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PostPosted: Thu Jul 07, 2016 12:57 pm 
Mark I

Joined: Fri Oct 16, 2009 3:25 am
Posts: 33
Location: Austin, TX
I'll preface this by saying that I have a 1X12 Stiletto Ace combo, with stock Celestion V30 not the best speaker for this amp. I will probably switch out for a Scumback speaker. After contacting them they recommended an M75-LD as well as a J75-LD which Jim said would be a better choice of tight bass and balance. A speaker change would help this amp out immensely but finding the right one can get expensive so some guys opt for modding the amp circuit. The components needed for the mods can be had for under $20.

****If you don't feel comfortable working inside an amplifier then please do not attempt these mods. Always discharge your capacitors and check your amp for any remaining voltage before working inside an amp.****

Here are a few mods to help tame the 'brightness' and 'ice pick' that some of the Stilettos have.

Bias Mod: R83 (39K) is the resistor that will be removed when installing a bias mod kit. Make sure you measure your plate voltage of power tube (pin 3)
and use bias probe to measure the the bias voltage in milliamps. I bought a kit from Grainger with a 25K pot and 15K resistor but even at the lowest setting I was unable to get the tube biased up to 30ma, so based on your starting bias (without kit installed) if it's too 'cold' you may opt for 25K pot and 10K resistor. Mine was initially a 17mA way cold so Grainger sent me replacement resistors. The schematic states that voltage measurements are taken on Channel 1 on Crunch in Bold Power with diodes but be aware that in the 2 clean modes the bias voltage(mA) will go up significantly.

Onto the other mods for Channel 2.

1. Change R38 from the 39K value to 56K. This is the "slope" resistor and sets the eq slope range. I then wired that to the center lugs of a 3way double pole switch and wired a 330K resistor on one side and an 82K to the other side. So you get roughly 47K/56K/33K values. It affects the perceived thickness of the mids and tightness in feel.

2. You can also remove C6 (180) and C7 (750) caps. These are a 'bright' cap system tied to the gain pot most active at lower settings <5, at max gain they are shorted out and have little to no effect. Removing will thicken the tone but you loose a tiny bit of gain and cut. You can also remove both caps and add a .0047 cap that keeps the gain but adds a little more mids but can be a bit much.

3. At C3 (500pf) and R5 (475K) you have the first treble peaking circuit. You can piggyback another 470K resistor on the R5 which will bring the value down to about 236K. Basically adding a bit of gain to just the frequencies lower than the cap and makes the tone balance with the high end with the inherent upper mid spike of the amp/speaker combo. Keeps it sounding aggressive without being brittle.
You can also play with just the capacitor and increase it to a .0022 which will feed a bit more mids a bit punchier tone.

Here are some other mods I have have performed on the amp but since removed.

1. On the Input Jack I wired a 220k resistor from Input wire to ground. This helps tame the treble the amp is seeing from the guitar and will beef up the tone.

2. Next I wired a 270p cap to the outer lugs of of the Treble Pot. 330p is recommended for the JCM. This cuts the ratty highs. You can now go past 9 o'clock on the treble knob woooooooooeeeee!!!

3. Change R44 from 10M to 4.7M. No more piercing highs. The Presence knob is now usable past 9 o'clock. I think any value between 4.7M and 10M will work.

When checking the affect of each mod make sure you get the power amp working, loop bypassed and Master to about 4 (past 10 o'clock) so you can actually hear what the amp sounds like and not just the preamp section.

If anyone needs the schematic for the amp feel free to message me and I can email it to you.

Isaac Berumen
Berumen Guitars

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