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previous: kg Re: bigass dissertation on cascodes and jfets -- 3/13/2000 7:07 PM View Thread

Cascode? Pentode? Same thing

3/14/2000 1:27 PM
Stephen Conner
Cascode? Pentode? Same thing
Hi folks,  
It's obviously cascode season! Now as far as I am concerned, there is no difference between a cascode of two triodes, and a pentode. You are simply splitting the innards of a pentode between two bottles (or two halves of one bottle) with the grid of the upper triode effectively being the screen.  
As KG pointed out, the behaviour of the cascode (high voltage gain, high output impedance, reduced output voltage swing) is identical to a pentode like the EF86. Except with the pentode, you don't have to worry about heater/cathode breakdown. Now if you used an EF86 with a MOSFET source follower hung off the plate, it would probably be just as good as the JFET/tube hybrid. And still only one tube socket!  
Now, when you're rigging up that setup with the JFET in the guitar lead, y'all think safety! The tip contact of the input jack will be at a high voltage. If you use self-bias, when the special JFET lead is unplugged it will rise to B+ voltage - ouch! Anyone who plugs in a normal lead will be in for a surprise. And you will get huge pops and thuds when plugging the JFET lead into the amp. I would love to hear this used with an acoustic guitar with Piezo pickups though.  
P.S. Remember to protect your JFETs from static.  
Steve C.

kg q{It's obviously cascode season! No... -- 3/14/2000 2:58 PM
Santiago The problem is the noise. Pentodes ... -- 3/14/2000 3:01 PM