[Alex Pogossov]

"John L Stewart" wrote in message
...

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.

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.

With Vg2=250V (as per your load line plot) yeah, AB2 might be justified to
suck everything from the poor 6V6. But for Vg2=+320V in pentode (as per your
schematic) I guess AB2 is too much for a poor 6V6..


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.

This makes sense. Since in PP you do not have 2H anyway (in theory), you
have nothing to lose.
Then, the control grid characteristic emphasizes the peaks because of the
"law 3/2" curvature, but plate-to-screen current distribution tends to
compress the peaks. These two effects somewhat cancel each other at a
certain load, thus minimising 3H.


Cheers, John


+-------------------------------------------------------------------+
|Filename: 6V6GTA 14W 4 PushPull C 5W.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=345|
|Filename: 2013_03_21_001 Peter Gabriel Salsbury Hill 8W.jpg |
|Download: http://www.audiobanter.com/attachment.php?attachmentid=346|
+-------------------------------------------------------------------+



--
John L Stewart