C30 "Presence" and resonance mods R56 & R57

[tubeswell]

Hi there

I am confused about replacing R56 and R57 with variable resistors in relation to cable shielding.

Which side of the R56 and R57 'replacements' should be connected to the shield? R56 and R57 are not grounded - so does it matter (or should they EACH be shielded at their number "1" end OR their number "2" end, or can one of them be shielded at one end and the other at the 'other' end?) ?

Also I note that for the "presence" mod, the Blue Guitar Mod recommends a 100k resistor between the 250k pot and the connection to C36/C53/C54. If I haven't got a 100k resistor, will a 91K or a 110K suffice? - and if so which would give an 'optimum' outcome in terms of presence "range"?

Lastly, in replacing R56 for a 'resonance' control, I note that the stock resistor is 220k. If I use a 250k pot, will I have 'much to play with' in terms of sound difference, or should I use a 500k pot? If I use the latter, should I have a resistor in series with the outside lug, and if so what value would be suitable?

So many questions?

[Enzo]

Neither end is grounded in the circuit, so neither end is connected to the shield. My Belden 8451 is a twisted pair with shield around - the same idea as mic cable. Ground the shield so the wires within do not pick up or radiate unwanted signal.

I referred you to the 5150 series schematics for a reason, you will find different value pots than the resistors they replace. Isn't it the case that only one of the resistors becomes a pot? The one with the cap in parallel?

100k/91k/110k? I never even worry about a 10% variation in part values. I surely doubt you'd hear any difference. When Leo Fender first made amps, the resistors were of 20% tolerance, and the filter caps had ratings of -20/+80%. DOn't sweat the small stuff.

Shielding is not grounding. The shield itself is usually grounded, but it is not connected to the signal lead inside. If you are running wires from both ends of a resistor to something elsewhere, then shield the wires, yes, but there is no shielding one end but not the other. Both wires are to be shielded - either both within one shield, or with a pair of shielded wires.

[tubeswell]

Ground the shield so the wires within do not pick up or radiate unwanted signal. Both wires are to be shielded - either both within one shield, or with a pair of shielded wires.

Thanks Enzo

So is grounding the shield to the back of the pot appropiate? (The pot is in an aluminimum case that will be grounded to the main chassis). What about at the PC Board end - where should I ground the shield there? (Wire to the chassis?)

[Enzo]

Pick a ground point that is convenient and accessible inside teh amp. The box needs to be grounded, but don't rely on that for the signal path or shielding.

[daz]

Be sure to report back...

....on the resonance control. i'd really like to hear how well it does or doesn't work before i do it. Been planning to do that since enzo mentioned it, but i was going to wait till i have the new OT installed so i can evaluate it with nothing else having been changed.

[tubeswell]

....on the resonance control. i'd really like to hear how well it does or doesn't work before i do it. Been planning to do that since enzo mentioned it, but i was going to wait till i have the new OT installed so i can evaluate it with nothing else having been changed.

Yep will do, but I got to get few more 250kA pots, I had two but found out that one was a dud, after I had rigged up the project box (putting the MV, Pseudo presence and resonance controls in one box)- I should have tested the pots before I put them in! (let that be a lesson to me) - the arrangement I've got with single shielded cables looks a bit messy in hindsight, so I'll get some twin-shielded cable at the same time and re-wire the box to make it a bit tidier - I'll post a pic of it when I'm done.

BTW I have a new custom-made 12" AlNiCo magnet speaker arriving next week from Axent Audio (In Auckland), I think I'll delay installing these controls till I've had a play with the speaker

[tubeswell]

MV, Presence, resonance box ready to go in...

Run out of zing tonight, but 4 pics showing installation:

1) Box w/out vero board showing backing spacers for board

2) Detail of board backing spacer beside vero board

3) Vero Board in Box pre-wiring

4) Finished wired-up box, MV at top of pick, White wire at rear is Ground (to Amp Chassis), The three twin shielded cables to various bits of the amp PC board

When I get it all sorted in the amp, I'll post another pic

I could only get a linear taper '250K' pot for the resonance control, so I'm keen to see how that stacks up...

[Enzo]

Here's hoping it works right off. If it howls, the high signal levels of the speaker circuit, of which the resonance/presence are part, are coupling into the MV. A little barrier across the box making it into two chamers ought to cure that. Not suggesting you will have any such problem, just in case you do.

[tubeswell]

Thanks for the tip Enzo, but I'm not sure what you mean by "coupling" into the MV?, how might that happen?

Also, the 250k Linear taper pot reads from 0k to 228k - basically lower than teh range of the stock resistor - will that be enough range for the resonance? (i.e. what should I expect to notice with that range - more sparkle or more mellowness?), or should I add (say upto a 100K) resistor in series with the pot (to balance it more around the existing 220k resistor setting)? - (The presence pot measures between 100K and 344K to give a higher range that the stock resistor)

.Furthermore, the MV pot measures from 100k to 602K - is that enough or did I do something wrong - shouldn't it be measuring 100k to 1,100k? - it has dual wipers and I connected them in parallel, like the guy in the shop said, duh....)

[tubeswell]

I should add that I have taken the shield from the LH side of the MV pot back through a separate circuit to the amp's PC board connection (as shown on the Aloha mod pictures), and I do not belive it is grounded to the same circuit as the shields from the Presence and Resonance pots, (although all the pots are bolted to the little aluminum box). Will that prevent 'coupling', or is coupling a microphonic phenomenon?

[tubeswell]

Here's hoping it works right off. If it howls, the high signal levels of the speaker circuit, of which the resonance/presence are part, are coupling into the MV. A little barrier across the box making it into two chamers ought to cure that. Not suggesting you will have any such problem, just in case you do.

Well Enzo, I took your advice with the usual precautionary approach and i have re-worked the box. TA-DAHHH!

Verion # 2 with vero board cut in half and box divided into two chambers (one for MV and t'other for Presence & resonance) separated by a piece of slightly oversized Polystyrene expanded foam with aluminium foil wrapped round it a few times and squeezed in there to make it nice'n'tight - now that's got to put paid to any risk of unwanted coupling from the box. I took the vero board out of the MV bit to make the leads to the pot nice'n'short to try to minimse unshielded cable in there. The cable is secure 'nough on the pot lugs IMHO. Coming up next episode - the installation!

[tubeswell]

I should add that I have taken the shield from the LH side of the MV pot back through a separate circuit to the amp's PC board connection (as shown on the Aloha mod pictures), and I do not belive it is grounded to the same circuit as the shields from the Presence and Resonance pots, (although all the pots are bolted to the little aluminum box). Will that prevent 'coupling', or is coupling a microphonic phenomenon?

Sorry Steve - That should be Ahola (not Aloha - how come that happened? - must be something wrong with the BB)

[tubeswell]

Well the mod's al finished and installed - works fine - gotta try out the resonance and presence/feedback loop cut a bit more tho'. (Might end up changing the resistor values - which won't be too hard. cos its all in the little box.

[tubeswell]

Installation detail

heres some little pics of the installation detail - I have to try out the presence and resonance a bit for a while. The 'presence/feedback-loop-cut' is okay, the resonance doesn't seem to make much diff with these values. I can't get the Tone Stack Calculator to work on my gay Mac.

[tubeswell]

One more tweek (Resonance control mod)

Well I have since replaced the 250kN pot I had in series with the 110k resistor (which I initially installed as a substitute for R56) with a 500kA pot (& no resistor in series). Now it works like a proper resonance control. With the pot turned 'off' the signal bypasses C37 (the little .0047 cap that's in parallel with R56 in the original circuit), and with the pot turned up full, a good deal of the signal virtually goes thru that little cap. So you actually get a noticeable difference in "resonance". So I'm keeping the 500kA pot there for now. Good tip about putting in that project box Enzo - its made experimenting alot easier.