For 6V6s (or 6L6s):
Pin 1 = no connection
Pin 2 = heater filament
Pin 3 = Plate
Pin 4 = Screen
Pin 5 = Grid
Pin 6 = No connection
Pin 7 = heater filament
Pin 8 = cathode
The pin socket tabs for Pins 1 and 6 (having no internal connection to electrodes inside the tube) are often used to anchor 'other' lead(s) - i.e. for the screen resistor or grid resistor (when these resistors are tied directly to Pins 5 or 4 respectively). But make sure, if using these pin tabs, that the connection for each pin tab is only to one of the resistor leads (and not to the leads on both the screen and grid resistor at the same time to the same pin tab - because that would be bad).
However, sometimes Pin 1 can be tied to Pin 8, and so be tied to the cathode. (when this is done, it is to help 'eat up' AC from the adjoining heater filament pin tabs). If this is done, make sure the Pin 1 tab isn't tied to the cathode as well as to the screen (or grid) pin tab at the same time.
In a PR, which is fixed bias, the Pin 8 tab needs to go to ground (usually either directly, or via a 1R cathode-current measuring resistor in series between the Pin 8 tab and the chassis ground). This connection is needed for the output tube to conduct - even though this pin voltage should be sitting at about 0VDC
Pin 5 needs to be connected directly to the bias voltage source (as well as to the coupling cap(s) from the Phase inverter). If using a grid resistor, this can be in series between the bias supply and the socket pin tab - usually with the other grid resistor lead connected to the Pin 1 tab (as described above). This pin voltage should be sitting at about -20VDC to -27VDC (so that you get about 18 to 24mA of tube current - measured at the cathode current measuring resistor)
Pin 4 needs to go to the screen grid resistor, (usually with the other resistor lead going to the Pin 6 tab - as described above). This connection is needed to turn the tube current 'on' (If the other connections are all connected, but the screen is at 0V, it will suppress tube current to such an extent that the tube won't conduct). The voltage on this should be close to or new the same as, the B+ voltage - or the same as the HT supply voltage for the screen supply node i.e. 410ish VDC.
Pin 3 needs to go to one of the output transformer primary winding ends. (and the Pin 3 tab on the other socket goes to the opposite OT primary end). This connection is also needed for the tube to conduct and supply current to the tube (through the OT primary winding back to the primary centre tap, which is connected to the B+ (incl reservoir cap). The voltage on this should be within a volt or two of the B+ voltage (i.e. 410- 430Vish)
The tabs for Pins 2 and 7 go (one each) to each side of the 6.3VAC heater circuit - which needs to be connected to the heater winding on the Power transformer. These connections are needed to heat up the cathode to the required temperature to ensure easy release of electrons. If the heaters aren't connected, the tube won't go 'on'. you should measure 6.3ish VAC between Pins 2 and 7, if everything is working properly.
Topic: AB763, Bias voltage issue? (Read 7278 times)