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Old 12th October 2010, 02:46 PM   #1
wwenze is offline wwenze  Singapore
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Default Quick question on this schematic

Something I found while reading the net:
Click the image to open in full size.

In the top half the left most tube (12AU7), the grid is connected to a 1k resistor then a 1M resistor connects it to ground - what is the purpose of this 1M resistor? Something like the resistor found in the same position in a SS/chip amp but at lower value? Why 1M then? (Sorry I am new to tubes)

Then, in different versions of the same amp there is no potentiometer at the input, would the 100k potentiometer result in any trouble in this case? Particularly when the volume is set to lowest?
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Old 12th October 2010, 03:16 PM   #2
DF96 is online now DF96  England
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The 1M resistor should be for grid bias. Unfortunately the circuit has a mistake. A capacitor is missing, which should go from the 1M resistor to the volume pot slider. With the capacitor missing, most of the grid current from the valve (small but non-zero) will go through the slider-track contact and could generate noise. At present all the 1M does is protect against pot failure. At least that is better than some circuits which omit that too.

The resistor is 1M because the higher the better (to avoid loading the signal) within reason (too high causes bias drift in the valve).

I can't quite see the point of this circuit. It has high gain (approx 250?) then throws it away with negative feedback, yet still ends up with far too much gain (x21) for a line amp so will probably have to be followed by an attenuator before the power amp.
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Old 12th October 2010, 03:29 PM   #3
wwenze is offline wwenze  Singapore
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Grid bias, thanks, I'll read up on that.

This is the schematic of a clone of CAT SL1 preamp, I can't quite see the point of preamps anyway... except for the tube sound which would need the tubes to be soft-clipping hence the gain, am I right?
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Old 12th October 2010, 03:33 PM   #4
DougL is offline DougL  United States
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Here is the relevant section of the 12AU7 data sheet.
http://www.triodeel.com/12au7p2.gif
Note the specification for grid resistor. The Maximum value is listed as 1 meg.

If they were using a stepped attenuator, the cap is probably unnecessary. On a carbon pot, the cap would be best practice.

Just my 2 cents.

Doug
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Old 12th October 2010, 03:44 PM   #5
kmtang is offline kmtang  Canada
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Using a tube amp or not is really your cup of tea. It's all your personal preference.

Build one and see if you like it or not.

My friend built one of this Convergent SL1 clone and it sound pretty good.

Johnny
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Old 12th October 2010, 03:54 PM   #6
DF96 is online now DF96  England
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No, the high gain reduced by feedback leads to hard clipping. If you want a soft-clipping effects box then design for low gain without feedback. Better still, design for low gain without distortion.

This circuit seems to run the input ECC82 at quite a low current, yet with a lowish anode load too. The result will be low-order distortion, which the feedback will turn into high-order distortion. However, the input stage needs a low impedance output because the second stage is being run right on the edge of grid current so could distort on peaks. My guess is that this was designed by someone who's day job is solid-state. The active load on the second stage will help, but its main function is probably to impress journalists.
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Old 16th September 2013, 01:57 PM   #7
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Default Convergent SL1

Quote:
Originally Posted by kmtang View Post
Using a tube amp or not is really your cup of tea. It's all your personal preference.

Build one and see if you like it or not.

My friend built one of this Convergent SL1 clone and it sound pretty good.

Johnny
I would like to also receive personally the original scheme of Convergent SL1.

thanks virigino

my mail: vegasi@tiscali.it
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