I'm currently planning to convert my AX84 SEL amp into some kind of Mesa Mk topology and would like to add presence and resonance circuits.
Also I think of placing the MV outside the feedback loop because dimmed it would decrease the available feedback.
A bootstrapped MV comes to my mind as the tail R is rather small so I think the volume can be turned almost of at minimum setting which sometimes seems to be a problem with that kind of MV.
What is unclear to me is the feedback circuit. The voltage divider can be scaled down according to Merlin B..
So the divider consists of R4 (100R) and R10(4k7). The values currently chosen are from DV Nator's website and his projects but I'm not sure if the scaling is right in the schematic.
I have to add a resonance pot (normally 1Meg // 4n7) and a presence pot (normally 5k //25k + 100n)
Shouldn't be all resistors divided and all caps multiplied by the same factor?
Let's say I stay with R4 100R the factor would be 50.
So presence should be 500R pot with 5u cap? For a 680n presence cap it's 33u.
Resonance therefore had to be 20k (25k) pot with 235n (220n) cap?
How large would be the feedback R (R10) in a SE PA? 4k7 seems rather large to me, this would equal 235k normally which is way to little feedback IMO.
I guess presence and resonance would'd have much of an effect..
What do you guys think?
Thanks a lot!
Resonance and Presence KT88 SE PA?
Moderators: pompeiisneaks, Colossal
Resonance and Presence KT88 SE PA?
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Re: Resonance and Presence KT88 SE PA?
I missed the 220k grid leak resistor on the power tube.
Updated the schematic with calculated values.
Now I'm unsure about the bootstrapped MV. Does the feedback loop suffer from the shared MV connection?
Thanks a lot!
Updated the schematic with calculated values.
Now I'm unsure about the bootstrapped MV. Does the feedback loop suffer from the shared MV connection?
Thanks a lot!
You do not have the required permissions to view the files attached to this post.
Re: Resonance and Presence KT88 SE PA?
No one's got experience with such a feedback circuit?
The Fender Princeton has a cathodyne PI and the previous stage has got feedback applied very similar. It's voltage divider consists of 2k7 feedback R and 47R Tail R which equals in 3 times less feedback than my circuit.
I guess I should aim for less feedback and consider a 2k7 feedback R as well?
The resonance circuit of course is missing a link between C5 and R10!
Still not sure about the bootstrapped MV.
The Fender Princeton has a cathodyne PI and the previous stage has got feedback applied very similar. It's voltage divider consists of 2k7 feedback R and 47R Tail R which equals in 3 times less feedback than my circuit.
I guess I should aim for less feedback and consider a 2k7 feedback R as well?
The resonance circuit of course is missing a link between C5 and R10!
Still not sure about the bootstrapped MV.