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Old 2nd June 2009, 04:33 PM   #11
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Hmm, I think I found an error on the tubelab website.

I was trying to check if the value of C4 (rectifier output cap) was ok for my 300B application and I found that there two different numbers spec'd on different pages.

The parts list shows 47uf, and mentions a table that I can't find.
The tubes and transformers page (where I think the mystery table should be) only mentions cap values for the 45 & 2A3 - but the suggested initial value is 4.7uf - a decade different than the parts list...

So - my question is what is the right cap value for the Rknize transformer (330-260-0-260-330) for my 300b's. I'm using a hammond 10h choke @ 155ohms - and eventually a motor run cap
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Old 2nd June 2009, 04:59 PM   #12
rknize is online now rknize  United States
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If you look carefully, what he is talking about is starting with a smaller cap to drop the B+ slightly if you have too much voltage on the HT winding, such as those "hot" Hammond transformers. When you lower the value of the first C in a CLC (or "pi") filter, the filter starts acting more like an LC filter with more of the work being offloaded to the choke. While a CLC filter will give you nearly 1.414 times the RMS AC voltage of the transformer winding, an LC filter will give you about the same DC voltage as the RMS AC voltage of the winding. A great way to play around with this is to fire-up PSUD and pay around with various values on the first cap.

To answer your question, I am using the specified 47 (forty-seven) uF.
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Old 2nd June 2009, 05:07 PM   #13
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Quote:
Originally posted by rknize
If you look carefully, what he is talking about is starting with a smaller cap to drop the B+ slightly if you have too much voltage on the HT winding, such as those "hot" Hammond transformers. When you lower the value of the first C in a CLC (or "pi") filter, the filter starts acting more like an LC filter with more of the work being offloaded to the choke. While a CLC filter will give you nearly 1.414 times the RMS AC voltage of the transformer winding, an LC filter will give you about the same DC voltage as the RMS AC voltage of the winding. A great way to play around with this is to fire-up PSUD and pay around with various values on the first cap.
Good info... Can you model transformers with PSUD? I suppose it is a windoze program too...

Quote:
Originally posted by rknize
To answer your question, I am using the specified 47 (forty-seven) uF.
Excellent ! That I have. What did your B+ work out to be?
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Old 2nd June 2009, 05:51 PM   #14
rknize is online now rknize  United States
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Quote:
Originally posted by oldmanStrat


Good info... Can you model transformers with PSUD? I suppose it is a windoze program too...


It is, but it works just fine in WINE.

Quote:
Excellent ! That I have. What did your B+ work out to be?
This post shows that transformer in both modes on the TSE with this CLC arrangement:

47uF - 10H, 82ohm - 220uF

http://www.diyaudio.com/forums/showt...17#post1803717

The 300B results are a little high because I wasn't biasing them very hard (they weren't my tubes). I was later brave enough to set them at 80mA each:

http://www.diyaudio.com/forums/showt...60#post1804760
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Old 2nd June 2009, 06:26 PM   #15
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I don't mean to hijack the thread but I have a related question on the motor run caps.

are there any smaller options for this supplemental capacitor? my real estate is bit tight so i'm looking for something that is less than 1.5" in diameter. all the caps that i've come across are about 2-2.5". i would prefer to mount it under the chassis (which is only 1.5" high).

TIA
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Old 2nd June 2009, 07:16 PM   #16
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Quote:
Originally posted by discomonkey
I don't mean to hijack the thread but I have a related question on the motor run caps.

are there any smaller options for this supplemental capacitor? my real estate is bit tight so i'm looking for something that is less than 1.5" in diameter. all the caps that i've come across are about 2-2.5". i would prefer to mount it under the chassis (which is only 1.5" high).

TIA

Try an oval, the one rknize just posted: ebay oval cap might be the ticket. email the guy and ask him for dimensions...
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Old 2nd June 2009, 07:36 PM   #17
rknize is online now rknize  United States
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http://www.mouser.com/Search/Refine....=UNL6W&FS=True

I've used the 30uF @ 600V before. It's about 1.3" x 2.1". They get expensive quickly above that, but they are quite compact. The 80uF is 2" x 2.5".


These are not oil-filled like the motor run caps are, which is probably why they are smaller. They will work just fine in our application.
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Old 2nd June 2009, 07:52 PM   #18
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thanks for the suggestions

i've sent the ebay seller an email

can a solen fast cap be used in this application? what is the benefit to the oil?
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Old 2nd June 2009, 08:04 PM   #19
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Quote:
can a solen fast cap be used in this application?
Yes, they work very well as a filter cap.

Quote:
what is the benefit to the oil? [/B]
Cooling. A motor run cap has to swing the full 370/440 or whatever volts 60 times per second. They are used for balancing the Q-factor of electrical motors so that they don't cause phase shifts in the power service (since induction motors are purely inductors). Some utilities actually have a surcharge for industrial power if a site has too much inductive loads.

We are just using them in a filter network, in this case on the "easy" end of a CLC filter. Heating is not really an issue.
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Old 2nd June 2009, 08:22 PM   #20
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Quote:
Originally posted by rknize
You can get mylar kits at Radio Shack, but the little thermal pads from Mouser/Digikey are better (no grease).
Following this thread since I'm starting my build too... Stupid question, what's a representative part # from Digikey for this? They have a lot of variants on the heat sink pads, not sure what specifics to look for.
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