Let's build a tube preamp

[cfsporn]

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".

The transformer is readily available on eBay.

Also, this seems to be a really good choice PT wise. Lots of current and lots of heater taps provide good expandability for future revisions. For $65 shipped what's so bad?

 

[rufus.K]

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.

I read that too. I'm not sure. The power switch turns on the amp, and the standby switch kills downstream B+ while allowing the heaters to get hot. Trying to keep it simple, I was going to stick to the old school with details like that.
Post what you come up with in regards and we can talk about it, I'd like to hear more.

 

[rufus.K]

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.

So should we leave it in?

 

[cfsporn]

@rufus.K Please don't take this as an insult or anything more than benign curiosity; are you an EE?

 

[cfsporn]

So should we leave it in?

I'd say leave it out. More parts=more places to mess up.

 

[rufus.K]

@rufus.K Please don't take this as an insult or anything more than benign curiosity; are you an EE?

Nope. Just a man.

 

[whatizitman]

@rufus.K Please don't take this as an insult or anything more than benign curiosity; are you an EE?

An Everything Engineer?

 

[beans-on-toast]

Designing a preamp isn'teasy.

Find what works, adopt that, and modify it. There are schematic sites with studio mic preamps and various EQ's. It helps be see how things were done.

I like a regulated heater to keep the hum down.
I like some wobble in the B+. It helps the amp breath and be less sterile. My personal taste.

There is so much to study before proceeding. At the same time, you gain insight by doing. But a plan is needed, a set of requirements is good start. The potential list can be long: input; output - what do you want to drive?, transformer output?; type of EQ; headroom requirements, do you want it ultra clean, capable of dirt, compression, ...

There are some useful design tools and tutorials here: amp books.

This chapter will show you how to design a simple triode gain stage using load lines: http://www.valvewizard.co.uk/Common_Gain_Stage.pdf. Good start.

 

[rufus.K]

Well for me, Ive built one amp, three times already (same design, three builds). They were guitar amps. I think my goal is more to spread what I know around to whom know less, and let everyone else who knows more educate me further. In the end, there would be a preamp build from my spare parts bin. There would be a non radical design done along the way that others who havent built before could follow (and modify).

 

[rufus.K]

Designing a preamp isn'teasy.

Find what works, adopt that, and modify it. There are schematic sites with studio mic preamps and various EQ's. It helps be see how things were done.

I like a regulated heater to keep the hum down.
I like some wobble in the B+. It helps the amp breath and be less sterile. My personal taste.

There is so much to study before proceeding. At the same time, you gain insight by doing. But a plan is needed, a set of requirements is good start. The potential list can be long: input; output - what do you want to drive?, transformer output?; type of EQ; headroom requirements, do you want it ultra clean, capable of dirt, compression, ...

There are some useful design tools and tutorials here: amp books.

This chapter will show you how to design a simple triode gain stage using load lines: http://www.valvewizard.co.uk/Common_Gain_Stage.pdf. Good start.

Please talk us through a regulated heater supply, vesus an unregulated one.

 

[mbelue]

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

Since we're on the subject of SAFETY. I think its prudent to mention the One Hand Rule.

Whenever possible do not work on live circuits. That includes unplugging mains and DISCHARGING CAPACITORS. When working on a circuit that is that is questionably live only use one hand at at time. The other hand should be well clear of the circuit and any possible ground paths. That includes pipes, workbenches wiring and metal racks and supports.
The idea is to not present yourself as a path for electricity to travel to ground.

Really the One Hand Rule is difficult to get used to and often impractical, but I strongly recommend its use while discharging capacitors at the minimum.

Use your brains folks, electricity can hurt and kill.


Now back to the fun.(Y'all are getting to the math part soon right?)

@rufus.K Please don't take this as an insult or anything more than benign curiosity; are you an EE?

Nope. Just a man.

Neither was Hartley Peavey when he built his first amp.

 

[rufus.K]

I was thinking on my drive home, that if I went with a Full Wave rectifier rather than Bridge, i'd be just shy of 300volts. Thats well within 12ax7 range.
It'd be more work to bring down the 465vdc id have from a Bridge rect.

 

[rufus.K]

Since we're on the subject of SAFETY. I think its prudent to mention the One Hand Rule.

Whenever possible do not work on live circuits. That includes unplugging mains and DISCHARGING CAPACITORS. When working on a circuit that is that is questionably live only use one hand at at time. The other hand should be well clear of the circuit and any possible ground paths. That includes pipes, workbenches wiring and metal racks and supports.
The idea is to not present yourself as a path for electricity to travel to ground.

Really the One Hand Rule is difficult to get used to and often impractical, but I strongly recommend its use while discharging capacitors at the minimum.

Use your brains folks, electricity can hurt and kill.
Now back to the fun.(Y'all are getting to the math part soon right?)
Neither was Hartley Peavey when he built his first amp.

Cant wait till the math part, hope you can help with it

 

[nbtone]

I don't want to stick my neck out too far, but what does your design have on say an ART tube MP? Is there going to be more in the low end? Those guys try hard to be unbeatable.

 

[rufus.K]

I don't want to stick my neck out too far, but what does your design have on say an ART tube MP? Is there going to be more in the low end? Those guys try hard to be unbeatable.

It will have an EQ.

 

[winegamd]

And be a true high voltage design, not a starved plate like the MP. This means better linearity, potentially less distortion, more headroom, better touch response, and more tube "magic".

 

[rufus.K]

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

 

[cfsporn]

I don't want to stick my neck out too far, but what does your design have on say an ART tube MP? Is there going to be more in the low end? Those guys try hard to be unbeatable.

It's DIY.

 

[beans-on-toast]

Please talk us through a regulated heater supply, vesus an unregulated one.

You don't want an unregulated heater supply in a modern preamp design. A DC heater or a regulated supply reduces or eliminates hum in the amp related to the heater circuit. Regulated supplies are also inexpensive to implement. A DC heater can be as simple as a couple of capacitors and some diodes. It can be more complicated and incorperate regulation which maintains the DC voltage at a fixed level as the power line supply varies.

Concerning the amount of ripple in the high voltage supply, see what you think of this: Class AB Power Supply Ripple

 

[cfsporn]

For the record, I'm not an EE either. I am currently in school to be one though!

Fun fact: If it wasn't for a particular thread on TB, I never would've gone back to school