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Dirty-South Blues Harp forum: wail on! >
changing gain
changing gain
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garry
102 posts
Aug 21, 2011
6:53 PM
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in another thread, someone mentioned that you can't change the gain on SS amps like you can by swapping tubes in a tube amp. clearly true. but it reminded me of something i've never quite understood. given a tube amp with separate gain and volume (master?) controls, what is the difference between swapping in a lower gain tube and just turning the gain down? why would you do one vs. the other?
for that matter, my vt30 has separate gain, volume, and master controls. they control input gain, the volume that goes to the power section, and the power amp section. i've read descriptions of how these interact, but in terms of guitar. any insight into how they work with regard to harp would be appreciated.
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Greg Heumann
1237 posts
Aug 21, 2011
9:52 PM
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I've only had limited experience with such amps, and you are on the right track - but what happened for me was that you couldn't get the gain low enough even with the control all the way down, or the control was extremely sensitive immediately off of 0.
The analogy I like best (and I have used it here before) - It is like owning a 500 hp sports car- too much gain is when the throttle pedal moves only 1/2" from idle to full throttle. Lowering the gain gives you 5" of pedal travel. Idle is still idle, full throttle is still full throttle, but the damn thing is a helluva lot easier to control in between.
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/Greg
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5F6H
815 posts
Aug 22, 2011
2:47 AM
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Further to what Greg says, changing a 12AX7 for a tube with less gain, or reducing gain by manipulating the circuit (or a combination of both), actually changes the operating conditions that the tube has to live with. The "master" & "gain" pots are just voltage dividers, letting a certain amount of signal through to the next stage (in many MV circuits having the master fully up aims to recreate the "stock" circuit...but there are lots of MV circuits, all different)...no matter how you work it, if you have too much gain with 12AX7s, you end up just robbing Peter to pay Paul with the pot balance. If that extra gain is at the first tube stage in the circuit, then you cannot "turn it down" so to speak, as the "gain" pot is essentially just the same as your regular "volume" pot in a non-master amp...A master volume amp is typically designed to allow you drive the preamp harder at an overall lower volume (dB out at the speaker, not pot setting #), it can't let you drive the preamp less at a higher overall volume.
Master vol, or no, it really makes no difference to the practice of reducing gain by tube subs/circuit mods. Also remember that lower gain tubes change the overall tone too, not just volume pot position and THIS is the primary reason for employing this technique (if you wanted to keep stock gain and just decide on a point where you wanted to turn the volume pot to, you can do this easily with a resistor or two...but if the amp was too stiff & aggressive to start with, at say "2", it will still be too stiff & aggressive at "7").
Last Edited by on Aug 22, 2011 3:11 AM
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jimbo-G
26 posts
Aug 22, 2011
10:17 AM
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I wired just a 50k pot to the bass pot, no capacitor or anything on my solid state and turned it right down and it now gives off a nice deep sound.
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"Everyone has to start somewhere."
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