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Let's build a tube preamp

#41
Of the varieties of rectifier, we will be using a Bridge rectifier to make the B+ voltage for the plates.
diode-rectifier-full-wave-bridge-03.gif


Those four diodes shown make a Bridge rectifier. The symbol to the left of them is the transformer. The transformer's secondoary winding (the part connected to the rectifier) has no Center tap
 
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#42
diode-bridge-split-supply.png

This one shows a Bridge rectifier connected to a transformer with a Center Tap on its winding.
After the rectifier to the right, there are two symbols for Capacitors. They store energy and release it. Since the diodes chop off half of the waveform, its not pure direct current, its choppy. the capacitors fill in the missing bits (ideally), and we call them smoothing capacitors in this situation.

Im oversimplifying so this doesnt become belabored.
 
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#43
If youre transformer's secondary has a center tap (wire tapped into the center of the winding), one should use it in the circuit rather than floating it, when making a powersupply. If your transformer doesnt have a C.T., then wire it like the first picture. Does anyone disagree with that statement?

As @cfsporn said, the output voltage of the Bridge Rectifier will be 1.4 times the input voltage, with no load on the circuit.
 
#45
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#46
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#48
seamonkey said:
Not to underthink this, but you could take an existing amp and mod it for the masses.

$50. and it has DC to DC converter to step up the voltage.
http://www.amazon.com/Nobsound®-Vacuum-Integrated-Amplifier-Headphone/dp/B014FASL1A/ref=sr_1_1

Here's a reverse engineering. You can see how to bypass the SS parts in the signal path.


Keeps a mass of people from touching mains voltage at least. ;)
Click to expand...

I see your point.
EVERYBODY... These Voltages may kill you.

And yes, its not the point of the thread. Its a nice recommendation though
 
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#49
So rather than starting with a certain transformer because it's the one sitting in the "parts bin" for one person, why not find a decently priced alternative so it doesn't end up costing more for no real benefit for everyone?

Most of the mediocre builds I've done in the past were centered around "I've got this part here, let's make everything else fit around it".
 
#50
With the transformer I have on hand (330-0-330), using a bridge rectifier, I should get 465 volts out the other end. Thats a bit high. Id like to hear anyone advise on dropping some volts, otherwise its just gonna be resistors.
What do the experienced cats cat say about maximum plate voltage for 12ax7/12au7?
 
#52
JackANSI said:
So rather than starting with a certain transformer because it's the one sitting in the "parts bin" for one person, why not find a decently priced alternative so it doesn't end up costing more for no real benefit for everyone?

Most of the mediocre builds I've done in the past were centered around "I've got this part here, let's make everything else fit around it".
Click to expand...
Well the price is right, theyre on eBay for 60 bucks.
They also have all the different voltage taps/windings, to make a full on amplifier. I could stop the thread, go buy another tranny after we agree on which is the perfect one, and proceed.
What transformer would you recommend?
You do have a good point, but is this PT a bad starting point?
 
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#53
B-string said:
Yes. Good time to also recommend bleeder resistors (220K 1 watt are a "standard"). They not only bleed off high voltage at power down but serve as a filter component.
Click to expand...
Good thinking. We're almost to that point.
Gotta dash out of the house now, maybe someone want to sketch whats been covered to this point, maybe add in standby switch...
 
#54
The uf4007 are more quite than the 1n4007 so less potential for top end hiss.

The cap after the bridge rectifier is the resivior that fills in your gaps between diode switches, and acts as your first filter component. Cap size is a bit of a balancing act as you want it big enough for the job, small enough that it charges quickly, and not too big as to tax the transformer.
 
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#56
rufus.K said:
Well the price is right, theyre on eBay for 60 bucks.
They also have all the different voltage taps/windings, to make a full on amplifier. I could stop the thread, go buy another tranny after we agree on which is the perfect one, and proceed.
What transformer would you recommend?
You do have a good point, but is this PT a bad starting point?
Click to expand...

Not enough to make a decent bass amp, too much for a preamp.

I'm not interested in smart assed, non-constructive comments from someone who started the thread claiming they didn't want that stuff on the thread.

Good luck with the build.

Unsubbed
 
#57
rufus.K said:
On my sketch was standby switch
Click to expand...
Since you're examining the design process here I'm going to ask, why the standby switch? I've seen several experienced designers write about why they consider the traditional standby switch is neither useful or necessary - I'll see if I can dig out the links in the morning. A simple mute switch might be more elegant.
 
#60
For a preamp a standby is unnecessary, and really it is unnecessary in general, as long as you are keeping the amp on for a period of time. Other wise you can potentially degrade the life of your tubes due to cathode stripping by not letting your filaments come up to temp, which may or may not be a problem depending on the quality of the tube. It is more a better safe than sorry approach.
 
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