• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

12B4 Preamp

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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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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.
 
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.
 
Ok. This is so simple I'm going to make it.

Questions though... 47nF... how many uF is that? What kind of cap?

The Resistors, what voltage should they be good for where not indicated?

The xformer... how many volts at how many amps on the 120VAC and 12VAC secs?

Sorry I'm a bit thick when it comes to this stuff.
 
The inexpensive O/P coupling cap. setup is a 5 μF. GE 40L3501 MPP bypassed by a 470 nF. film and foil part. 470 nF. = 0.47 μF.

Jim McShane has the GE MPPs and some of the now NOS Panasonic ECQ-P(U)s. Other candidates for the film and foil bypass are a Dayton from Parts Express and a 716P series Orange Drop.

The diodes in the B+ bridge are 600 PIV Schottkys. Church mice are noisier. :D
 
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