Audio Note Interstage Mono Block 300D Instruction Manual page 4

I explained in the interstage transformer article about the virtues of transformer coupling to the output valves, and here the
bifilar interstage transformer is used to full advantage. The extremely high impedance AC load to the 6SH7 means that
you get more or less the full gain of the valve, and the loadline is horizontal, which means because the 6SH7 is so linear
the distortion produced by the driver stage is very low, and predominantly even order. It's an innate characteristi c that a
good triode has a constant mu over a wide range of operating conditions, therefore, as has been known for a long time, if
a triode is run in constant current mode (either by active load, or by transformer or choke loading) then the valve is as
linear as it can be. The secondary side of the transformer provides a low impedance path to ground for grid current.
A triode-coupled 6SH7 has a mu of 40 odd and an anode impedance below 10K, also it's nicely linear, both as a triode
and as a pentode. The high gain, and low impedance means that the input sensitivity of the amp with only one stage, and
with a pair of 300Bs is still reasonable, if we were to look at using a 6J5 say, the gain is less than half and you start
needing serious volts to drive the thing, also the 6J5's anode impedance is significantly higher and that would limit
bandwidth due to loading by the transformer and by the 300Bs. There is some kind of phobia, caused by a lack of
understanding about triode connecting pentodes, it's really a case of choosing the right valve, and then using it in the right
way to get good sound.
There are some of the old telephone triodes out there, such as the 417A and 437A, but these are pretty scarce, and by all
means if you are up to it you can experiment and modify and try replacing the 6SH7 but if you melt your Western 300Bs
don't call Brian!
Andy Grove - Audio Note design engineer
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