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Posts: 2422
Location: No. California
Joined: October 13, 2002
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Hi Mike, this is a very unusual design, it uses the 6ar8 beam deflection tube! I got to thinking about this after hearing Allen Wright's talk at VSAC where he talks about a PP stage with CCS on the cathodes. Well the 6ar8 does this automatically in one tube. If you aren't familiar with these tubes they are basically a pentode with a split plate and a pair of deflection electrodes. The pentode part gives a very good CCS action, with 0V across the deflectors the cathode current is split evenly between the two plates. A difference across the defelectors causes the "beam" to defelct so one plate gets more current, the other gets exactly the same amount less. Perfect differential behavior. The transfer function from the deflectors to the plates is also probably the most linear transfer function of any tube, period. Any way I thought this would make an ideal low power amp. The only problem is that its a high voltage low current device, hence the high impedance. I don't know of anyone that has used a transformer with this tube, its always used with a pair of 100k-200k plate resistors, which work fine for its intended purpose as a differential demodulator, but not so good for a power amp. I know this is very unusual, but its characteristics are so perfect for this type of amp I have to give it a try. I could go with a lower primary impedance, but then I have to change the operating point and get less power output, which would probably be fine for a preamp, but not so good for driving speakers or headphones. Thanks for looking at this, John S.
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