Deisel401 said:
When you say tone stack, are you referring to the cap directly behind each pot? Or the c's and r's throughout the pcb ? :shock:
The network of resistors and capacitors after the last preamp triode is considered the tone stack. it is the section of passive filters that control the gain of certain frequencies. The potentiometers are used to vary the gain. It isn't really confined to a specific section of the PCB. If you want to find out what caps and resistors make up the tonestack you would have to map out the circuit board (only the part after V2). To fine out the effect of each component you would have to learn more about filters, tone stacks, and the gain stages the precede the tone stack.
The rest of the resistors and capacitors are associated with the gain stages themselves and other sections that make the modules function properly. The components that make up the gain stages also contribute to the tone of the module (i.e. what frequencies are cut/boosted). I recommend finding a book on preamps to learn more. I've posted recommended links and books on a previous thread. I hope i have cleared it up a little bit.
To the topic at hand...considering what i have said about tone shaping it is evident that there are many different ways to boost the treble of the module. Some of them involve the tone stack but you can also look at the cathode bias circuits, anode load circuits, gain stage coupling circuits, and the pre-gain stage circuit (bright switch/gain pot).
To shift the mid range you should look for the filter in the tone stack that controls the mid range frequencies and change the value(s) to satisfy your tastes. There are simple equations that you can use to find out what values will work based on the filter arrangement. You should be able to find these online.
I think a formal definition of gain should be mentioned. The gain of a circuit is defined as the ratio of a circuit property from input to output. In amplifiers this is usually voltage. Therefore the gain of any circuit is simply:
Gain=Output Voltage/Input Voltage=Vout/Vin
Now because the guitar input is composed of many different frequencies and the gain stages themselves introduce other frequencies (harmonics) the gain of the preamp is going to be different for each frequency. This depends on the component values used. For example after V2 the output may yield a +3dB boost in treble frequencies, a +6dB boost in midrange frequencies, and a -3dB cut in bass frequencies. It gets more complicated than this but for simplicity this is basically how it works.
Anyway my point is that the term gain has been associated with distortion but in actuality distortion is merely a result of manipulating the gain characteristics of the vacuum tubes. Distortion or clipping is a result of the output signal exceeding the limit of the tube. In other words the gain of the circuit is higher than the threshold for clean amplification. Therefore distortion is a result of gain but gain does not always generate distortion.
My other point is that it is probably best at this point that you look at the tone stack for your tone manipulation. While it can be done in the gain stages, unless you know exactly how they are designed (both in general and for the specific case) it is best to leave the gain stages alone.
Before you turn on the soldering iron have you tried the presence knob? I know on my RM80 if it is below 3 o'clock then my tone is unbelievably hollow. It sounds like there is no treble range at all.
One final note. If i have presented any incorrect or partial information then please correct me. I am still learning about audio tech and have taken a several month break so i may be misinformed. If you have any questions then let me know.
Take Care,
John
