I've been doing some tweaking on an early version of the a Scott LK72 amp.
This version uses the 7199 phase inverter circuit and originally did not have any provision for adjusting DC Balance or Bias voltage level.
I've added pots to adjust bias and balance and they are working well.
The voltage at the screen lug of the power tube sockets has always run about 10V lower than the plate voltage which is running at about 428V with the bias adjusted at 35ma per tube.
The schematic shows 450V at the plates and 430V at the screens.
The screen voltage goes thru a 1.2K 10watt resistor per the schematic and the resistance on the installed resistor checks good.
I noticed that the schematic for the LK72b has an additional 270 ohm resistor in series with a 1.2K resistor and shows a voltage drop of 43V between screen and plate.
I've replaced the 1.2K resistor in my early LK72 with a 1.5K 10watt and it lowers the screen by 5V over it's original value.
So the screens in this amp do not draw enough current to lower the screen voltage to the schematic value for either the LK72 or the LK72B using the schematic dropping resistor values.
My question is:
Does running the screen voltage this close to the plate voltage create a durability risk with my precious old stock US made 7591 tubes ?
Also, since the original tube application data for the 7591 shows a 50V to 100V lower voltage for the screen what are the advantages of lowering it ?
This version uses the 7199 phase inverter circuit and originally did not have any provision for adjusting DC Balance or Bias voltage level.
I've added pots to adjust bias and balance and they are working well.
The voltage at the screen lug of the power tube sockets has always run about 10V lower than the plate voltage which is running at about 428V with the bias adjusted at 35ma per tube.
The schematic shows 450V at the plates and 430V at the screens.
The screen voltage goes thru a 1.2K 10watt resistor per the schematic and the resistance on the installed resistor checks good.
I noticed that the schematic for the LK72b has an additional 270 ohm resistor in series with a 1.2K resistor and shows a voltage drop of 43V between screen and plate.
I've replaced the 1.2K resistor in my early LK72 with a 1.5K 10watt and it lowers the screen by 5V over it's original value.
So the screens in this amp do not draw enough current to lower the screen voltage to the schematic value for either the LK72 or the LK72B using the schematic dropping resistor values.
My question is:
Does running the screen voltage this close to the plate voltage create a durability risk with my precious old stock US made 7591 tubes ?
Also, since the original tube application data for the 7591 shows a 50V to 100V lower voltage for the screen what are the advantages of lowering it ?