Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blues man on April 17, 2011, 09:27:09 pm
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quote: from Hot Switch Mod on dougs site. http://www.el34world.com/projects/hotswitch.htm
"The 470k resistor connected to the end of the .022 sets the gain level that you send to the valve two tube".
This would be the 470k that goes to ground. right? If I want to raise the gain I increase the value and if I want to lower the gain I decrease the value. Am I right?
The 2 470k res make a voltage divider. so could I lower the value of the 470k that feeds the input of valve two instead of changing the 470k that goes to ground to increase the gain.
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Your analysis is correct, if you raise the value of the 470K resistor to ground you will increase the gain of the signal fed into the third stage. There already is a lot of gain here though, increasing it too much will probably result in instability/ oscillations and/or blocking distortion in the third stage which you will have to address is some way, like with a snubber cap . You could replace the 470K to ground with a 1M pot to try and dial it in - put the outer legs of the pot in place of the 470K and take the signal off the wiper that goes to the switch. The other 470K resistor is in the circuit weather or not the switch is engaged, so changing that is going to change the tone somewhat when the extra stage is not engaged because of the Miller effect : http://www.aikenamps.com/MillerCapacitance.html. i.e., lowering the 470K grid resistor will result in increase in highs somewhat because the resistor in combination with internal tube capacitances forms a low pass filter that will allow more highs the lower the value of the resistor.
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This would be the 470k that goes to ground. right? If I want to raise the gain I increase the value and if I want to lower the gain I decrease the value. Am I right?
Yes and yes.
The 2 470k res make a voltage divider. so could I lower the value of the 470k that feeds the input of valve two instead of changing the 470k that goes to ground to increase the gain.
There is no voltage divider so changing the 470K that feeds the next stage will not affect gain.
Inca Roads covered it well. Here's a pic for reinforcement. Doug's circuit is like figure 1. There is no voltage divider. The 470K to ground is effectively in parallel with the previous plate load resistor. Lowering it's value will lower the load resistance the previous tube sees, which will lower the gain of that tube.
Figure 2 shows a typical voltage divider. Changing the ratio of the two 470Ks will change the amount of signal being fed to the next stage. This does not change the gain of either stage as long as the total resistance of the divider remains 940K. The voltage divider works just like a typical volume control.
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Thanks for the replies. I am starting to understand more with each build I do. As soon as I get time I am going to dive into my electronics book and at least finish the book even If I don't go back to school.