Hi John,
What is the purpose of the cathode followers in front of 6V6s? At 320Vdc
they can deliver as much as they are capable safely in AB1. Overdiriving the
amp with Vg2=320V and going into control grid positive zone will either:
a) burn the screen grids if nominal Ra-a load is present;
b) burn the plates if the load happens to short out.[/quote]
You are correct Alex, the loadline needs to be adjusted from Class AB1 conditions. Otherwise the screen grids draw excessive current & are in danger of failing. For the example I gave in the previous post I set the loadline at 5.6K. The attached PP 6V6 plate family illustrates this but these are not the actual loadlines.
From the trace we see the B+ changes by about 13 volts in 9 ms. The current is coming from a 100 microF cap. If CE = IT, then the current pulse on the PS is 144 mA. So each 6V6 sees a total plate & screen current increase from nominal of 72 mA. The standing current in each is about 38 mA. So peak current during this test was 110 mA. And Ik adds a bit more to account for the G1 current while +ve. Running as a vertical deflection amplifier the 6V6 cathode in peak rated at 115 mA, so looks OK. But in a real audio situation it is unlikely the amplifier would be run at this level. The program material would see to that.
I’ve attached here another trace, this time of music by Peter Gabreil, the beginning of his performance of Salsbury Hill. This one drives into a loudspeaker load. All done while my wife was out, being very loud, far more than one needs while inside! For this one the loadline was at 6.8K.
This trial in Class AB2 is similar to what has been published in the older tube manuals showing 6F6s running PP Class AB2 in both triode & pentode connexion.
My practice is to set the loadline in a PP pentode stage to be somewhat lower impedance than that which results in the maximum output. It is claimed that this helps to lower the 3H. It is also useful where the loudspeaker impedance rises somewhat above nominal at the resonances in some speaker systems.
Cheers, John
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