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Quick question on this schematic

wwenze

Member
2008-03-07 12:46 pm
Something I found while reading the net:
[IMGHTTPDEAD]http://pic.hifidiy.net/forumid_10/100122085042a750a88c3ffb99.jpg[/IMGHTTPDEAD]

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?
 
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.
 
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.