hiwatt dr103 build presence control sweep

[togstn666]

I recently build a hiwatt dr103 clone that works and sounds awesome. I ended up using the layout from ceriatone and referenced the schematics from hiwatt.org, tubetown and one i found here to make some changes (No external bias jacks, HT wires on the opposite side, HT fuse between bridge and ground)

http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/10/HeyWhat103.jpg

Here are some pics from the build

https://drive.google.com/file/d/1mhQqoGfQLSchknBGQBgAV-_EVi0gk-dr/view?usp=sharing
https://drive.google.com/file/d/1a0t8QZ-TAbFw7f23JpND9dOh-AJVqAjv/view?usp=sharing
https://drive.google.com/file/d/1jpJ5eQD7rmZIraRUzQBweokWsMg5uhV1/view?usp=sharing
https://drive.google.com/file/d/157_fK9r3aHYUBKKV5gAimI3kbhPypbQP/view?usp=sharing
https://drive.google.com/file/d/17p0ozXstfi7R33axabB-nGRBJvU7x3bd/view?usp=sharing
https://drive.google.com/file/d/1cRSzDArEXAkjjEQFqiJ7b2wvpXma5_LN/view?usp=sharing
https://drive.google.com/file/d/1qJF74Q7AlVpuU8o7dwJUqAVGc92PnHhH/view?usp=sharing
https://drive.google.com/file/d/1DmcOGRtRRixhOJztUUfSOTK6mFL_-vfx/view?usp=sharing
https://drive.google.com/file/d/1MPQ5pKz_qJg9_rPcoCszoEJNzcw_ikmi/view?usp=sharing

The problem is that the sweep of the presence pot is not evenly distributed. seems like all the changes are happening on the last quarter of the pot. no noticeable difference in the first 3/4 of the pot.

here is some more info i found online about the Hiwatt presence circuit

"A presence effect works by cutting some of the highs out of that negative feedback signal. Removing highs from the negative feedback signal means less highs are cancelled. The presence circuit is dumping the highs from the NFB to ground through a capacitor (100n). It can only do this when it has a low resistance path to ground. That path to ground is the 100 ohm resistor coming off the wiper of the presence pot. With the pot turned down that pathway is blocked off to the capacitor since the full 100k resistance of the pot is between them, so no presence effect happens. Rotating the pot to the right gradually reduces this resistance so the capacitor can start dumpimg more and more highs to ground. But the Hiwatt presence control does double duty. Connected to the other end of the pot is the exact same kind of treble dumping circuit, but it's connected to the last stage of the preamp (1n capacitor). Rotating the pot all the way to the left cuts highs from the preamp signal, and rotating it to the right cuts highs from the negative feedback signal. That means at one end of the control treble is cut and at the other treble is boosted."

I have looked at a couple  other schematics and noticed 2 variations:

lower NFB filter cap from 100n to 47n

https://hiwatt.org/Schematics/DR_Pre4Input_v1a.gif

increased wiper resistance to ground from 100R to 470R

https://hiwatt.org/Schematics/DR_Pre4Input405.gif

My question is if any of these changes will make the sweep more evenly distributed across the pot without affecting the way the NFB works. I dont want to go the trial and error way, I prefer to have a better understanding of what I am doing.

[sluckey]

The problem is that the sweep of the presence pot is not evenly distributed. seems like all the changes are happening on the last quarter of the pot. no noticeable difference in the first 3/4 of the pot.

Many times the fix is to use a pot with a different taper. Having a schematic rather than a layout would be extremely helpful to analyze your presence circuit.

[72Blazer]

That is amazing work-very nice.  I'll bet Pete Townsend would jam on this!

Vr
J.

[togstn666]

The problem is that the sweep of the presence pot is not evenly distributed. seems like all the changes are happening on the last quarter of the pot. no noticeable difference in the first 3/4 of the pot.

Many times the fix is to use a pot with a different taper. Having a schematic rather than a layout would be extremely helpful to analyze your presence circuit.

Yes you are right . Here is the one I used for the preamp, it contains the NFB loop

https://hiwatt.org/Schematics/DR_Pre4Input_v2.gif

My first thought was changing the pot too. Replace it with a 25k like marshall. But then i saw the variations above and got curious. Do they have an effect on the sweep? Because in every schematic I looked there is a 100K Linear Pot. I am also using a 12ax7 instead of 12at7 for the PI. I ordered a couple of 12at7s to see if there is any difference.

I haven't played a real hiwatt yet so maybe this is normal operation? But it seems odd the man ovelooked this

[PRR]

It wants a Reverse Audio. At least. Maybe an extreme custom. Or maybe the users just gotta deal with it.

The value is a compromise, so varying it will just disfavor one end or the other.

[pdf64]

It’s effectively 2 tone circuits combined, the input passive LPF would work best with a log taper, the global NFB LPF would work best with an antilog taper.
Hence I suggest to try dual track pot with 47k log and 47k antilog tracks, and wire them in series.

[Vonskee]

The problem is that the sweep of the presence pot is not evenly distributed. seems like all the changes are happening on the last quarter of the pot. no noticeable difference in the first 3/4 of the pot.

Many times the fix is to use a pot with a different taper. Having a schematic rather than a layout would be extremely helpful to analyze your presence circuit.

Yes you are right . Here is the one I used for the preamp, it contains the NFB loop

https://hiwatt.org/Schematics/DR_Pre4Input_v2.gif

My first thought was changing the pot too. Replace it with a 25k like marshall. But then i saw the variations above and got curious. Do they have an effect on the sweep? Because in every schematic I looked there is a 100K Linear Pot. I am also using a 12ax7 instead of 12at7 for the PI. I ordered a couple of 12at7s to see if there is any difference.

I haven't played a real hiwatt yet so maybe this is normal operation? But it seems odd the man ovelooked this

Hi, did you found what was the problem? I have the same exact in my copy and I'm curious if I need to change pot for smaller or it is something different in wiring

[frank57]

In the Hiwatt custom 20 tube I have, they used a B100k pot.
The presence is always"on".
 One way darker, other way brighter.
Usually put it in the middle.