As you can tell from the question, I'm not too experienced in this field, though I have built a couple of simpler tube amps successfully.
In discussions on various forums the comment has been made about amps having a shelf or notch frequency of 350 -400 for guitar and 1000 - 1500 for steel. Can someone explain to me how this actually works and which part of the preamp circuit controls this. I believe it comes before the tone stack/controls but where? Also how would I change the notch on a SF Deluxe to 1k or 1.5k? The idea is to make the high end sweeter and ideally be switchable between stock and (steel) sweeter.
I have a number of textbooks on amp building, but none seem to clarify this, as far as I can see.
Thanks
if it sounds too good to be true, then it probably is...................
Bananafist wrote:As you can tell from the question, I'm not too experienced in this field, though I have built a couple of simpler tube amps successfully.
In discussions on various forums the comment has been made about amps having a shelf or notch frequency of 350 -400 for guitar and 1000 - 1500 for steel. Can someone explain to me how this actually works and which part of the preamp circuit controls this. I believe it comes before the tone stack/controls but where? Also how would I change the notch on a SF Deluxe to 1k or 1.5k? The idea is to make the high end sweeter and ideally be switchable between stock and (steel) sweeter.
I have a number of textbooks on amp building, but none seem to clarify this, as far as I can see.
Thanks
Duncan amps has a tone stack calculator were you can change the values and see the results on a frequency/DB graph. Go to duncanamps.com and click on the technical link, download it and install.
I'd think that it you get get the curve you're looking for you could use push/pull pots to switch between the difference eq curves.
The frequency notch you are referring to is sort of built in to the typical FMV tone stack. At most positions of the Treble and Bass pots, there is a big crevice in the mid-range. The Mid control never really gets rid on the crevice or gives a true mid boost. There are old Gibson designs with Twin T or Bridged T networks that give a notched frequency response but you don't see those very often. Then again, Steele amps have sort of a black art mistique and there isn't very much info on what are considered the Holy Grail of Steele amps. Changing cap values and the 'slope' resistor (100K on a blackface Fender) will move the notch around. Use the Tone Stack calculator to see the effect.
The shelving type response you refer to is what the bright switch does on a Blackface Fender. In the 5F6A this is accomplished by the two input channels. Marshall took it one step further with different caps on the first tube cathodes. The presence control does basically the same thing in variable fashon. Just change the associated cap value to shift the frequency. Smaller cap moves frequency up, bigger moves it down. On a SF Deluxe there is a 47pF on the Vibrato channel Volume control, change that. Also change the 25uF cap on the first 12AX7 in the Vibrato channel to something between 0.22uF and 1uF. There is also a 47 ohm resistor in the power amp. To act like a presence control, connect a cap in parallel with it. Something between 1uF and 10uF. If it acts too "wild", connect a 10 or 22 ohm resistor in series with the cap.
