 of the output tubes and take a resistance reading from Pin 3 to ground the output tubes...Do the resistance check with it unplugged of course but we need to find out why these tubes aren't pulling current.
Karl
Did you measure the actual voltage coming out of the variac or just use the dial reading or meter on the variac reading? If so,measure the actual AC voltage coming out of the variac.
I have 2 meters, both give virtually the same readings.
Unfortunately the Scott is just one of the projects (not all audio) I have going, and it is one that my wife really doesn't care much about. I think that officially puts it on back-burner status for the time being.
Thank everyone for the input and when I get back to it I will give this thread a bump and hit you all up with my rookie questions. Maybe I'll even read something on the subject in the meantime, besides the tube datasheets.
Unfortunately the Scott is just one of the projects (not all audio) I have going, and it is one that my wife really doesn't care much about. I think that officially puts it on back-burner status for the time being.
Thank everyone for the input and when I get back to it I will give this thread a bump and hit you all up with my rookie questions. Maybe I'll even read something on the subject in the meantime, besides the tube datasheets.
This is an interesting thread for me.
Have just completed a repair/upgrade on an HH Scott LK-48 for a friend.
This is my first experience with this amp, which I consider to be one of the best of the era.
This amp started life in the original version with 7189/7199 tubes but has been modified to 6U8A and 6BQ5/EL84 tubes at some time in its life.
Apart from the direct plug-in tube change there have been no other modifications.
I have the schematics for both the LK-48 and LK-48B designs, as well as the similar 222C.
The power amps and power supply sections in these amps are similar.
CONTROL GRID BIAS SUPPLY:
A design point that seems to have been overlooked in the previous discussion in this thread is that the heaters for the 4 x 12AX7 pre-amp tubes are supplied with DC from the bias rectifier.
DC current is about 600 milliamps so a 1A FWB silicon rectifier is required for reliability.
The voltage is set to 50 VCD by the design and should ideally be fixed at about that.
The actual bias voltage at the 6BQ5/7199 grids (Grid #1) is somewhat less due to the voltage divider network in the bias balancing circuit and grid current in the power tubes.
Note the bias adjustment pots do not adjust the bias up or down as such but are intended to balance the cathode current in the power tubes for push-pull circuit optimisation.
Since the G1 circuit resistance should be minimised as preferred design practice it follows that to vary the applied G1 DC voltage the circuit must be modified between the bias supply voltage (-50VDC) and the inputs to G1.
But given the maximum G1circuit resistance is limited to 220k to ground by the grid resistors, the bias supply components can be modified to suit without affecting that basic fixed network or the load on the previous driver stage.(7199/6U8A)
Although the 6BQ5 is rated for up to 300K G1 circuit resistance it is wise to err on the side of caution and stay with the HH Scott design value of 220k.
PLATE AND SCREEN GRID SUPPLIES:
In the case of the B+ voltage there is a problem with this series of designs and that is excessive plate and screen supply voltages.
The power transformer secondary design centre voltage of 350/360 VAC ensures a DC voltage of up to 500VDC approx. peak at the 5AR4 rectifier output at switch-on, reducing as the power tubes warm up and conduct.
If the power tube bias in too positive (i.e less negative) the amp will operate in Class B and draw minimal cathode current - resulting in the B+ remaining excessively high during normal operation.
After extensive experimentation I have found that a mains input voltage of not more than 110 VAC is appropriate. Any more than that is likely to reduce tube life and lead to reducing performance over time.
Excessive G2 Screen Grid voltages may produce self-oscillation in the 6BQ5/7189 output stage and cause many problems with the sound quality. Red-hot plates is an indicator of this issue.
If the plate voltage exceeds the circuit values I would reduce the G2 voltage by increasing the voltage drop in the screen supply circuit - i.e increase the 1.2k resistor in the series voltage divider network to a higher value (with an appropriate increase in rated wattage)
My amp is happy with a 105VAC mains input.
For those who do not like the sound of the 6BQ5/7189 pentodes and prefer beam power tubes, the 6973/6CZ5 is a fine alternative. The pinout is different - refer the tube data sheets before proceeding.
Have just completed a repair/upgrade on an HH Scott LK-48 for a friend.
This is my first experience with this amp, which I consider to be one of the best of the era.
This amp started life in the original version with 7189/7199 tubes but has been modified to 6U8A and 6BQ5/EL84 tubes at some time in its life.
Apart from the direct plug-in tube change there have been no other modifications.
I have the schematics for both the LK-48 and LK-48B designs, as well as the similar 222C.
The power amps and power supply sections in these amps are similar.
CONTROL GRID BIAS SUPPLY:
A design point that seems to have been overlooked in the previous discussion in this thread is that the heaters for the 4 x 12AX7 pre-amp tubes are supplied with DC from the bias rectifier.
DC current is about 600 milliamps so a 1A FWB silicon rectifier is required for reliability.
The voltage is set to 50 VCD by the design and should ideally be fixed at about that.
The actual bias voltage at the 6BQ5/7199 grids (Grid #1) is somewhat less due to the voltage divider network in the bias balancing circuit and grid current in the power tubes.
Note the bias adjustment pots do not adjust the bias up or down as such but are intended to balance the cathode current in the power tubes for push-pull circuit optimisation.
Since the G1 circuit resistance should be minimised as preferred design practice it follows that to vary the applied G1 DC voltage the circuit must be modified between the bias supply voltage (-50VDC) and the inputs to G1.
But given the maximum G1circuit resistance is limited to 220k to ground by the grid resistors, the bias supply components can be modified to suit without affecting that basic fixed network or the load on the previous driver stage.(7199/6U8A)
Although the 6BQ5 is rated for up to 300K G1 circuit resistance it is wise to err on the side of caution and stay with the HH Scott design value of 220k.
PLATE AND SCREEN GRID SUPPLIES:
In the case of the B+ voltage there is a problem with this series of designs and that is excessive plate and screen supply voltages.
The power transformer secondary design centre voltage of 350/360 VAC ensures a DC voltage of up to 500VDC approx. peak at the 5AR4 rectifier output at switch-on, reducing as the power tubes warm up and conduct.
If the power tube bias in too positive (i.e less negative) the amp will operate in Class B and draw minimal cathode current - resulting in the B+ remaining excessively high during normal operation.
After extensive experimentation I have found that a mains input voltage of not more than 110 VAC is appropriate. Any more than that is likely to reduce tube life and lead to reducing performance over time.
Excessive G2 Screen Grid voltages may produce self-oscillation in the 6BQ5/7189 output stage and cause many problems with the sound quality. Red-hot plates is an indicator of this issue.
If the plate voltage exceeds the circuit values I would reduce the G2 voltage by increasing the voltage drop in the screen supply circuit - i.e increase the 1.2k resistor in the series voltage divider network to a higher value (with an appropriate increase in rated wattage)
My amp is happy with a 105VAC mains input.
For those who do not like the sound of the 6BQ5/7189 pentodes and prefer beam power tubes, the 6973/6CZ5 is a fine alternative. The pinout is different - refer the tube data sheets before proceeding.