Patrick Turner
March 25th 10, 02:05 AM
Next time I have to build a line preamp I will try using a 6U8A triode-
pentode.
I can see that the pentode maybe set up with bypassed Rk and with 10k
between anode and the follower triode section with B+ at 300V. The
triode can be biased from +160V with pentode Ia
= 4mA and 10k betwen Panode Tcathode. Ouptut is via a cap off
Tcathode.
The Panode may be left unstrapped to the Pscreen. The Pscreen may be
connected to the Tcathode which then supplies the anode signal back
into the pentode via a low resistance source.
The result is that the Rout would be lower than if the P were normally
triode strapped. The P gain would be the same as normal triode
strapping.
Normal shunt FB can be arranged to reduce the gain to 12dB for a line
preamp.
The other alternative is to use 50k load for the P anode to+300V so
that Ia = 4mA.
The triode is direct coupled to the P anode and feeds 1.5mA for Ig2 to
the pentode.
Pcathode is bypassed to 0V. If someone forces T cathode 1V negative,
the pentode screen is also forced negative thus causing the P anode to
go a couple of volts positive, depending of course on the screen grid
gm.
So you'd get say +3V Vg-k of the follower and this perhaps +12mA of
cathode current so Rout = 1V / 12mA = 83 ohms which is a lot lower
than a normal follower. Gain overall would be the same as conventional
triode strapping and it could be reduced with a normal shunt FB
network.
Patrick Turner.
pentode.
I can see that the pentode maybe set up with bypassed Rk and with 10k
between anode and the follower triode section with B+ at 300V. The
triode can be biased from +160V with pentode Ia
= 4mA and 10k betwen Panode Tcathode. Ouptut is via a cap off
Tcathode.
The Panode may be left unstrapped to the Pscreen. The Pscreen may be
connected to the Tcathode which then supplies the anode signal back
into the pentode via a low resistance source.
The result is that the Rout would be lower than if the P were normally
triode strapped. The P gain would be the same as normal triode
strapping.
Normal shunt FB can be arranged to reduce the gain to 12dB for a line
preamp.
The other alternative is to use 50k load for the P anode to+300V so
that Ia = 4mA.
The triode is direct coupled to the P anode and feeds 1.5mA for Ig2 to
the pentode.
Pcathode is bypassed to 0V. If someone forces T cathode 1V negative,
the pentode screen is also forced negative thus causing the P anode to
go a couple of volts positive, depending of course on the screen grid
gm.
So you'd get say +3V Vg-k of the follower and this perhaps +12mA of
cathode current so Rout = 1V / 12mA = 83 ohms which is a lot lower
than a normal follower. Gain overall would be the same as conventional
triode strapping and it could be reduced with a normal shunt FB
network.
Patrick Turner.