galactron amp schema problem?

[tadiam]

I buid this amp but no have 1,3v zener diode ..the small value is 2,2v any solution?

http://www.pi.infn.it/~federico/Immagini/gl.zip

[Mooly]

What value zener is it ?

[Mooly]

Use silicon diodes in series for small values. Each diode is approx 0.6 volts. Use LED's, different colours have different volt drops.

[lineup]

I would use two 1N4148 in series like Mooly suggested.

Use like 2-3 mA. This will give close to 1.3 V.
This makes R7 could be 22k or 15k.
I should try 15k = 3mA
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The precise voltage (1.3V) at Q4 base is not the important thing.
The thing is to get close to 20 mA (0.020 Ampere) through Q4.

After you have got something like 1-1.5 Volt at Q4 base, measure voltage across R12.
If is, for example 0.60 Volt.
Then current is = 0.60V/27Ohm = 22.2mA. Close enough!

If voltage across R12 is 0.40V, then we get current is:
0.40V/27Ohm = 14.8 mA
... so we will change R12 to 20 Ohm.
Current is now:
0.40V/20 = 20mA. Which is the current we want in Q4

Formula, to get 20mA (0.020 Ampere):
R, Ohm = Voltage across R12 / 0.020

[tadiam]

diodes no work for me and red and green led too. have an ac sound in the speaker(cheap speaker)
i see again the formula thanks to all

[Mooly]

Diodes must work What's not happening ? What volt drop have you across them, and R12 ?

[tadiam]

28ma

[Mooly]

Whats 28ma. Do you mean the current through R12. The absolute value in itself is not too critical but I do see that the amp has no adjustment for quiescent current.
I would be happier with R10 replaced by a 2.2k preset set initially to it's highest value ie 2.2k. That will give no current through the outputs, much safer for testing.
Anyway I would use I think 3 IN4148's ( you realise they go the other way round to a zener !! ) to give a little compliance and adjust R12 to give around 20 ma.

[Mooly]

Like this, the 3 diodes give 1.8 volt, the resistor always see 1.8-Vbe which is 1.8- 0.6 =1.2. R =V/I = 1.2/0.020 = 60 ohm.

[lineup]

Originally posted by tadiam
have an ac sound in the speaker (cheap speaker)

How old is the amplifier?
The AC sound, how does it sound? Can you tell more.

The adjustment of Q4 and R12 has nothing to do with the AC sound.
Must be something else.