question about mods to valve amps

[blindboybenton]

Hello

My knowledge on valve amps is not brilliant but i get by. so i have a few questions for all you experts

My question is about tweaking and modding
Im asked more and more now. Can you add more gain or decrease gain or more bass etc

Increasing or decreasing the value of components around a pre amp tube how will that change the sound?

I have noticed that the values of decoupling caps differ from make to make, So by increasing or decreasing that value
How with that change the sound ?

and lastly. If i want to reduce or increase the treble or bass etc the best way of doing it ?

many thanks guys

Bassman

[KevinOConnor]

Hi Guys

You'll notice that most preamps in tube amps use the same 12AX7 tube, but the amps sound quite different. The Rs and Cs around the tube tell it how to behave and perform, so in a given circuit you can change those values quite liberally and have different sounds and gains. Tubes are electrically rugged and will withstand mistakes you might make like swapping a plate and cathode resistor value. The sound will be low or choked, but once you notice the error and swap the parts everything will work fine.

Power tubes affect the overall tone at all signal levels, not just at full tilt. Every tube adds a veil of character to the sound and the sound is a bit different with soft attack versus hard attack because of the curved transfer function of every tube. This gives tubes "touch response" and is a dynamic quality that tubes have that transistors do not. Power tubes can make good and bad distortions and tones just as preamps can, so do not fall for the hype that all the tone is in the output stage.

How much gain you can add depends on the specific circuit. You can always decrease gain just by swapping in a lower-mu tube from the 12AU_7 family. You have to balance gain against other attributes such as sustain. Sustain means you can hit a note and rings for a long time even when clean. It has nothing to do with compression, like the Boss Compressor-Sustainer pedal, which is a compression and expansion device that props up the level as the note decays (fades out).

There are too many possibilities of mods to outline here. Best to begin with a specific circuit and a specific change you want to achieve. There are many good books about modifying tube amps and also online resources.

Have fun