Tony said:
Edrel is winding me a 220V primary 9V secondary to be used for parafeed OPT.
If 6sl7 has too much gain, then I have a pair of 6j5 that I can use and a lot of 12au7 and 5670.
Thanks.
mach1 said:
Potential, not current. The potential comes from the fact that one winding of the transformer is at plate voltage, the other is at ground. If you take two sheets of metal, put a gap between them, and apply a potential to one side while the other is grounded, you have a charged capacitor.
To answer the question you didn't ask, where you get saturation in a parafeed circuit due to DC current is when the cap is too big -- then the process of charging and discharging the cap pulls current across the primary.
I think if you look at the schematic you will see that one winding is at plate voltage, the other is floating.
Re point 2, I concur.
Re point 2, I concur.
For safety reasons, I always learned to tie one side of the secondary to ground, so I assumed you did this too. If you don't, then I have a different, and distinct, objection to the schematic. 🙂
Anyhow, I don't care where you put your parafeed cap -- I was told by someone I respect, and who has forgotten more about transformers than most of us on this board will ever know, that one option was considerably better than the other, and that placing the cap to ground will hurt frequency response. I was simply sharing that bit of knowledge.
Anyhow, I don't care where you put your parafeed cap -- I was told by someone I respect, and who has forgotten more about transformers than most of us on this board will ever know, that one option was considerably better than the other, and that placing the cap to ground will hurt frequency response. I was simply sharing that bit of knowledge.
And you will find that Mike Lefevre of Magnequest has no such misgiving and concedes that many of his customers prefer results with the cap in the ground leg.
Big difference between 'knowledge' and opinion.
Big difference between 'knowledge' and opinion.
dsavitsk said:
The thing to be aware of here is that the capacitance between primary and secondary is always there no matter what DC potential exists between them. Likewise, that capacitance will always be charged/discharged by the signal in the same way, no matter what DC bias.
It's true that if you put the primary at 400VDC then the capacitance needs to be charged to that level, but it's done only once when you power up the amp, and then it's done directly by the power supply not by the output tube.
In the end, the capacitance seen by the output tube (as far as signals are concerned) will be the same either way. It works the same way as a coupling cap between stages inside an amp. If the cap is .1uF, then it's always .1uF. Putting one end at a higher voltage doesn't change that.
The above assumes that the capacitance is 'ideal.' In fact, it seems that some capacitors - purpose made or stray - behave more ideally when there is a large DC bias on them. Something to do with the dielectric being nonlinear as the field swings back and forth around (or near) zero. I don't pretend to understand the physics of the dielectrics, but it all seems entirely plausible to me.
In any case, more than one person has reported a slight but noticeable improvement in sound when the parafeed cap is placed in the ground leg. That's the most common report. The second most common is that there is no difference. I don't remember anyone saying that it sounds better in the input end, but I have not talked to everyone.
-- Dave
- Status
- Not open for further replies.