[Eike Lantzsch, ZP6CGE]

Big Bad Bob wrote:
On 2020-03-03 02:18, Eike Lantzsch, ZP6CGE wrote:
When the EL84 start pulling grid current the difference of
impedance of the cathode side and the anode side of the phase
splitter comes into play. To balance the grid current the anode
side grid resistor is of lower value than the cathode side
resistor. That is careful design. The engineers probably found
out that, for consumer grade equipment, the distortion due to this
effect are negligable compared to the the distortion due to
overload and canceled the resistors to save some cents.

interesting. normally if you care enough to add the series resistors on
the output tubes, you'd have both halves of a 12AU7 driving them so that
it isolates grid current issues from the splitter circuit.

But yeah that's one more tube in the design, and extra $$.

Yes that would be a better design. For more costly amps with more
oomph it was applied - such amps equipped with EL34 or KT88 and the likes.
But for the more economic ones with around 15W output and EL84 tubes the
engineers usually didn't do that. But the latter were good amps
anyway.

For a guitar amp, I'd do it this way (with the extra 12AU7) becaue a
'totem pole' type of splitter does really weird things at the overload
point. if you WANT that sound, go with it. Otherwise, the somewhat
cleaner "final stage distortion" of the splitter - 12AU7 - power tube
combination would be better. Also a series resistor on the splitter's
grid would be needed. Then you'll have very nice predictable behavior
at the clipping point, and WAY beyond that.

aside from that, the different resistor values on the grid resistors is
a bit silly, probably why they were just removed, later. "Muntzed"




Kind regards, Eike