ODS presence issues

[txbluesboy]

I have a 50 watt ODS, built using pretty much the 101 HRM schematic. My presence pot doesn't seem to have much effect. I'm using a 7.2 K feedback resistor to a 2K pot and 1uf cap paralleled with a 470 ohm resistor to ground. Should I use a 1k resistor in place of the 470 ohm?

[heisthl]

which tap are you using to feed the 7.2 k resistor? Bear in mind this is just a simple voltage divider and the 4k7/390 values were what HAD liked for 100watt Fender type iron and using the 4 ohm tap (somebody correct me if I'm wrong).
So anyway if you want more high end kick out of your presence control using the 1uF cap, just adjust the ratio of the 2 resistor values to get more voltage at their junction. 1k instead of the 390 is a good T&E place to start, easier yet would be to cliplead some higher value resistor or pot across the 7.2k to play with bringing it's value down slightly until you find the point that you like. Make sure you're only using 2 legs of the presence pot with the wiper feeding the 1uF to ground, no connection on pin1 and the junction of the divider resistors on P3.

[talbany]

Blues boy

The 7.2 is a bit high for the feedback resistor.. I would start at spec..4.7k..on the 4 ohm tap..Resistor substition boxes work great for this type of tweaking..Also would try to stick with the 390 to ground or as close to that as possible.. I have messed around with the presence control on these amps to try and squeeze more range out of them.. I always came back to the stock Dumble...As I fiddle around with Dumble's designs most of the time I come right back D-is the man...


Tony VVT Amps

[txbluesboy]

I'm on the 4 ohm tap. Ill try your suggestion about a pot across the 7.2K. Thanks!

[Bob-I]

I have a 50 watt ODS, built using pretty much the 101 HRM schematic. My presence pot doesn't seem to have much effect. I'm using a 7.2 K feedback resistor to a 2K pot and 1uf cap paralleled with a 470 ohm resistor to ground. Should I use a 1k resistor in place of the 470 ohm?

Just to make sure you're not wired wrong. The pot is wired as a variable resistor, top to the connection of the NFB, 470R and .1uF to the PI, wiper to the 1uF to ground. There's no connection from the pot to ground, a common mistake.

If you're wired this way, then the ratio of NFB to the 470R shunt resistor is wrong. NFB is a factor of the OT tap used and the shunt resistor. Typically I start with 4.7K/390 and adjust the 4.7K from there.

All that said, the more important factor is the NFB. I seldom use the presence at all, it stays at 0 99% of the time.

[txbluesboy]

OK, I replaced the 7.2K resistor with a 10K trimpot in series with a 2.4K resistor. With a l setting of about 5K, the presence pot does make a difference in sound from lowest to highest setting, but its a very subtle change. If I adjust the trimpot down to where its just seeing the 2.4K resistor, the change is negligable/subtle from 1 to about 7 on the presence dial, then changes a lot from 8 to 10. It is a linear pot. I didn't change out the 470 ohm resistor to 1K, would this spread out the change at the pot?

[Funkalicousgroove]

the same ratio of nfb using the 7K2 resistor would have a 600 ohm tail. If you want more range try a bigger pot, and perhaps a smaller cap(.68-.1)

[txbluesboy]

I noticed on the latest ODS 101 HRM layout a 2meg pot is called out. Is this a misprint or is this a usable alternative. That's a long way from 2K! Also its audio taper

[Bob-I]

I noticed on the latest ODS 101 HRM layout a 2meg pot is called out. Is this a misprint or is this a usable alternative. That's a long way from 2K! Also its audio taper

Misprint for sure. It should be 2K linear. Your response of 0-7 being subtle and 7-10 more effective is normal.

[txbluesboy]

Thanks, I thought that was probably a mistake. I'm going to leave the NFB trimpot in to play with for a while.