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Old 14th August 2003, 12:01 AM   #1
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Default PSUD modelling of a filament supply

Hi,

A 2A3 filament is about 1 ohm, right (2.5V, 2.5A). If I use the defaults in PSUD, the transformer secondary has a DCR of ~ 30 ohms, the filter capacitor has a DCR of 2 ohms, and everything goes totally haywire, the voltage across the load doesn't go above a few millivolts. This happens with a constant current load and a resistor load. So...

* What are reasonable values for the DCR of a 2-10V filament transformer secondary and filter caps in the 5000-10000uF/10-30V range?

* Modelling the current and voltage regulators - the current regulator will have a voltage drop across the sense resistor. Is there a drop inside the regulator itself too? And how do I model the voltage regulator?

* I'm starting to come to the conclusion that there are no Schottky bridge rectifiers that can handle 2.5A, and I'll have to settle for regular silicon rectifiers. Is that correct? If not, could someone please give me part numbers for Schottky rectifiers that I can use for 2A3 filament supplies? I guess full-wave rectifiers would work too, I'd just need a higher output trasformer.

I finally found part numbers for the common mode chokes (I found one that's 150uH at 2.8A - should be good enough, right?) and the regulators themselves, so I think that's everything I need to build this.

Thanks in advance,

Saurav
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Old 14th August 2003, 09:53 AM   #2
dhaen is offline dhaen  Europe
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Hi,

Quote:
* What are reasonable values for the DCR of a 2-10V filament transformer secondary and filter caps in the 5000-10000uF/10-30V range?
Calculate the secondary resistance from the transformer's regulation figures: Typically <1 ohm
Look at a capacitor datasheet: Typically <50m ohm

Quote:
* Modelling the current and voltage regulators - the current regulator will have a voltage drop across the sense resistor. Is there a drop inside the regulator itself too? And how do I model the voltage regulator?
Allow 4-5 volts drop across a reguator.
Quote:
* I'm starting to come to the conclusion that there are no Schottky bridge rectifiers that can handle 2.5A, and I'll have to settle for regular silicon rectifiers. Is that correct? If not, could someone please give me part numbers for Schottky rectifiers that I can use for 2A3 filament supplies? I guess full-wave rectifiers would work too, I'd just need a higher output trasformer.
There are plenty of 2 pin TO220 types. Choose a type with an isolated case. Just tried to read the number off some of mine, but the eyesights failing

I finally found part numbers for the common mode chokes (I found one that's 150uH at 2.8A - should be good enough, right?) and the regulators themselves, so I think that's everything I need to build this.

That's similar to what I've used in the past. I'll be interested in other's opinions.
IMO there is nothing to beat an electrostatic shield between the primary and secondary of the transformer.

Cheers,
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Old 14th August 2003, 03:54 PM   #3
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A 2-pin TO-220 would be a single diode then, right? So I'd have to use 4 of them to build the bridge. I was hoping I could find a 4-pin packahe where someone else had already done that work for me

Thanks for the other responses. I don't think the Hammond filament transformers I've been looking at have shields.
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Old 14th August 2003, 04:23 PM   #4
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Quote:
Originally posted by Saurav
A 2-pin TO-220 would be a single diode then, right? So I'd have to use 4 of them to build the bridge. I was hoping I could find a 4-pin packahe where someone else had already done that work for me
Five minutes work to make a dicrete bridge from 4 individual diodes. You usually get better single diodes too.

Quote:
Thanks for the other responses. I don't think the Hammond filament transformers I've been looking at have shields.
None of the Hammonds I've ever seen have a shield.
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Old 14th August 2003, 04:44 PM   #5
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Quote:
Five minutes work to make a dicrete bridge from 4 individual diodes. You usually get better single diodes too.
Alright alright, I'll get off my lazy butt and do it
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Old 14th August 2003, 04:53 PM   #6
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Saurav,

For a direct heated tube, use Schottky's as they have lower switching noise and overrating them won't hurt either. The filament on a DHT is just as sensitive as the grid because the tube works off the difference between the two.
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Old 14th August 2003, 05:22 PM   #7
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Alright, do these look usable?

http://www.digikey.com/scripts/us/dk...684&Row=306252

Technical/Catalog Information: MBR735-ND
Diode/Rectifier Type: Schottky
Voltage-Rated: 35V
Current Rating: 7.5A
Package / Case: TO-220AC

So 8 of those, or something similar. The TO-220AC package has that little hole in it - does that mean that I need to individually heatsink these? I didn't see anything in the datasheet about the current rating being valid when used with a heatsink.

And this would be the regulator, which I'd need 4 of:

http://www.digikey.com/scripts/us/dk...836&Row=293545

Technical/Catalog Information: LT1085CT-ND
Current: 3A
Voltage: Adjustable
Features: Positive
Package / Case: TO-220

And I should probably heatsink these, right? My Pass Pearl phono stage used this heatsink: http://www.digikey.com/scripts/us/dk...8465&Row=77986, so I guess I'll get something similar.

CMC - saw these part numbers posted on an AA thread: http://www.digikey.com/scripts/us/dk...982&Row=184893. That's 150uH @ 2.8A, maybe I should pick the other one that's 47uH @ 3.5A.

I know I'm being really pedantic here
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Old 14th August 2003, 08:09 PM   #8
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Hi,

The trouble with posting links from Digikey is that this doesn't work...

From the description you put in, it should be O.K and when you have TO220 you can often use the chassis as your heatsink.

Check the datasheet before you decide to do this and use some heat conductive paste.

That should save some $ on the budget.

Cheers,
__________________
Frank
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Old 14th August 2003, 08:32 PM   #9
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Ouch, I didn't even check that. My apologies.

Using the chassis will get just a little tricky construction-wise, because it'll be a little harder to build it on a PCB. I could find longish pieces of scrap aluminum and attach that to the diodes and regulators, if I lay them all out in a line on the PCB.

On an unrelated note - I was thinking about advice I got here about increasing the first cap in my PS in order to improve regulation. The reason I didn't want to do that was that it would raise voltages. However, now that I think about it, if I got B+ up to the 400V range, I could try the Loftin-White schematic you showed me, with the 6SL7 SRPP direct coupled to the 2A3. That would get rid of one cap in the signal path.

And speaking of caps - given the fact that my speakers show a pretty significant IMD increase below about 60Hz or so, I think it makes sense to keep an input cap in the amp to high-pass everything at around 90Hz, right? The negative impact of that cap should be less than the improved clarity in the speaker's output. I have a polyester or polystyrene in there now (don't remember what I picked up at the surplus store), I'll probably play with other types too.

I'm also thinking of trying better OPTs than the Hammond 125ESEs I have in there... this seems to be a nice enough amp to make it worth the expense. The One Electron UBT OPTs are on sale for $80 each, and if I go with those, I'll need to decide on whether to get the 3K one or the 5K one.
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Old 14th August 2003, 08:53 PM   #10
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Alright, do these look usable?

Yes, but the cathodes of the diodes are the tab on the package so if you're going to mount them on a common heatsink, they'll need to be isolated. Simiarly the LT1085CT has the output on the tab too so same problem. Electrically isolated and a big heatsink should be fine. Well at least I think they will be. I have the flu and am less than coherent.

3K for the 2A3's.
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