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Old 27th August 2011, 04:34 PM   #1161
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by regal View Post
Those #'s are very good, this was on the mu out? What was the load? Most soundcard have a terrible load for testing preamps (~1k).
No, normal out. Just realised that haven't measured the bias voltage properly as I did it from ground to the top of the LED array. Will need to add 1V

Have used Pete Millett's interface, but my Win7/EMU-202 don't get well at all. What I was more interested in was to identify the type/brands of tubes with the lowest distortion in relation to the rest.
cheers,
ale
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Old 27th August 2011, 05:32 PM   #1162
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Originally Posted by jamesdb View Post
Andy, vt4c.com has purpose built plate chokes that are 200H 30ma DCR 2800 ohms at comparable prices. I wonder if those would be better than Hammond general purpose chokes. Plus two of those in series would increase your inductance to 400H total.
Weight :1410 (gram/Each)
Size (mm): 68L x 57H x 78W

Those are pretty big!!

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Andy
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Old 31st August 2011, 10:58 AM   #1163
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Yesterday I bought some UX-201A and CX-301A tubes on Ebay and want to build a DHT preamp. I know that some of you have used 01A tubes and it’s obvious that it’s characteristics differ from 26. Could some of you give me an advise what a good starting point is for tube settings: –Vg, Ia and Va?
I am going to use a CCS anode load and battery grid bias or LED cathode bias (I have not decided which yet).

Peter

Last edited by pietjers; 31st August 2011 at 11:15 AM.
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Old 31st August 2011, 12:49 PM   #1164
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The max anode voltage is 135v, and the data sheet suggests -9v and 3mA for that. Since that fits a 9v battery nicely for grid bias, I'd start there.

Filament is 250ma at 5v so you need some kind of DC supply per tube. Rod Coleman's boards are very popular, but also consider choke input. A hammond 159T choke is 2.5H at 300mA which would be well worth putting in the circuit. You could follow that with a 158T which is 1 Henry at 300mA. That would be a nice solution. Keep all filament chokes way out of the signal chassis in another box, because they can be a source of hum.

I like two 156C anode chokes in series as an anode load. I also like filament bias, which works nicely with choke input. Avoids a cap on the input. As you can see, I find Hammond is a cost effective source of components for these kind of preamps. I'm in the OK and use Bluebell Audio who are good to deal with.

andy

Last edited by andyjevans; 31st August 2011 at 12:52 PM.
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Old 31st August 2011, 03:15 PM   #1165
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Hi Andy, thank you for the reply. I will start with Ė9V and 3 mA!

I have already decided to put the transformers including rectifiers and first filter caps in a separate box.

The problem with chokes is that they are rather big, so I think I will use Rodís solution, or something similar. I am handy with transistors (and prefer them over Mosfets in many applications) and not afraid of using them in combination with tubes, even when these tubes are from the mid twenties.
I have once used Lundahl LL1668 anode chokes in a 6N6p preamp. It sounded good, but not as good as a really good transistor CCS.

Peter
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Old 31st August 2011, 03:33 PM   #1166
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Hi Peter, for the low current filament regulators (up to 2,5A), you'll be pleased to hear that BJT pass transistors are used, instead of FETs! They do have very useful advantages, including lowest Cob, and highest gm.

Please send some email via the board, if you would like the PDF application notes, etc.
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Old 31st August 2011, 04:41 PM   #1167
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As Rod knows (he's very patient) I'm pretty hopeless with solid state technology and despite the size of chokes, they have this useful habit of only having two connections and just sitting there forever with no blue smoke coming out of them. I also think they sound divine, which is fortunate given my knowledge of solid state.....

andy
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Old 31st August 2011, 10:06 PM   #1168
FRUGIVO is offline FRUGIVO  Spain
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My first version 26 pre amp
Click the image to open in full size.Click the image to open in full size.
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Old 31st August 2011, 10:12 PM   #1169
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Is that some sort of copper tape around the chokes? Details please!

Does this cut down on hum in chokes and the 26 tubes? What did you find?

Andy
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Old 1st September 2011, 07:26 AM   #1170
FRUGIVO is offline FRUGIVO  Spain
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It's just a sheet of copper recycled from a power supply
SMPS for RF amplification
designed to achieve the maximum shielding
with the tubes is the same but with aluminum paper
The result of the shield is great
practically zero noise

I'm working on another version
upload photos soon

Pedro
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