[Premium-Rx] 6DJ8 / ECC88 / 6922 / E88CC [WAS: siemens e311]

[Charles Steinmetz]

Terry O' wrote:

>I was unaware audio guys went for these.  I wonder why.  A friend 
>who designs guitar amps for Sovtek found them inadequate for audio use.

They have high transconductance, which means relatively low noise and 
decent gain.  They were originally designed as cascode RF amplifiers, 
so they also have relatively low interelectrode capacitance.  Two 
sections in cascode work much like a pentode, but with significantly 
lower noise.

BUT: they are quite nonlinear, when used as simple (non-cascoded) 
RC-coupled triode amplifiers with several volts of output (it is 
truly educational to plot the plate dissipation limit on the plate 
characteristic curves -- all of the beautiful, linear-looking area in 
the top three-quarters of the graph turns out to be unusable, far in 
excess of the allowable plate dissipation).  They are also 
notoriously microphonic.

Best regards,

Charles