Bluesmaster PI mod issue

[heisthl]

Or to say it another way - Don't ground the presence pot anywhere, the ground is for the cap to make.

[heisthl]

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

I agree on the grounding but I've had 2 builds lately that had a small hum that went away when I moved the PI tail ground (and Presence Ground) off of the I/O jacks and put them on the Front ground bus. It makes no sense as the B+3 PS cap was grounded at the transformer (main star) lug along with B+1 and 2.

[chris_sanford]

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

I agree on the grounding but I've had 2 builds lately that had a small hum that went away when I moved the PI tail ground (and Presence Ground) off of the I/O jacks and put them on the Front ground bus. It makes no sense as the B+3 PS cap was grounded at the transformer (main star) lug along with B+1 and 2.

I ground both my screen B+ feed cap and my PI B+ feed cap to a point on the chassis that is between the V1 and V2 output tubes, while the ground for the caps that feed V1 and V2 preamp tubes is tied to the preamp ground 'bus' which (iirc) is how Dumble does it. If you think in terms of current return paths through the chassis, you want to make sure there there are no two paths which cross, as that will tend to generate hum.

chris

[heisthl]

I ground like you do (I use a torch to solder weld a ground lug at or near the transformer leg next to the tubes) -as I said it makes no sense.

[Robert]

I used the chop stick technique today and discovered that my OT Secondary wires were touching my effects return wire. Moving them apart stopped the annoying distortion effect.

FWIW I've grounded my presence cap to the front ground bus. I may try moving it while I'm in there next.

I'll thank Bob-I for this problem as I had read a post he made recently about the interaction between two wires having a positive effect. While I was performing my PI mods I moved the plate and cathode wires around and ended up moving things closer to the OT wires. I can also credit him for the solution.

[mlp-mx6]

Just an FYI - and I would like your input on it as well.

I just completed a Komet/Trainwreck blend where I did not use the chassis for ground at all, except for one connection. I used a ground buss and EVERY ground connection went there - power caps, speakers, circuit grounds, input, everything. The connection to the chassis was at the input jack end. It is a very quiet amp (for a Wreck-style circuit). It IS quieter than the previous Wreck-style that I built.

The power inlet is also directly connected to ground.

[chris_sanford]

I ground like you do (I use a torch to solder weld a ground lug at or near the transformer leg next to the tubes) -as I said it makes no sense.

Hmmm. But don't you have your main filter bank connected to the same point? Maybe I misunderstood you.

chris

[heisthl]

I generally use 3 grounds in an amp. 1 point for AC input ground wire and sometimes heater ref ground another for every thing B+3 and below (including output tubes and rear panel jacks) and the last is the pot bus with b+4 and b+5. Like I said it makes no sense why moving the PI and presence would lessen the hum, and I've only seen it twice. The loudest hum one that got fixed with this is covered in the files section under "total butchery".

[chris_sanford]

Like I said it makes no sense why moving the PI and presence would lessen the hum

Well, I'm not sure I'm following your ground scheme exactly, but if you have any current return paths that cross, it *would* makes sense to move or separate a connection (guess I would need to see a map of your grounds to be sure). Also, if you aren't doing so already, I would suggest that you separate the main filter bank/PT secondary ground from the other ground connections, as that one can induce noise.

chris

[chris_sanford]

Just an FYI - and I would like your input on it as well.

I just completed a Komet/Trainwreck blend where I did not use the chassis for ground at all, except for one connection. I used a ground buss and EVERY ground connection went there - power caps, speakers, circuit grounds, input, everything. The connection to the chassis was at the input jack end. It is a very quiet amp (for a Wreck-style circuit). It IS quieter than the previous Wreck-style that I built.

The power inlet is also directly connected to ground.

I don't know if you were asking for my opinion or not, but in case you were, I guess I'll jump in.

Buss grounding schemes (as opposed to star-ground types), can be quiet too, but like anything, the devil is in the details. If you haven't already, go read the tech info piece on Randy Aiken's site called 'Star Grounding'. He does a much better job explaining things than I could ever do.

FWIW, Dumbles employ a modified star-grounding scheme (sometimes referred to as 'ground follows signal') that results in a very quiet amp. If you copy that scheme or do what RA recommends (they are very similar), you'll be fine.

chris

[novosibir]

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.

Since it's a variable resistor - and not a voltage divider - it doesn't matter, from which of both lugs the pres cap goes to ground.

Larry