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PostPosted: Wed Jul 12, 2017 10:45 pm 
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Mark I

Joined: Fri Oct 16, 2009 3:25 am
Posts: 32
Location: Austin, TX
****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 Stilettos have.

Most of the 'icepick' high end that some owners complain about is in the Crunch/Tite Gain/Fluid Drive. These modes are tied in together through gain stages, even Crunch in Channel 1 is tied to them. Looking at the schematic is like looking as a modded cascaded Marshall amp...Superlead, JTM, JCM 800. The Marshall DNA is definitely in this Mesa design. What is surprising is that there are a number of Treble Peaking components in the circuit that are meant to retain clarity but can result in a peaky high end that is almost impossible to dial out as it is continually passed from gain stage to gain stage. Here are a couple mods that can alleviate this and maybe even thicken up the tone.

Onto the mods for Channel 2 of Stage II for ACE, Deuce, Trident

1. You can mod the mid slope resistor, this can thicken up the tone and add a bit more gain to what's going to power section. A higher number will give a tighter feel. Early Marshalls had a 56K or a 33K resistor so that's a cool place to start. Change R38 from the stock 39K value to a 56K resistor. This is the "slope" resistor and sets the eq slope range. I then soldered two wires in parallel to that and connected them to the center lugs of a 3way double pole switch. I 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. There is voltage going to this component so be aware and be careful!

2. You can also remove C6 (180) and C7 (750) caps. These are a 'bright' cap system tied to the Gain pot of Channel 2 and are most active at lower settings of the control <8, at max gain they are shorted out and have little to no effect. Removing these will thicken the tone but you loose a tiny bit of gain and cut. You can also remove both caps and add a 470pf (471) cap that keeps some of the gain on the upper frequencies (500pf) or even (.005) were older Marshall values and helped to retain clarity at lower settings of the gain control. Try any value from 250pf-.005 (4700pf) and see what works best. Bogner XTC uses 470pf and a 4700pf to have two bright cap voices. The 4700pf cap will up the highs as well as mids and definitely give it a considerable gain increase.

On a side note, on Gain of Channel 1 you have two caps as well. a 120pf and a 500pf. Now the 120pf is always connected but the 500pf gets connected in and out via a relay and depending on the mode of Channel 1. This is why the two Crunch modes sound slightly different. Now, I find it weird that there are also two caps on Gain of Channel 2 but they are both connected to the pot, there is no relay that select one in or out. Not sure if the Stage 1 is like this. Why not just use one cap with a total value of both (180pf+750pf+930pf) or even a .001cap?

3. At C3 (500pf) and R5 (475K) you have the first treble peaking circuit. The capacitor is basically passing frequencies above that value through from the first stage and the resistor limits the frequencies below that to even out the tone. 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 below the capacitor and makes the tone a bit smoother/thicker and helps to balance with the high end with the inherent upper mid spike. You can also use a jumper wire across the R5 and bypass the entire treble peaking circuit. This adds gain to the mids and lower frequencies and it will affect all the gain modes, it may be too much low end so listen and play all modes before and after. You can also remove just the capacitor or play with just the capacitor value and increase it which will feed a bit more mids and give a bit punchier tone and more gain. I left this stock after experimenting with cap/resistor values and jumping was a bit too woofy with the added lows going through to the next stage.

4. At C14 (500pf) and R19 (475K) you have yet another treble peaking circuit. You can do the same mods as in above in step 3. I actually chose to use the jumper wire and bypass it and left the first treble peaking circuit C3/R5 unmodded. This was pretty much enough to kill the peakyness of the gain modes and by keeping the first treble peaking circuit I was able to retain the initial note definition and clarity, the ability to role down the volume on the guitar and clean up. Now it will add some perceived gain and a thicker feel so see if this is something you like. Tite Gain mode becomes thicker and is really cool for classic gained up Marshall tone.

5. A speaker change can also do wonders. I went from a Vintage 30 to a Warehouse ET65 and it really thickened up the tone. The problem was that even after a speaker change there is no way to dial out the icepick until doing some of the mods. The great thing about these mods is that the components are easily accessible and the mods are reversible if done cleanly. In some case you can use alligator clips to test components before soldering down.

Let me know if you have questions or need the schematic for Stage 2 Ace, Deuce, Trident.

Isaac Berumen
Berumen Guitars


Last edited by berumen77 on Thu Jul 13, 2017 10:52 am, edited 1 time in total.

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PostPosted: Thu Jul 13, 2017 8:20 am 
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Mark I

Joined: Fri Oct 16, 2009 3:25 am
Posts: 32
Location: Austin, TX
Older Marshall 4hole amps usually ran the Bright and Normal channels in parallel and guys would sometimes plug into one channel and then jump the channels via a cable plugged into remaining inputs which allowed them to blend in both channels' characteristics with the Normal and Bright Volume controls. This resulted in a bit more gain and a tone that was not as extreme in dullness or brightness that could be tailored a bit better. In regards to cascaded "one wire' mods to say Marshal Plexi's and other later models, the channels are run in series into one another and there is a significant increase in gain. Basically the foundation for higher gained amps and what you see in the Stiletto. Usually the Normal channel with .022 coupler is run into the Bright channel with the .0022 cap and the bright cap on the Volume (Gain) control. I've seen guys also swap the coupler caps around so .0022 is seen first stage and feeds into the next channel/stage with .022 which now has a bright cap on the Volume (Gain) pot. So it amounts to having a kind of treble boost circuit/stage into a Normal Channel that now has a bright cap on it's volume (Gain) pot. This will yield more gain on a Marshall like a regular 'one wire' mod but also a very fast clean attack like a Van Halen, Nuno type tone. These amps had no master volumes so it is basically like cranking you amp to max and using the Gain control as a volume/tone control.

Here's that take on a one wire mod.

https://www.youtube.com/watch?v=W_4pCaG4z8o

With the Stiletto though, you have other modes that are tied together so you must test and be aware of what affect any changes are going to have on the other Gain modes.


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PostPosted: Wed Jul 19, 2017 11:53 am 
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Mark III

Joined: Sat Jul 01, 2006 1:23 pm
Posts: 263
berumen77,
thanks for Posting these mods, I'm very interested in trying some of them.

I agree with you on the speaker choice, a Vintage 30 and Stiletto Stage II are a terrible match.

I have a question, if you don't mind.

If you've ever monitored the bias as you are switching channel modes, in the First channel Clean modes, the Bias is considerably higher. I wanted to put a Bias pot in place of the resistor, but I didn't want to have a situation to Redplate the power tubes if I set the bias for the Gain sounds, but then switched into a Mode that would take the bias above 70%.

Does that make sense?

Steve


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PostPosted: Fri Aug 18, 2017 2:38 pm 
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Bottle Rocket

Joined: Wed Jan 05, 2011 10:45 am
Posts: 19
So jumping r19 alone is a fairly simple way to impact / balance out the EQ?


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PostPosted: Sun Aug 20, 2017 7:08 pm 
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Mark I

Joined: Fri Oct 16, 2009 3:25 am
Posts: 32
Location: Austin, TX
kramerxxx wrote:
berumen77,
thanks for Posting these mods, I'm very interested in trying some of them.

I agree with you on the speaker choice, a Vintage 30 and Stiletto Stage II are a terrible match.

I have a question, if you don't mind.

If you've ever monitored the bias as you are switching channel modes, in the First channel Clean modes, the Bias is considerably higher. I wanted to put a Bias pot in place of the resistor, but I didn't want to have a situation to Redplate the power tubes if I set the bias for the Gain sounds, but then switched into a Mode that would take the bias above 70%.

Does that make sense?

Steve


Yes, I have monitored them while switching modes/channels. I bias the amp based on Channel 1 Crunch Mode as that is the mode that "All Voltages Measured" is specified in the schematic. I bias my amps at about 62-65% max dissipation not the 70, This is what Bogner does on amps like the Shiva (they go for a 29mA). The bias will drift as the amp warms up and is played. I found that biasing at this range it will drift right to slightly below the 70%. A lot of the newer production tubes can not take the high voltages and being biased hotter. With the drift, they can go even hotter and start to red plate.

Isaac Berumen


Last edited by berumen77 on Mon Aug 21, 2017 9:39 am, edited 1 time in total.

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PostPosted: Sun Aug 20, 2017 7:35 pm 
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Mark I

Joined: Fri Oct 16, 2009 3:25 am
Posts: 32
Location: Austin, TX
Guys, also make sure to check the switching JFETs, there are two J175's and a 2N6426 that applies voltage to the two located to the upper left of the V1 tube. One of the J175's (lower one) switches in an additional cathode resistor for the Tite Gain and Fluid Drive and the other switches in an additional cathode bypass capacitor for the Fluid Drive. The soldering spots for their lugs are extremely close together and it is very easy for the cap to become engaged for all gain modes by just having too much solder too close together. Basically giving all the high frequencies a gain boost for all gain modes. Apparently Mesa had some of these switching JFETS that can become 'leaky' as well. That means the components they engage may be partially in circuit. They have 3 lugs, one of the outer lugs connects to ground, the other outer connects to the component. The two outer lugs are always connected to each other until voltage is applied to the middle lug at which point the switch opens and the component is taken out of the path from ground.

I had issues with one recently, the one that switches in the cathode bypass cap. I clipped the JFET off since I was planning to replace it and was surprised to find I really liked the tone. Basically made Fluid Drive an extension of the Tite Gain mode. More gain as usual but without the added highend that it had before.

By the way, the Stilleto switch scheme is a real brain twister as each mode/channel engages and disengages various relays and switching JFETs at once with the flip of a switch.


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PostPosted: Sun Aug 20, 2017 7:39 pm 
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Mark I

Joined: Fri Oct 16, 2009 3:25 am
Posts: 32
Location: Austin, TX
timgman wrote:
So jumping r19 alone is a fairly simple way to impact / balance out the EQ?


This is an option as well as just removing the C14 cap. Try adding a 500pf mica across R18 first. This will smooth out some of the high end bite. These will have some charge even with amp off so discharge the caps properly.


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