Last question(promise) on 3rd trafo. Would throwing a hotter b+ at the pp center tap compensate for the added impedance. So that by the time the voltage ran through half the PP trafo and the SS trafo it would be down to what we're looking for at the EL84 plate, or am I completely misunderstanding how it works?
For real last question on that.
Monty, let me know if you're up to the Schematic Challenge.
Adding a third center channel would make it impossible to use negative feedback from the secondary, so there would be higher output impedance and higher distortion, and poorer frequency response. Plate-to grid feedback would be possible, though that introduces its own complications. Also recall that the two channels need to be drive in reverse phase - so you'd need some sort of phase inverter for any conventional stereo source.
One complete schematic for you to compare is in the link I mentioned earlier, post # 103.
One complete schematic for you to compare is in the link I mentioned earlier, post # 103.
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Tom, is this the one you're referencing?
If so, yes the three topologies I've been considering are the 8601, the RH84 and the UL schematic Monty and Eli have been bouncing back and forth.
The reference to the third PP Trafo is just going back to the original crap topology of the Philco. Once and a while I feel salty about leaving the original scheme completely out of the reimagined Whale Guano amp.
Why the buffer? It's (IMO) a simple progression. The original power trafo comes up "short" in the B+ rail voltage dept., even if SS rectification is used. In order to take maximum advantage of the available B+ voltage, "fixed" bias via a C- supply has to be used. When fixed bias is used, the grid to ground resistance limit of the 6BQ5/EL84 is lower. Combine that 300 Kohm limit with the increased CMiller of UL mode vs. full pentode mode and the 'X7 triode voltage amplifier is being pushed to or past its limits. While, as Tom Bavis stated, the buffer may not be essential, the use of the FET buffer guarantees that things will not fall apart.
Being in a can't lose situation is NICE.Contact member Jeff Yourison for pictures and tips about P2P wiring with ZVN0545As. Jeff did a terrific job of building a tweaked RCA phono preamp, which employs ZVN0545A buffers.
Harlon, do you own an oscilloscope and signal generator? The issue of phase compensation comes to the fore any time (like now) a GNFB loop is employed. Phase compensation is best fine tuned with the aid of those 2 instruments. However, I have a brute force method of phase compensation that works reasonably well for folks whose benches are not well stocked.
Would it improve things if I went ahead and utilized a new power trafo? I like the idea of saving $$$, but if it will help ensure success. What's another $50 give or take in the name turning
Those are on the list, but gotta say no. Not at this time.
Why spend, when you may not get exactly what you need? The FET and associated parts are much less expensive and you will get a good result. Recycling appeals to my cheap nature. To be fair, AnTek's $37.50 AS-2T230 looks just about right for self (cathode) biased O/P tubes.
The values of some parts can't be specified, until the PSU is breadboarded and the B+ rail voltage, under load, determined.
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