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Old 22nd July 2003, 02:34 AM   #21
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Quote:
Originally posted by dhaen


Chokes used in choke-input filters should be 'specially designed. They have some DC across them. Ordinary chokes buzz in this application.

Cheers,
no amount of DC will make a choke buzz... the buzz is the effect of to large of an AC voltage.

chokes for choke input filters deal with very large AC voltages, and must be designed both electrically and physically to be quiet.

dave
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Old 22nd July 2003, 02:41 AM   #22
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Quote:
What's the point of the 3uF cap?
I used it to get the voltages where I needed them to be. If I make it a choke input filter, the voltages go down quite a bit. Or are you saying that I should get a higher voltage power transformer, and start from there?

I've seen this advised in a few places though, to use a small first cap to mimic some of the behavior of a choke input filter, even though it's really a capacitor input filter.

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It only makes for bad regulation, effectively increasing the critical inductance value.
Would you mind explaining that?
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Old 22nd July 2003, 07:43 AM   #23
dhaen is offline dhaen  Europe
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Default Correction

Quote:
Originally posted by dave slagle


no amount of DC will make a choke buzz... the buzz is the effect of to large of an AC voltage.

chokes for choke input filters deal with very large AC voltages, and must be designed both electrically and physically to be quiet.

dave
Dave,

You are right. I was considering the ourput voltage difference between CLC and LC, and getting mixed up with "swinging" chokes.
The buzz, is of course due to the considerably greater AC component.
Premium manufacturers such as Sowter, make special chokes to cope with this.

Cheers,
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Old 22nd July 2003, 09:32 AM   #24
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Quote:
Originally posted by Saurav
I've seen this advised in a few places though, to use a small first cap to mimic some of the behavior of a choke input filter, even though it's really a capacitor input filter.
Actually it's halfway between.

Quote:
Would you mind explaining that?
Think of a cap input filter: with no load, it charges to peak voltage. As it's loaded down, the cap discharges more and more between charging peaks. At worst case, the cap does nothing and the output ripple form resembles a continuous train of positive sine wave peaks, this comes at the point where the rate of discharge is greater than the rate at which the sine wave falls to the valley between peaks. Anyway, the voltage output in this condition is the average, .9*RMS for a sine wave.
Now, add a choke on after that, and you remove the AC component, resulting in the average voltage being seen. Thus at no load, the first cap is charged to peak voltage, and has no ripple on it since remember we have no load. Likewise the next cap will have the same voltage on it... Now under load, that first cap nearly disappears as described above, and the choke does its job essentially as a choke input supply. The first cap is just extra luggage...

Now of course this is all fine for a class A design but even so a PSU should have good regulation. It's good design practice.

Tim
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Old 22nd July 2003, 03:40 PM   #25
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I see what you're saying, but if I remove the first cap and model it in PSUD, the output voltages drop by about 70V. I bumped the 100uF capacitors up to about 470uF, and that didn't make a difference. So, short of replacing my 300V transformer with a 350V transformer, I think I'll have to keep a cap at the input to get the voltages I need. Or can you suggest some other way around it?

Thanks for the explanation.
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Old 22nd July 2003, 11:23 PM   #26
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You can:
- Leave it as-is (and stick with the bad regulation)
- Use full cap-input (which will net better regulation than this) and drop the remaining voltage with a regulator or resistor (er, so much for regulation)
- Drop the primary's AC voltage with a buck transformer, and use cap input. This is N/A if the PT has filament windings in use.
- Or, of course, get a new PT.

Anyway, what's it powering?

Tim
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Old 22nd July 2003, 11:37 PM   #27
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Quote:
Use full cap-input (which will net better regulation than this) and drop the remaining voltage with a regulator or resistor (er, so much for regulation)
That's what I thought, resistors would make the voltage sag with current, and I thought that would be worse. I hadn't thought through the effects of a small cap.

Quote:
Drop the primary's AC voltage with a buck transformer, and use cap input. This is N/A if the PT has filament windings in use.
What's a buck transformer? The filament windings are in use, but that could be changed. I've been thinking of trying regulated DC heaters, which would need higher voltage transformers, I think.

Quote:
Anyway, what's it powering?
A stereo SRPP 6SL7 into SE 2A3. I took the design from the JE Labs/Angela website.

http://www.angela.com/catalog/how-to/EZ.2A3.html

I used the same values as the original desgn for the audio section, but decided to try my own PSUD model for the power supply.

Pictures: http://mywebpages.comcast.net/saurav/2a3/
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Old 22nd July 2003, 11:44 PM   #28
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Hi,

Quote:
A stereo SRPP 6SL7 into SE 2A3. I took the design from the JE Labs/Angela website.
Haven't looked yet but if memory serves, that's a Loftin-White circuit direct coupling the 6SL7 to the 2A3, right?

Better take care of the voltages on that one and pay a lot of attention to the PSU.
It's going to love you for using the best possible PSU...no kidding.

Cheers,
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Old 22nd July 2003, 11:55 PM   #29
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Nooo.... I think I saw that Loftin-White circuit when I was looking for a circuit to build (or at least, I found another schematic that said Loftin-White 6SL7 SRPP direct coupled to 2A3" or something like that). This one's cap coupled. The L-W looked a little more complicated, because the 2A3's biased up higher in order to meet the voltage on the 6SL7, and that biasing is done with a slightly more complex resistor arrangement. It also needed a higher B+, which meant a more expensive transformer, and.... well, I ended up picking the other one. I also read about risks with a direct coupled design if someone turns the amp on with some tubes not inserted, and things like that (don't remember the details any more). And more solder joints = more mistakes I could make. My amp is laid out pretty much symmetrically, and I'd swapped grid and plate connections on one 2A3 socket because I was automatically mirorring the other side. Luckily I spotted it before I powered the thing up.

I compared the SRPP stages of the two, and the L-W amp uses different resistor values. They both have equal resistors for the cathode bias of the lower tube and the plate-cathode between tubes, but the resistor values are different, IIRC.
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Old 23rd July 2003, 01:07 AM   #30
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Default L-W

Hi,

Quote:
The L-W looked a little more complicated, because the 2A3's biased up higher in order to meet the voltage on the 6SL7, and that biasing is done with a slightly more complex resistor arrangement.
Don't be afraid of direct coupling...it looks a bit more complicated but is quite simple to implement in SE designs.

It only gets complicated when you have to DC couple various stages in a row.

To give you a modest example: DC coupled input voltage amp DC coupled into phase splitter, DC coupled into a pair of CF buffers cap coupled to powertubes...Now that's no bed of roses.

Quote:
I compared the SRPP stages of the two, and the L-W amp uses different resistor values. They both have equal resistors for the cathode bias of the lower tube and the plate-cathode between tubes, but the resistor values are different, IIRC.
That's quite normal, the regular SRPP circuit will always have equal cathode Rs for both triodes. They only are oprating at different operating points in the two circuits.

Here's the Loftin-White SE 2A3 amp I refered to a Japanese design from looking at the way it's drawn.)

Cheers,
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File Type: gif 2a3-se_2.gif (17.4 KB, 102 views)
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