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In Reply to: RE: Question on the B- power for fixed bias posted by hifipaul on April 24, 2015 at 10:27:47
Paul
I agree 100% with the full wave supply with Schotkys and higher quality capacitors.So many people engineers included used to discard doing anything with the C minus supply because they felt it didn't draw enough current to warrant anything other than a single diode and a couple caps separated by a resistor. All one has to do is put their scope on the C minus supply source with that simple scheme and you will see audio riding on the rail and being that audio is an AC voltage,certain audio frequencies can get filtered out in a poorly designed C minus supply.I've even contemplated putting a small choke in the C minus line.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Follow Ups:
An tech that I know explained that any ripple, or noise, in the bias supply manifests itself as intermodulation distortion. This may be why some people like self bias so much.
"All one has to do is put their scope on the C minus supply source with that simple scheme and you will see audio riding on the rail "
Where would that be coming from? Even without the bias supply output filter conducting the AC to ground - something it's *very* good at - the differential audio signals from the phase splitter should sum to near-zero on the bias rail. If the tiny amount of remaining signal is bothersome (not to be confused with "audible"), a CCS in series with the bias supply or a better phase splitter might be in order. Installing a FW rectifier and Schottkys won't do a thing.
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Buy Chinese. Bury freedom.
Jack
What I was referring to was when you put the scope on the bias supply source while music is playing,you can sometimes see audio riding on the C- rail even tho it may be little..Since the C- rail is joined to the output side of the output coupler thru a 270k grid leak resistor,that's where I was able to see audio.The caps were old obviously and I realize it's not a lot of audio but it was there and even tho it's separated with a grid leak resistor to the last filter cap in the C- supply,it makes you wonder how much of the audio signal may getting filtered out by that cap when it's good.
The only reason it made me take more interest in the C- rail is when I had a Citation 2 I was working on that had an open bias winding and Jim just had me put small toroid in its place with a bridge and when I did that,it was made a sonic improvement over my Citation 2s that were built the same way. After that I just put that little toroid in all four of my Citation 2s even tho the bias winding was good..I figure a well executed C- supply with a lower impedance may have a positive effect? Then again maybe I'm over analyzing this.It won't be the first time.LOL
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
Edits: 04/25/15
"it makes you wonder how much of the audio signal may [be] getting filtered out by that cap when it's good [and with a large enough value to have a low reactance down to the lowest frequency of interest WRT the AC signal]."None. The better the cap the more it acts like a piece of wire and the grid resistor will act like it IS grounded.
When the cap is bad, or it's reactance is too high, then the signal to the grid of the tube will be altered by the cap. Not the other way around.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/26/15
"it makes you wonder how much of the audio signal may getting filtered out by that cap when it's good."
In terms of grid signal, I don't see the difference between grounding a tiny portion through the C- rail filter, or connecting the grid resistor directly to ground. In any event, it's the C- filter that the grid resistor sees, not the rectifier ahead of it. Maybe you're hearing a reduction in IM due to more effective filtering at 120 Hz. Even with a half-wave rectifier though, ripple should be really tiny. Difficult to imagine how it could produce audible levels of IM unless the filter is defective...
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Buy Chinese. Bury freedom.
In terms of grid signal, I don't see the difference between grounding a tiny portion through the C- rail filter, or connecting the grid resistor directly to ground.
I agree with you because the grid leak resistor sees AC ground when it hooks to the C- supply but to be honest,I only saw the audio on a couple dyna MK3s and one other fixed bias amp. As far as effective filtering goes that would make sense but being there is so little current involved how effective does it have to be?..Mcintosh used a single 10uf cap in their C- rail but for some reason the the full wave bridge with the little toroid seemed so effective.
Nobody ever went broke underestimating the intelligence of the American public
H. L. Mencken
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