I recently tried the floating paraphase inverter, but it provided too much gain for the amp I was building. For the 5C3, it should work just fine.
I'm having trouble understanding the concept of adjusting the balance on the paraphase inverter on V2 (the second 6SC7).
Here is a terrific article on this type of PI, and he gives you the math you'll need to work on balancing.
http://www.freewebs.com/valvewizard/flo ... phase.html What is it that makes you think "built as drawn" isn't right?
Do I measure for voltage at the #5 pins of the 6V6's?
Which resistors/caps would I change off pins 2 or 3 of the pi?
Should I sub a pot or two in there to get a ballpark value?
Pin 5 of the 6V6's is the grid/input point. There isn't much voltage there.
Pins 2 and 6 of the 6SC7 are the plates. Plate voltage is controlled first by the plate load resistor, in this case a pair of 250K, and second by the cathode resistor, which is 2.5K. Raising the 250K resistors will drop voltage and lowering the value will increase voltage. You don't say what you've got with a pair of 250K resistors, so it is impossible to comment on whether something could/should be done. Lowering the plate resistor will raise the noise level and a tube like a 6SC7 is a candidate for noise, so I wouldn't be quick to move in this direction. You might tweak the cathode resistor a bit. Try 2.2K or 2.7K and see if it makes a difference. For testing purposes, taking in a 18K or 22K resistor across the 2.5K will reduce it to around 2.2K; if you want to raise it, lift one end and tack in a resistor between 100 ohms and 250 ohms. Raising the value of the Rk will decrease plate voltage a bit and vice-versa. I would not do this with a pot; more work than it's worth and choices are rather limited to begin with.
As for balancing, you should probably look at the voltage divider that feeds the second grid on the 6SC7. There is a wire that goes between a 1M and a 25K+250K resistor. Changing those will change the characteristics of the PI
As for reversing the primaries on the OT for NFB, that is simply to correct the phasing on that part of the circuit. If the primaries are "backwards" you end up providing positive feedback instead of NFB and the amp will howl. You'll know right away if the leads need to be reversed. If it howls and then stops when you disconnect the NFB loop, reverse the leads.
Also-the grid leak bias on V1.
I am getting -.5VDC on pins 3 and 4, with 90 volts on the plates (pins 2 and 5). Schem calls for -.8VDC...with 85 Volts on the plates. Should I mess with this, or is it "close enough"?
Are you kidding? These numbers are right on the money. Leave it alone.
It has been the biggest PITA I have ever dealt with.
That's only because you are not familiar with it. Chalk this off to a learning experience. I understand it is frustrating, but keep working with it and you will get a great amp out of it.
