presence controls
Moderators: pompeiisneaks, Colossal
presence controls
Hi, I have a Kt66 fixed bias push pull amp about 40 watts that has a cathode follower tone stack going to a LTP phase inverter. What is a suitable design presence control for this. Thanks.
Re: presence controls
Hi Andrew
Presence controls are typically active tone controls that are run off a global feedback loop, from the OT secondary to a cathode in one of the pre-amp tubes (like a Driver in a cathodyne PI, or) in this your case, at the bottom of a tail resistor in the LTP.
The presence control works as a lo-pass filter in the feedback loop, by preventing higher frequencies from being fed back to the pre-amp, thus allowing higher frequencies to be accentuated (if you follow my mangled explanation).
So take a look at various GFB loops where presence controls are included in LTPs (like various fenders) and you'll get an idea about what to try. Bear in mind that the presence control in a 5F6A bassman is designed having DC on the pot, so it is typically scratchy when turned, whereas the later Bassmans changed this design.
Presence controls are typically active tone controls that are run off a global feedback loop, from the OT secondary to a cathode in one of the pre-amp tubes (like a Driver in a cathodyne PI, or) in this your case, at the bottom of a tail resistor in the LTP.
The presence control works as a lo-pass filter in the feedback loop, by preventing higher frequencies from being fed back to the pre-amp, thus allowing higher frequencies to be accentuated (if you follow my mangled explanation).
So take a look at various GFB loops where presence controls are included in LTPs (like various fenders) and you'll get an idea about what to try. Bear in mind that the presence control in a 5F6A bassman is designed having DC on the pot, so it is typically scratchy when turned, whereas the later Bassmans changed this design.
Re: presence controls
Thanks tubeswell, I'll probably use an Express presence control but I see a Hiwatt design from mhuss that has a presence control on
the cathode of a triode (I have an unused triode)
http://hiwatt.org/Schematics/DR103sn727pre.jpg
I would like to try it if possible. I'm open to ideas. Thanks
the cathode of a triode (I have an unused triode)
http://hiwatt.org/Schematics/DR103sn727pre.jpg
I would like to try it if possible. I'm open to ideas. Thanks
Re: presence controls
I wired up an Express presence circuit and I could hear no change at all. Yikes.
Re: presence controls
It all depends on the voltage strength at your OT secondary and the size of the NFB resistor. The stronger the secondary voltage is, the bigger the NFB resistor needs to be to avoid nullifying the signal too much, and vice versa. Then you have to pick the Pot/Cap RC combination for the presence control that lifts the 'right' amount of hi-freqs from the feedback loop.andrew wrote:I wired up an Express presence circuit and I could hear no change at all. Yikes.
Dropping the NFB circuit from one amp into another isn't necessarily going to achieve much (as you can see)
Re: presence controls
It sounds good enough without a presence circuit. I'll leave it alone. Thanks.