andy-h-h wrote: ↑08 Aug 2020, 21:03
Can anyone explain what's happening with the tone control section? I have limited technical knowledge, and I'm confused as to why one side of the treble control connects to the collector of T4.
BOTH pots connect to the collector.
It's a baxandall bass + treble control: Both tone pots have their "cold"/lug1 connected to the output/collector of Q4, their wipers/lug2 connected to the input/base of Q4, and their "hot"/lug3 connected to the output of the previous stage.
A baxandall can cut or boost a range of frequencies determined by the cap values - You can choose appropriate cap values and use a third pot to allow cut or boost of bass, treble AND middle if you so desire, e.g. EQD Talons.
The lug1 connections to the collector will direct feedback from Q4 back into the base: The collector is out of phase/inverted WRT the base of the same transistor, so this is NEGATIVE feedback. You can more easily understand how it works if you analyse one pot at a time: Look at the bass pot - At maximum rotation (full bass boost), the output from Q3 goes through a 10k resistor (slight attenuation of all frequencies, but full audio spectrum allowed through) then directly through lug3->lug2 (zero resistance because the pot is max'ed) and through a 33k to the base. At the same time, the bass pot is also applying 100k (plus the 10k on lug1 and the 33k) of resistance to the negative feedback via lug1->lug2->Q4 base. Increasing resistance here reduces the amount of negative feedback, therefore increases the gain of Q4 (this is how opamps work).
But why is it BASS control? The 22n cap across the pot will short/bypass the 100k pot resistance for all but the lowest frequencies (i.e. higher frequencies in the feedback signal are NOT attenuated) so higher frequency feedback is large (and so not boosted) but lower frequencies in the feedback cannot bypass the pot, feel the full 143k resistance and are therefore significantly boosted as they pass through Q4.
You can see the same thing going on for the treble control, but with the caps arranged to select the highest frequencies. The 2n2 cap from the Q3 output cap (4u7) means that only very high frequencies can pass into lug3 of the treble pot, through lug2/wiper and on into the base via that 10k fixed resistor. If the treble pot is max'd, there is also a large resistance between the collector and base through lug1+2 of the treble pot, but this time the (2n2) cap in the feedback path is in SERIES with the pot resistance (not in parallel like the bass pot): This means that only the highest frequencies in the feedback signal make it through this pot back to the base and therefore only higher freqs are boosted when the pot is rotated clockwise.
When the pots are turned counterclockwise, the reverse effect occurs: The input signal from Q3 is attenuated by larger resistances between lug2 and 3 of each pot, AND the amount of frequency-specific negative feedback allowed through is increased by the reduced resistance between lug1+2: There is therefore a major cut in those frequencies.
You can get a similar effect without using a gain stage (by connecting each lug1 to ground instead of the output) and this is called the James tonestack: It is effectively a "passive Baxandall" tonestack. This retains some of the benefits (e.g. there is not much interaction between the two controls like you get with the standard Fender/Marshall/Vox amp tonestack) but it cannot actually boost any frequency above the level that existed at the input to the tonestack.
TLDR: This is a baxandall tonestack: Frequency-specific pots can both cut and boost selected frequencies using negative feedback
seeing as i build them.
Fallout Cloud.pdf
