Ever complete an amp build only to find your eardrums bleed from the bright tone? Maybe your guitar sounds murky? It could be multiple things, but let’s look at the first connection to the amp. The input resistor has a huge impact on your tone, and may be the main culprit for bright or murky sounding amps. Lets learn how to take advantage of the input resistors tonal possibilities.
Look inside your 5F1 champ kit https://crunchytubeamps.com/product/5f1-tweed-champ-guitar-tube-amp-amplifier-kit-diy/ and you will see 68K input resistors. When you’re in the bright channel the resistance might read 34K while normal channel reads 68K. Before you go changing your resistors to brighten or muddy the sound of your amp we should talk.
The input resistor stops unwanted noises from entering along with your intended signal. It also acts along with the preamp tube to filter out high frequencies. How high depends on the a few factors. The metal parts in the tube along with the tube socket connections actually hold a small charge (Miller Effect). That charge is measured in capacitance, and works with the input resistor to filter out higher frequencies. Higher resistance equals lower frequencies (murky). Lower resistance equals higher frequencies (bright).
I mentioned above that our input resistor stops unwanted noise. Well, here’s where we must compromise our tonal endeavor. Lowering the input resistance can reduce unwanted noise or hiss. The opposite is true of raising your input resistance. Now we have to decide how much noise is acceptable for our application.
Now we can adjust our bright or murky sounding amps, so it’s time to experiment. I have linked my Facebook group below if you’re interested in using my crude excel calculators. You can download them if you have Microsoft. My next post will be a how to on these calculators. I find that most of the time, a 12AX7 requires an input no lower than 22K ohms and no higher than 82K ohms. Tone is a flavor, so everyone will have different likes. Now go get the tone you’re after!
