Some brief notes on the monoblock power amps
Support Information from Andy Grove
These amps are fronted by a 6SH7, connected as a triode, which may seem a little unusual, but apart from the 6SH7 and the similar, but in a smaller package 6AU6 both being good sounding valves, and the fact that they are available at reasonable prices there are good technical reasons for using it. The idea with these amps is to use an interstage transformer, and only ONE amplifying stage before the output valves. This makes for an incredibly direct route, and the amps sound better for it. Really, listening to such an amp is a revelation, there is only one gain stage, and there are no coupling capacitors. The underlying colouration which coupling caps of all sorts produce is absent, and there is a far greater sense of immediacy and dynamics. If you can imagine, the effect of the bifilar interstage is to knit together the anode circuit of the driver valve and the grid circuit of output valve, what happens in one happens in the other.
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. Itís an innate characteristic 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 itís 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 6J5ís 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, itís 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 donít call Brian!
Andy Grove - Audio Note design engineer