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Posted: 5/25/2002 8:32:18 AM EDT
Ok, you experts --  

I'm building a little project - a 1-tube preamp

1. It uses 2 transformers back-to-back isolating the power supply, taking the filament voltage front in between.  I could only find unshielded transformers.  So do I need to build enclosures for them to keep the hum from getting inducted into other parts of the circuit?

2. Should I try to distance them physically
from any of the other components?  I'm starting with a large enclosure, but will probably put it in something smaller eventually.

3. Stranded or solid wire?
Link Posted: 5/25/2002 8:48:17 AM EDT
Why a tube?

You can get a good pre-amp by using an op-amp, little 8 pin IC. Toss in a couple Transistor style voltage regulators (plus and minus) and that should set you up. All these should be readily available at any Radio Shack.

1.If you insist on a tube design, I would start with just a metal plate inbetween the transformers and the tube, with it being just a pre-amp, you shouldn't be drawing a lot of current (the more, the current draw, the larger the magnetic field from the transformers will be).

2. I wouldn't worry about the distance, being a pre-amp.

3. Unless you're talking about some real exotic frequency use, then you should be fine with any old wire.

Off the top of my head, been along time since i've thought about circuit design.
Link Posted: 5/25/2002 11:03:58 AM EDT
Why - This is for my electric guitar, I want that imperfect, distorted sound.

The tranformers are 2 12.6v and I will be using the center taps on each to get the 6.3v

The store didn't have the 6.3v kind, so I figured I could just use the center taps.

Something I should have thought of before:

The second one is placed to step the voltage back up after drawing off some for the filaments.  Then the 110v or so goes to a bridge rectifier.

Since the 6.3v from T1 is going into only HALF the secondary winding T2 which is actually being USED as a primary, I wonder how well the step-up will work.

Or, wait! Maybe I can use the full 12.6 for that, and just tap the 6.3 off.  Then it should work OK.

Or could I modify this and run 12.6 to pin 4 and ground pin 5 (or vice versa) instead of 6.3
into 9 and grounding 4 and 5?

The metal between the tube and T1,T2 is a good idea.   In fact, why didn't I think of this? - I could probably find one of those shields that snaps into a clip at the base.

I only thought of stranded wire because of extenral vibration & noise.  But really, solid copper is pretty ductile.

Link Posted: 5/25/2002 11:31:21 AM EDT
If I understand correctly what you're trying to do...

Connect the two 12.6 secondaries together. You don't need to connect the center tap of T2 to anything, just solder it to a terminal somewhere to stop it from shorting to anything else, or clip it off. Use the center tap of T1, and either of other secondary connections to get the 6.3V for the filament.

Link Posted: 5/25/2002 11:50:24 AM EDT
Stranded or solid wire will work the same. However, consider tightly twisting together any wires carrying AC current (thereby providing a self-shielding effect). Some manufacturers go to the extreme of rectifying and filtering the tube filament supply to minimize hum (shouldn't be too hard to do with a 3-terminal regulator such as a 7806).

Keep the tube away from both transformers, since the physical humming noise from from them can be picked up by the tube via microphonics. Also magnetic flux from the transformers can also be picked up by a nearby tube. Note that there are also slip-on metal shields available for tubes that will reduce this effect.

If an output transformer is present, it is often mounted at right angles to the power transformer, so as to minimize magnetic coupling between them — probably not a concern in your application.
Link Posted: 5/25/2002 11:54:54 AM EDT
sounds like the only thing u want to do is make the filaments light up.
Link Posted: 5/25/2002 12:02:22 PM EDT
wheres ure B+ source voltage gonna come from ,and the bias for the tube?
Link Posted: 5/25/2002 12:36:45 PM EDT
wheres ure B+ source voltage gonna come from ,and the bias for the tube?
View Quote

Reread his posts. It uses two transformers "back to back" to step the voltage down and then back up, then rectifies it for B+. Basically it makes a low power isolation transformer, with a 6.3V filament output, using easy to find parts.
Link Posted: 5/25/2002 11:50:21 PM EDT
Maybe I over simplified this by leaving out the rest of the circuit.

Anyway, the tube shielding sounds like a good idea - I'll have to hit the flea market looking for old non-functioning electronics I can cannibalize.

Lots of good tips here.  Thanks!!
Link Posted: 5/26/2002 12:02:04 AM EDT
Jeeze I'm ignorant!!![B)]
Link Posted: 5/26/2002 1:54:57 AM EDT
My sorta thing1

Yes, you should keep the transformers from any other inductors (obviously). If you can find epoxy & carbon filled inductors (maybe hard to find) this will minimize some of the effect and reduce possible ringing. Shielding is a good idea.

For a pre-amp, I suggest a 12AX7 or a 12AU7.
They are similar and interchangable but have somewhat different audio qualities.

Stranded wire, copper.
Solid wire (especially aluminum) will break easily.

Use a good high voltage on the plate. Tubes are nice as they tend to compress the signal as it reached saturation whereas transistors will simply saturate (or cutoff) causing NASTY distortion.

Typically, odd numbered harmonics are whats pleasant to the ear but.... don't make a device which enhances these harmonics and mixes them back into the audio chain. Doing that would violate a certain patent for a device called an [u]Aural Exciter[/u] by a company named APHEX. [;)]

Ahh, a nice warm TUBE sound!!! How much I miss that!!
Link Posted: 5/26/2002 1:59:07 AM EDT
wheres ure B+ source voltage gonna come from ,and the bias for the tube?
View Quote

Apply a high voltage to the plate from one of the transformers and attach a large value resistor to the cathode (1Meg. Ohm or greater) the other side of this resistor should be grounded. In theory this will pull the cathode below ground (negative voltage).

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