Bluesmaster PI mod issue

[Robert]

I absolutely love the change in tone and feel the bluesmaster PI mod made in my nonHRM 100watt clone. I changed the .1 caps to .022, altered the PI resistors and added the 100K NFB resistor, didn't change the presence pot though. However, I have a problem I need to correct. After hitting a note, within a short time, <.2 secs, I hear a low but audible fizzy, buzzing that swells a little and then dies away. It can be worse on some notes and in the higher, 12th fret treble region. It is also worse when hitting two notes at the same time. Adjusting the PI trimmer didn't seem to affect it. Increasing the preamp volume increases the volume of the buzzing. Playing fast notes it doesn't have time to develop.

Any ideas regarding what I am describing and or troubleshooting measures to fix it. It was not present at all before I made the changes. I have rechecked all values and resoldered all connections in the mods.

Thanks so much for sharing this mod with everyone!!!:)
Robert

[dogears]

You have to change the whole thing IMO. Did you change the feedback tail? Do the whole shabang....

[Robert]

Your up to late Except the presence pot I think I did change the whole shabang. Whats the "feedback tail"?

[heisthl]

Your up to late Except the presence pot I think I did change the whole shabang. Whats the "feedback tail"?

Probably the .1 cap, make sure you tie pins 2 and 3 together on the presence pot and put the .1 from there to ground. Pin 1 of the pot goes to the junction of the 9.1k and the 4.7k. It's impossible to make your 2k pot work because no combination of series resistors give you enough tuning range.

[mat]

Your up to late Except the presence pot I think I did change the whole shabang. Whats the "feedback tail"?

Probably the .1 cap, make sure you tie pins 2 and 3 together on the presence pot and put the .1 from there to ground. Pin 1 of the pot goes to the junction of the 9.1k and the 4.7k. It's impossible to make your 2k pot work because no combination of series resistors give you enough tuning range.

I have pin 1&2 tied together and pin 3 has the .1 cap to ground. Would there be any difference between how it is on mine and the way You discribed heisthl ? The layout mine build is based and the 101HRM has both different presence pot wiring.

[heisthl]

Assuming that the "pin 3" on a pot is the pin that normally goes to ground, like on a volume pot for example. Your way would work backwards and give you max "treble" all the way down instead of all the way up. That's fine, the important thing is not to do it like the 101 schematic. You don't want your presence pot to be a parallel DC path changing the static resistance value of the 4.7k tail.

[mat]

Assuming that the "pin 3" on a pot is the pin that normally goes to ground, like on a volume pot for example. Your way would work backwards and give you max "treble" all the way down instead of all the way up. That's fine, the important thing is not to do it like the 101 schematic. You don't want your presence pot to be a parallel DC path changing the static resistance value of the 4.7k tail.

Thanks heisthl for clearing that out. My presence pot works in a right way, I'll check the pins again next time I open the amp.

[Robert]

Probably the .1 cap, make sure you tie pins 2 and 3 together on the presence pot and put the .1 from there to ground. Pin 1 of the pot goes to the junction of the 9.1k and the 4.7k.

I do have my presence pot wired like that, however I was using a 1.0 uf cap instead of 0.1uF. I will give that a try shortly.

For future reference, anyone have a name for the symptom I described and the electrical reason for it?

Thank you.

Robert

[Edit] Tried the .1uF cap but it didn't resolve the distortion

[yeahyeah]

That's fine, the important thing is not to do it like the 101 schematic. You don't want your presence pot to be a parallel DC path changing the static resistance value of the 4.7k tail.

Are you saying that the presence pot is NOT wired in parallel with the 4.7k?

So is it supposed to be wired in series???

you have a PM.

[dogears]

What are you talking about? The Bluesmaster presence is wired exactly like the 101 schematic, just with the enhanced values. Maybe you did not install it correctly?

the important thing is not to do it like the 101 schematic. You don't want your presence pot to be a parallel DC path changing the static resistance value of the 4.7k tail.

[heisthl]

Maybe I'm looking at the Dumble "gut shot" pictures wrong but I believe the real ones are not wired like the 101 schematic. The 101 style does not have any pot legs tied together and you can clearly see Dumble tied pins 2 and 3 together. I have attached excerpts from the 101 and the alternate presence schematic.

[dogears]

Actually, Dumble usually did not wire the presence pot lugs together. No need to as it is a variable resistor. Here is a pic of a late model HRM presence pot. I have a lot more too.... Btw, it is exactly the same result in both of the schematics you posted. Tied or not tied.....

[img:933:756]http://www.scottlernermusic.com/ftp/PresencePot.jpg[/img]

[mat]

Is the .1 cap tied to the ground from end of the white or yellow wire - or does it matter ?. I suppose the cap is on the board.

[chris_sanford]

What are you talking about? The Bluesmaster presence is wired exactly like the 101 schematic, just with the enhanced values. Maybe you did not install it correctly?

the important thing is not to do it like the 101 schematic. You don't want your presence pot to be a parallel DC path changing the static resistance value of the 4.7k tail.

Heisthl is right re: the 101 schem... the presence pot should be wired in *series* with the cap. Also, the ground should connect to the same place where the pi input is grounded (which is to the loop return jack), not to the ground bus for the tone controls etc.

chris

[dogears]

Of course, I just noticed the issue of the tail resistor connecting to the pot and not ground with the cap. So I see your point..... Imagine alt schematic and just don't tie the lugs.... Or tie them... Doesn't matter. Or use the 101 layout and take the tail to the cap ground.....

Fwiw, all my amps have it correct and the tail resistor and Presence cap tie together at PI ground.

What are you talking about? The Bluesmaster presence is wired exactly like the 101 schematic, just with the enhanced values. Maybe you did not install it correctly?

the important thing is not to do it like the 101 schematic. You don't want your presence pot to be a parallel DC path changing the static resistance value of the 4.7k tail.

Heisthl is right re: the 101 schem... the presence pot should be wired in *series* with the cap. Also, the ground should connect to the same place where the pi input is grounded (which is to the loop return jack), not to the ground bus for the tone controls etc.

chris