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Old 27th January 2005, 09:22 PM   #1
coony is offline coony  Belgium
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Default Tube compressor problem!

Hi!

I've build this tube compressor with 12AX7's:

http://www.triodeel.com/comp1.jpg

but I can't get it to work. Has anybody build this and maybe adjusted it?

Thanks!

Koen
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Old 27th January 2005, 09:36 PM   #2
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Can you give some more details? Doesn't work as in no sound or that it doesn't add compression? What does your power supply look like?
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Old 28th January 2005, 06:47 AM   #3
coony is offline coony  Belgium
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There's only a very faint sound comming through and LOTS of hum.

The PS is the typical RC network, used by other preamps that work well, so I guess the PS isn't a problem.

On the drawing there seems to be a line added later on the second tube, connecting the plate with the grid. I've never seen this type of circuit before, is this added line 'right'?

Thanks,
Koen
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Old 28th January 2005, 07:57 AM   #4
Yvesm is offline Yvesm  France
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Hi Koen !

It seems that the right most tiode acts as a "controlled resistor" and shunts the signal to ground.

This added wire is intriging but right to emulate a diode ! ?
But, IMHO, the main typo is not here.
I'm bothered by the fact that the "controlled resistor" seems always hardly conducting.
May I suggest to insert a 3 to 5 volts bypassed Zener between its cathode and ground, feed with some 100K to B+ .
Doing so, it will not conduct nor shunt the signal until some positve voltage coming from the amplified and rectified signal will instruct it do do so !

Just my two pence idea

Yves.
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Old 28th January 2005, 07:48 PM   #5
coony is offline coony  Belgium
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Quote:
Originally posted by Yvesm
Hi Koen !

It seems that the right most tiode acts as a "controlled resistor" and shunts the signal to ground.

This added wire is intriging but right to emulate a diode ! ?
That's what I thought as well, a rectifier to get a 'steering' signal.
Is the added wire really needed, or would it work without it - the original drawing was without it so...?

Quote:

But, IMHO, the main typo is not here.
I'm bothered by the fact that the "controlled resistor" seems always hardly conducting.
May I suggest to insert a 3 to 5 volts bypassed Zener between its cathode and ground, feed with some 100K to B+ .
Doing so, it will not conduct nor shunt the signal until some positve voltage coming from the amplified and rectified signal will instruct it do do so !

Just my two pence idea

Yves.
So you think it's the last valve that's not working? I'll try you idea tomorrow! Does it have to be a zener or would a resistor work as well?

Thanks,
Koen
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Old 28th January 2005, 08:37 PM   #6
Yvesm is offline Yvesm  France
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Hi,

Quote:
Does it have to be a zener or would a resistor work as well?
Something that fix the cathode at a few volts above the ground should work, why not a resistor bridge, or some SS diodes in serie ?

Regards, Yves.
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Old 31st January 2005, 09:32 PM   #7
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Hi guys,

I followed this thread already for a while. I built the same compressor limiter, and in my case it also did not work well, although used between a guitar and a first tube (12ax7) of a guitar amp the 'music' gets through, without to much hum. Not much changes when turning the pots. After reading your posts I made a rectifier bridge so that 3 volts were put on the last triodes cathode. When connected to a scoop and tone-generator now I can see that at least some of what it should do finely happens. You see the sinus wave extend and/or the top of it flattend (making it more look like a square wave).
I say some because the effects are small. One would like to see more.
Maybe one of you knows how many volts may be put on that cathode, and if that should have effect.

Werner
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Old 1st February 2005, 08:08 AM   #8
Yvesm is offline Yvesm  France
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Hi Folks.

Looking on more time at this schemo, I note that the control voltage for the last triode is obtained from the input signal, rectified by the second one, but there is no kind of voltage gain anywhere, just cathode followers.

In turn, I beleive that this unit could work only at relatively high level.

It should be instructive to check the DC voltage at the cathode of the third tube with the resistor (15K) to the next grid removed, and at various input levels.

Also, why not pick the voltage for the fourth triode on the cursor of the 25K pot, rather than at top ?

Concerning the "offset" of the fourth cathode, it could be set so that just a vy small (if any) current flows throught the tube at no signal (easy to check by monitoring its anode voltage).
To do that, I suggest to feed the cathode with a 100K from B+ and having a adjustable resistor (say 2K) to ground.

IMHO, this compressor cannot work as is, either because the conceptor never tried it, or because the "copyist" had made too many typos or hazardous adaptations !!

Regards, Yves.
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