Building tube amp

[davent]

Here's some reprints of some 50's magazine amp design articles to get you started. Geared to hifi but will give you an idea of what may be  involved.  http://www.audioxpress.com/resource/audioclass/index.htm

[PRR]

> how does one design an amp without any math?

You WANT to do math? Read Kuehnel. Here's a taste:
http://www.ampbooks.com/home/tutorials/lesson-010/
FWIW, his techniques would NOT be practical in the 1950s. Cramer's Rule is computationally intense, and no private company could afford that many computers (asian grad students).

Of course with a Pentium such stuff takes longer to set-up than to solve.

For fun, I checked most of the math in his 5F6A book with paper, pencil, and $13 calculator (my slide-rule was at work). No significant mistakes.

How did Leo Fender do it? Here's a clue:



He cribbed values from the tube databook.

How were these values found? Tung-Sol paid a junior engineer to sit with tube, decade box, and notebook. Each company optimized a little different; that's how we know he peeped Tung-Sol or Westinghouse not GE or RCA.

> If its so easy, why can't I find a simple summary on designing an amp?

From scratch, it is overwhelming.

In a genunely new system, you define requirements and use tube-maker's suggested conditions, just as Leo Fender did.

In a rich and mature field, without corporate financing and constraints, you PLAGIARIZE. Read and digest and cross-compare EVERY schematic you can find. The simple pure amps pre-1960, and the baroque monstrosities of the 1980s. They all have something (even if only something to avoid).

Then you BUILD. I see a lot of guys agonize over design and never build. Build and then WORK with the amp. Leave the soldering iron hot, have lots of different parts on hand. We know from accounts that this is how early Mesa Bougie amps were "designed": some basic amp tinkered for months. We know from reading early Fender schematics that Fender, White et al build, sold, changed, sold, changed, sold. They didn't get it perfect the first time; they kept working. They had some very good amps, then screwed-up the next model. Design is a continuous process, not a single event.

[surfsup]

I think i just learned something, the fender schematic looks similar to the rca...! I dl'd the audioXpress pdfs thx

[surfsup]

This is an excellent document I would suggest to anyone to read first. Very good indeed.

http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf

[Derringer]

This is an excellent document I would suggest to anyone to read first. Very good indeed.

http://www.freewebs.com/valvewizard1/Common_Gain_Stage.pdf

+1

it wasn't until after I read this and then started playing with the fetzer concept that things really started to make some sense

[PRR]

> the fender schematic looks similar to the rca...!

RCA published thousands of schematics..... which one?

However, tube audio amps generally follow similar patterns.

The RCA book mentioned in this thread has a plan which "could be" used as a guitar amp.

RCA RC-27 plan 25-14 page 554. Open this copy and expand so you can read it:
http://i56.tinypic.com/2v9f7mr.jpg

6AU7 and 6AV6 are like 12AX7, just bundled differently.

There's a first preamp, volume control, a second preamp, a tone control, a driver amp and a power amp. To feed these there is a power transformer and a rectifier and DC filtering.

As a guitar amp it has these mis-fits:

The phono input does not have enough gain.
The mike input has too much gain, and distorts bad with anything other than the specified ceramic mike.
The tone controls are boost/cut; guitar usually needs more bass and treble boost, less cut.
The output transformer is expensive.
There's a cheaper way to get NFB around the power stage.
It overloads "gracefully"; when WE overload an amp we want RUDE.
The whole thing has more parts than it really needs! It is an example, not a refined product.

But there's nothing much odd about it. V1B and V2 are wired exactly like the generic voltage amplifier, with values similar to the chart suggestions. V3 is a plain power amp, plus a now-unstylish NFB connection. The power supply is over-elaborate but very conventional.

And that's how you do it. You find something close, then change it. I'm showing this RCA plan just to show that all audio amps are related. But if you are going for a guitar amp, you should peep most of the MANY guitar amp plans available, so you start closer to where you want to be.

[tubelectron]

Hi All,

Oh, I may have already shown it one dozen times... I designed and built this small tube amp from a vintage VOX Escort busker SS amp for my daughter in 2009 because she loves the look of the VOX amps. Lovely for a beginner, isn't it ? But...





Doing this seems quite simple, but it's not at all : as a beginner, to limit your chances of failure and deception, you ought to start with an easy to build kit like a Champ Amp. You will find a lot of support information on the web and from the kit seller (usually), and it stays an affordable project, both technically and financially. And in the meantime, you will learn, take a good start, and so be ready to go "bigger" next time with your acquired experience.

Good luck & A+!