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RE: 'Turbo' Western Electric 91 amp question...

Posted by Paul Joppa (P) on June 18, 2016 at 18:20:45

In Reply to: 'Turbo' Western Electric 91 amp question... posted by gilmorneau on June 18, 2016 at 17:47:56:

The major technical reason for using a cathode follower to drive an output tube is because it can supply grid current on positive drive swings - maybe for more power, or maybe just for softer clipping. If there is a coupling capacitor, it will get charged on those grid current peaks, and until it discharges the output tube's operating point will be disturbed.

As I recall, the late George Wright dealt with this by using a much larger cap (1.0uF? - I don't remember that detail, and being George he may have changed it from time to time :^) It would keep the shift small as long as the A2 grid current was infrequent.

In any case, if there will be grid current, the operating point should be adjusted from the pure Class A1 ideal.

I believe Thorsten has used a choke load for the CF, with its DCR serving as cathode bias resistor - a very clever way to get direct coupling while still keeping the quiescent output grid voltage close to zero.

Personally, I don't much like what cathode followers do to the sonics. It's not a huge problem, but it's real and audible to many critical listeners. If you must have them, I recommend playing better music which will allow the art to distract you from the sonics.

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