Let's build a tube preamp

[winegamd]

View attachment 802186
If you want to skip ahead, this is my 1st draft

I can tell you've built some guitar amps. For the input grid resistor I suggest less than 68k. That gives us a roll off in the topend starting around 15k. Not really a problem for bass, I am just more of a 20-20k guy. Also, it looks like there will be plenty of gain so we may not need the bypass cap on V2A. We will probably want larger coupling caps for a bit more low end, too.

 

[winegamd]

I think a baxandall tone stack is better, too. It has less loss than an FMV, and we can actually manipulate the mids in stead of just raising the gain.

 

[rufus.K]

I can tell you've built some guitar amps. For the input grid resistor I suggest less than 68k. That gives us a roll off in the topend starting around 15k. Not really a problem for bass, I am just more of a 20-20k guy. Also, it looks like there will be plenty of gain so we may not need the bypass cap on V2A. We will probably want larger coupling caps for a bit more low end, too.

Good input. Great Input actually.
Once I get through talking out the power supply, I will start at the input jack and go down the line. Your input in invaluable to me.
I should have one triode left to buffer the output, and I will need help there too.
What do you think of my question about voltages, Full Wave around 300vDC, or Bridge around 465?

 

[rufus.K]

I think a baxandall tone stack is better, too. It has less loss than an FMV, and we can actually manipulate the mids in stead of just raising the gain.

This is gonna be a fun thread

 

[winegamd]

300 V is definitely better for 12A_7 type tubes.

 

[rufus.K]

Okay, then Full Wave it is.

For those who have sub'd to the thread:

Full Wave rectifier with center tapped secondary
Only two Diodes this time, if you look back and compare the Bridge rectifier diagram you will see four

 

[BrianVengeance]

We won't need 5v supply because I'm planning to use diodes instead of a tube rectifier.

... which clearly means the remaining tap is meant to be rectified and used to power a USB port for charging your phone during longer shows / recording sessions. Let's dream big!

Great thread, btw!

 

[rufus.K]

... which clearly means the remaining tap is meant to be rectified and used to power a USB port for charging your phone during longer shows / recording sessions. Let's dream big!

Great thread, btw!

Hey, now that's a cool idea! USB charging port.

 

[rufus.K]

So here is our sample/draft power supply for the B+.
By using a full wave rect. instead of a bridge, I will be getting .9 times the input power, so about 297vDC

 

[rufus.K]

capacitors are used to supply extra juice when there is a big draw, and also to smooth out ripples in the newly created DC voltage. You can see after the transformer, there are two diodes. They come off each side of the high voltage winding of the secondary side of the PT (power transformer).
The diodes join up on their output side. The dark hard circles are junctions, you can see the two diodes joining, then after that there is another junction. Two capacitors in series are below that junction . I drew in the ratings of the caps I have on hand, those values will likely change after a bunch of discussion, but I drew em in so move foreword. Capacitors in series halve their capacitence, but sum their voltage rating. So, the two in series each being 220uF at 285v sum up to be 110uF at 570v.

 

[rufus.K]

A choke is like half a transformer, one winding. It resists change.
I have it lying around, and its not a required component. It also will smooth out ripples in the funky DC voltage we made with those diodes. The one I have on hand has a value of 6 henries. Its kind of a beast, but goo for smoothing power. It comes after the initial bank of capacitors, and is followed by a dropping resister.

 

[rufus.K]

They are hard to move through.
I didnt put any values in the diagram, cos I havent run any numbers yet. In this application theyre called Dropping Resisters cos they'll drop the voltage down. These are higher rated (in Watts) than resisters elsewhere in the audio portion of the preamp. There is no audio in the power supply as you may have guessed by now.

 

[rufus.K]

Here are 3 points of varying B+ voltage levels for different circuits. All will likely go to the plates of the tubes eventually. You can see each segment of the power supply has a capacitor (against ground) at the takeoff point for each voltage segment. If you think about it, you can understand why... each one has its own little buffer as not to disrupt the rest.
Each segment is between two resistors, the point between 2 resistors is ... a voltage divider, pretty clever name.

 

[rufus.K]

at the end is a whopper of a resistor, the bleeder. This helps de-energize the power supply when the power is cut, making it just that much safer.

Nothing about playing with this stuff is safe by the way.

 

[rufus.K]

A few folks mentioned , both in this thread and by PM, ideas about tone stacks... I cant wait to get into that. Someone mentioned Baxandall and someone else said Four Band EQ... Im down with that guys... please join in with sketches and explanations.
Really hoping the crowd-sourcing idea of this takes hold and everyone get input. Feel free to chime in with questions and comments. Definitely with corrections... I am not an engineer, nor am I a pro at this by any means, but its an accessible thing to build, not terribly hard to understand, and amazingly fascinating slice of technology.

 

[agedhorse]

Unless I am misunderstanding your comment, a full wave rectifier and bridge rectifier are 2 different ways to gain exactly the same thing, either 2 diodes and a center tap or 4 diodes and no center tap. Voltage should be the same assuming the same voltage transformer. The difference is how the current is steered on each 1/2-cycle.

 

[NKBassman]

A few folks mentioned , both in this thread and by PM, ideas about tone stacks... I cant wait to get into that. Someone mentioned Baxandall and someone else said Four Band EQ... Im down with that guys... please join in with sketches and explanations.
Really hoping the crowd-sourcing idea of this takes hold and everyone get input. Feel free to chime in with questions and comments. Definitely with corrections... I am not an engineer, nor am I a pro at this by any means, but its an accessible thing to build, not terribly hard to understand, and amazingly fascinating slice of technology.

I've read and kind of understand the function/operation of Baxandall, so I'd like to see something more exotic!

Hows an inductor EQ work?

 

[rufus.K]

Unless I am misunderstanding your comment, a full wave rectifier and bridge rectifier are 2 different ways to gain exactly the same thing, either 2 diodes and a center tap or 4 diodes and no center tap. Voltage should be the same assuming the same voltage transformer. The difference is how the current is steered on each 1/2-cycle.

You're so right. Do have the time to fill in a bit of detail for those who are following, as well as myself?
To clarify what I meant. If I used a bridge rect. on this tranny, I would end up with 465 volts, and if I use a full wave, I will get 297v. Am I correct?
Did I draw it wrong? I don't want to give out false information.
I really appreciate pro's chiming in and making this informative for all of us.
I am trying to simplify some things as I go for those unfamiliar, please excuse me if I oversimplify, or don't end up making a clear point.

 

[NKBassman]

View attachment 802268
capacitors are used to supply extra juice when there is a big draw, and also to smooth out ripples in the newly created DC voltage. You can see after the transformer, there are two diodes. They come off each side of the high voltage winding of the secondary side of the PT (power transformer).
The diodes join up on their output side. The dark hard circles are junctions, you can see the two diodes joining, then after that there is another junction. Two capacitors in series are below that junction . I drew in the ratings of the caps I have on hand, those values will likely change after a bunch of discussion, but I drew em in so move foreword. Capacitors in series halve their capacitence, but sum their voltage rating. So, the two in series each being 220uF at 285v sum up to be 110uF at 570v.

I think I read somewhere recently about also adding some resistors in around each cap when running two in series like that. Something about sharing the load equally between the two caps?

 

[NKBassman]

From Valve Wizard:

If you need a higher working voltage then the usual trick is to put two capacitors in series so their voltage ratings add up. However, the total capacitance will be halved, so two 100uF capacitors would amount to 50uF. Also, resistors must be added in parallel in order to encourage equal voltage sharing between the capacitors. The resistors should be equal to 50/C or less, so two 100uF capacitors would each need a 500000 ohm resistor (470k would be the obvous choice). These also act as bleeders when the amp is switched off.