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Old 2nd February 2009, 03:02 AM   #1
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Default 12B4 Preamp

I have searched and read pretty much everything on diyAudio regarding 12B4 preamps & now my head is spinning!

I would like to build one but am looking for schematics for both the pre & a good PSU to match (I'm a bit partial to tube rectifiers but can be easily convinced). Many of the earlier posts no longer have the attachments so I'm sorta flapping in the breeze here. It seems as if fdegrove & arnoldc are on to something but some information seems to be missing.

Any help for this pseudo-newbie would be much appreciated.

Thanks in advance.
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Old 2nd February 2009, 03:13 AM   #2
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Any help for this pseudo-newbie would be much appreciated.

Your wish is my command, but I'll toot my own horn in the process. Look here and here.
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Old 2nd February 2009, 02:37 PM   #3
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I read through the postings, this is your schematic yes?
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Old 2nd February 2009, 02:38 PM   #4
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Old 2nd February 2009, 02:42 PM   #5
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Is that two separate grids at p2 & p7?
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Old 2nd February 2009, 03:59 PM   #6
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One grid, two pins brought out for use. Best practice is a CC grid stopper at EACH pin, right at the pin. Your input will then connect to both resistors, on the other end of the resistor.
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Old 2nd February 2009, 04:55 PM   #7
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Here are the inevitable questions -

1) what is the value of the resistor after the 9H choke in the psu?
2) what are the resistor power ratings (want to check my math) and ANY type preferences?
3) the closest I could get to a 9H choke was the Hammond P-T193M at 10H/300mA, I see no reason why this expensive little puppy should not work?
4) AC heaters?
5) you like the Mundorf cap $70 more than the Solen :-) ?

Thanks again.
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Old 3rd February 2009, 01:44 AM   #8
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Yes, it's my design. The schematic drawing was made by Erick Bates, AKA Buzz, from a narrative I provided. The parts values and styles shown reflect Buzz's "pile" and strong predeliction for "boutique" items. I'm CHEAP and look for good performance at modest expense.

The critical current in mA. for proper choke I/P filter operation is approx. V/L. It works out that 1K Ohm of resistance is used for each Henry in the 1st inductor. So, a 9 KOhm resistor is appropriate with 9 H. and a 10 KOhm bleeder is appropriate with 10 H.

AC heaters should be fine, as they are biased off B+. If hum levels are unacceptable, switching to DC heater power is easy enough.

If an "oiler" is to be used as the O/P coupler, I'd look for Soviet surplus. Mundorf is good, but oh that price. No, I would not use Solen or any other SCR (France) made MPP. You can get GE 40L3501s and 470 nF. film and foil bypasses from Jim McShane for very little money.

A Hammond 193M is certainly massive enough for 1st inductor service in this design. You might get away with a Hammond 193JP, as the potting should muffle "singing". This PSU filter is pseudo-choke I/P. The "fudge" factor cap. takes some strain off the 1st inductor.

A 5 W. rated part provides a nice margin of safety, in the bleeder position. A 1 W. part is fine for the 500 Ω cathode bias resistor. All the other resistors can be 1/2 W. Carbon composition please, in the grid stopper positions. Except for the stoppers, low noise Vishay-Dale RN65 metal film parts get my nod.

IMO, the popular 10M45S is not up to snuff, for this job. Use a good cascode design for CCSes.
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Old 3rd February 2009, 09:49 AM   #9
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Hi guys,

Very interesting tube here
Please, can you say to me if these 12B4 are even slightly microphonic?
What brands are suggested ?

Thanks ,
Paolo
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Old 4th February 2009, 07:25 PM   #10
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what kind of diodes are those? i can't read it too well...
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