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Old 5th September 2002, 03:56 AM   #1
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Default learning, building and testing tube amp

Gentlemen/gentlewomen:

Would it possible to build from the ground up (tube out) a simple SE amp by taking directions from you (the learned ones) online--here?

For my benefit as well as other readers, could I test the components based on your suggestions/directions, and most importantly, learn the principles of tube amplification while building it? It is not important for me to have the amp at the end. Rather, the learning and experience is my goal. Perhaps there are others like me.

I have built from a kit a PP, PCB-type tube amp, but unfortunately, did not really learn a lot about the principles.

I have no doubt that I could assemble a tube amp from a schematic if given the values for the components i.e. resistance, voltage, capacitance, etc. That is not the point. I would like to eventually design/ select components.

Is it possible/advisable? Or should I stick with the textbooks (time consuming and boring).

Your comments appreciated.

Aspiring diyer,
Rick
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Old 5th September 2002, 05:50 AM   #2
Colt45 is offline Colt45  Serbia
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pretty tired, but here are my suggestions:

Look around at lots of different schemes for amps.. of different classes. see what changes between them.. and so forth.

read datasheets for the tubes involved. lots of info in there (even if you cant read the graphs.. which i'm still not too good at.)
see which tubes are similar, and what you'd have to change to make them work in the design.

read some of the NEET stuff.. (forget where it is now). its on the web somewhere. it's the military's electronics info i guess.. lots of tube theory, etc..

a real basic 2 tube SE mono deal should be very easy to do from scratch. ( I mean, youve got two tubes.. ~5 resistors.. a couple caps. two transformers. some filter caps and a tube or SS rectifier.)

I'll attatch a pic of the BASIC design of most SE amps (bare minimum deal.)
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Old 5th September 2002, 05:35 PM   #3
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Have a look here...http://home.t-online.de/home/MHuber/
I'm a big fan of Manfred's work...
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Old 7th September 2002, 12:49 AM   #4
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Default output from gain tube

Thanks for the site, Phil; maybe still over my head.

Thanks for schematic, Colt45. Pretty basic it appears! Just what I need.

Question: (Apologies for such a basic question, first.)

If the output from the gain tube is positive and is used as the control grid input to the output tube, how does it become negative thereby resisting the electron flow from the (negative) cathode?

I am certainly missing something here. I know it works, but how?\

Aspiring diyer,
Rick
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Old 7th September 2002, 01:58 AM   #5
Colt45 is offline Colt45  Serbia
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ok.. I'm not super good at this, but I'll try to explain it.

ooops.. forgot a coupling cap between the two tubes in the scheme.

anyways, in theory the grid has 0v DC on it. (and of course an AC signal [the audio]).

when the tube is in operation, with cathode/self bias (this configuration [the cathode resistor]) the cathode becomes positive! (for example we'll use say it's +10v. it really depends on the tube and resistor though.)

so if the cathode is +10v, and the grid is 0v, the grid IS negative in respect to the cathode!

neat huh?
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Old 8th September 2002, 01:30 AM   #6
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If you want to design your own valve amps go here now:

http://www.audioxpress.com/resource/...lass/index.htm

If you're familiar with the basics, these articles should tell you everything you need to know for a starting point.
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Old 8th September 2002, 01:57 AM   #7
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Rick,

Colt is correct with his explanation of the cathode biassing arrangement for the output tube, and also that you need a cap between the plate of the driver (gain) tube and the grid of the o/p tube.

Jeff has also given a truly excellent resource in the Classroom series by Norman Crowhurst. NC was a very knowledgable engineer, and there is so much condensed wisdom in his work you will find new things in it for many years to come.

Can I make a suggestion? At the moment what is happenning in this thead is a lot of general info for tube amps. Tell us what your requirements are for an amp in terms of power etc, and we can help you design an amp from scratch, or take another schematic and pull it apart for you to learn from.

With a poweramp, start at the output and work back in. So we need to know
- power output / load impedance
- preferred tube types: DHT, IDHT, triode, trioded pentode etc
- budget
- what parts you have in your partsbox.

Note, for a SET amp 8 - 10W is about the limit before you start getting into high voltage power supplies and transmitting tubes.

Cheers
Brett
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Old 8th September 2002, 05:32 AM   #8
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Default parts and questions

I have a lot of parts--transformers, tubes, resistors, caps, just about everything!

Tubes, resistors, caps, wire are all going to work. Since I know even less about transformers than tubes, I don't know if the transformers will be useful. Perhaps I can test?

I have a complete electronics shop and lots of old electronics laboratory equipment (decommissioning and demolishing it all soon!) but I have never worked in electronics--just facility (research and development) management and environmental, health and safety.

So, early suggestions to me were: SET el84 (Planet 10). Got the tubes. I have very efficient speakers: altec lansing 605a (102 db/1w/m; 16R) or jbls control 28 (93 db; 8R) or others...low amp power is fine; 8 watts is plenty, thought el 84 put out 1-2 watts. 12ax7's I have.

Budget? I think I can handle it--not looking for fancy--education and experience the goal.

I did print out and read NC's part 1 and still not getting it...

Here's what level I am:

The AC signal (audio) is going + and -, thereby attracting electrons (increasing current to the anode) and repelling (decreasing current to the anode)?

AC signal can be + as long as it is more negative than cathode?

Is the AC signal always repelling the cathode to anode flow?

Then at the anode, it has a fluctuating DC that has to pass the coupling cap to the output tube? How? Caps stop DC? There must be an AC signal out of the gain tube?

Thank you for your response and patience,
Rick
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Old 13th September 2002, 01:37 AM   #9
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OK, I'll take a shot at this, though I think others may be better able to explain it than me. I've only got a rudimentary understanding of things. Others please correct any mistakes!

>>I did print out and read NC's part 1 and still not getting it...<<

What specifically about it are you not "getting"?

>>The AC signal (audio) is going + and -, thereby attracting electrons (increasing current to the anode) and repelling (decreasing current to the anode)?<<

In a sense this is correct. Reality is that the grid is always repelling electrons in "normal" class A operation. If the grid goes more positive than the cathode you have class A2 operation. There are valves designed for this, but typical receiving valves (the sort most frequently used) are not. There are a few commonly used exceptions. The 211 is one.

>>AC signal can be + as long as it is more negative than cathode?<<

Yes, unless you want class A2 operation.

>>Is the AC signal always repelling the cathode to anode flow?<<

Yes, sort of. The AC signal on the grid does the repelling. See above.

>>Then at the anode, it has a fluctuating DC that has to pass the coupling cap to the output tube? How? Caps stop DC?<<

Yes, the cap blocks DC and allows the AC signal to pass. The cap must be of a certain minimum size to pass all audio frequencies. This depends on other circuit values.

>>There must be an AC signal out of the gain tube?<<

The fluctuating DC voltage is across the load resistor of the gain tube. The way I look at it is this: The tube is a variable resistor controlled by the grid - cathode voltage. This variable resistor forms a voltage divider with the load resistor. The power supply supplies a fixed (theoretically) voltage to the top of the voltage divider. When the grid is made more negative less current flows making the tube seem like a big value resistor. When the grid is made less negative more current flows making the tube seem like a small value resistor. If you think about the way a voltage divider works (the voltage drop across each resistor is proportional to the size of the resistor) you can see that the voltage across the fixed load resistor will vary with the input signal on the grid of the tube.

Hope that made at least a little sense!

Jeff
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Old 14th September 2002, 09:12 PM   #10
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Thank you for the reply, Jeff.

So, the audio signal (AC) that is input to the gain tube is amplified by the fluctuating DC passing from the cathode to the anode. Audio "rides" on top of the DC to the cap, where the DC is dropped. This amplified AC (audio) as measured in milliamps is input to the output tube as audio signal.

The process begins again, except this time the audio signal (expressed as amplified milliamps, along with some voltage value) is large enough to be transformed into a usable form to drive the speakers. Is this about right?

Generally, can you give me some example values of voltage and current at different points in the circuit? As below:

# Location Current (AC or DC) Voltage

1. audio signal at gain tube
2. gain tube cathode pin
3. gain tube anode pin
4. before coupling cap (between gain
and output tube)
5. after coupling cap (between gain
and output tube)
6. audion signal at output tube
7. output tube cathode pin
8 output tube anode pin
9. at the output transf. (amp side)
10. at the output trans. speakers side

As I look at this, if it's too hard, please don't. I'll try to measure on one of my PP tube amps. I'm sure it won't be the same as SET but may prove useful.

Quick question: the literature says that most transformers are single application constructed, that is, probably can't be used in another circuit. I have many transf. and wanted to know how you can tell (measure) which will be useful (PP or SE). SE's are big and expensive, I understand. What do you look for?

Thank you for your interest.

Aspiring diyer,
Rick
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