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In the Forum: Melquiades Amplifier
In the Thread: The 6E5P tube data.
Post Subject: 6sn7 = high output impedancePosted by hagtech on: 12/4/2007
The 6sn7 is a fine amplification tube for use in preamps or gain stages, but it has way too high of an output impedance for driving power tubes. This is my personal preference. It also depends greatly on the topology. Maybe George used it as a cathode follower?
For me, the best tube to drive a 2a3 is another 2a3, but that gets you nowhere.
My new amp is a push/pull/parallel, where I can dial in the ratio between single-ended (in parallel) operation and differential operation. One mode has a lot of even order harmonics, the other odd. The differential mode has better PSRR, but still to me, preserving harmonic balance is more important. The advantage to this method is that you can use a regular output transformer, no dc flux to worry about (or parafeeds). This circuit also employs a new trick I invented to provide inherent self-balancing of unmatched tubes. Using a CCS for each cathode maintains a perfect dc current balance in the transformer. Also, tube bias does not have to be adjusted (nothing does!). All this is done without servos or opamps or feedback or any of the usual garbage. However, it does require an interstage transformer. I run an SE driver stage with parafeed CCS to the interstage. This is where the super low plate impedance of the 6e5p comes in handy. It is the only way to drive the load with power, grace, and bass down past 20Hz. A 6sn7 in this position would fall flat on its face.
Basically, there is no right answer. The 6sn7 might work in another circuit. Just not mine. For driving output tubes I prefer the interstage tranny, as it does not suffer bias shift across a coupling capacitor caused by grid current.
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