Desertdweller
Roadie
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So I did a couple of mods to the 5150 III 50w EL34 a couple years ago that really make the amp quite a bit better. I posted this on another forum then, but it's a dead now so I figure why not update it and post it here?
Blue Channel C137 Mod
I have yet to hear anyone that truly likes the stock blue channel on the 50w EL34 amp, which is an absolute shame because all the other 5150 III heads have an amazing blue channel. Fender put in a fail-safe to fix this thankfully, and I'm not the first to do this simple and great mod. Simply snipping a small capacitor fixes this.
This is the easiest mod one can make to this amp. However, if you're not tech savvy at least a bit, hire a tech to do this. Innards of amplifiers have enough current to kill, so DO THIS AT YOUR OWN RISK as always. What I did is just snip out capacitor 137 and the blue channel comes to life, making it just as great as the blue channel on the other 5150 III amps.
Now that I've modded that blue channel, I screwed the amp back into the chassis and kept the front off so I could mess with tubes to modify the sound of the red channel.
Red Channel Gain kick-down through tube selection
This will be way easier, as it's just tube preference, they're preamp tubes so there's no worry about biasing, these tube changes will:
1) Make the gain knob more of a usable sweep so it doesn't hit full saturation at 10 o'clock, and
2) Get rid of the squeaky midrange the red channel can sometimes have.
I wound up with this configuration that accomplished both:
V1: First driver for all 3 channels. I used Mesa SPAX7-A (a decently high-end 12AX7) originally and now have a Tung Sol 12ax7 in there. I usually use Tung Sols in V1 of all my amps, just my preference, I can hear a difference even if Glenn Fricker can't. Lower noise floor and it just punches harder IMO.
Red channel specific tubes, V2-4 are all the additional gain stages for the red channel:
V2: JJ 12au7, much lower gain tube, makes the gain knob sweep come to life. 40 is the new 30, right? Well 2 or 3 o'clock is the new 10 o'clock on the dial.
V3: Sovtek 12ax7LPS, to sweeten the midrange and make the channel sound more balanced
V4: Mesa 12ax7A, just a good tube! A Tung Sol would do nicely here too.
Conclusion: made an already awesome amp a lot better with a few small tweaks. Slightly lower noise all around, tighter/thrashier blue channel, and a singing lead red channel that now has some variation.
Blue Channel C137 Mod
I have yet to hear anyone that truly likes the stock blue channel on the 50w EL34 amp, which is an absolute shame because all the other 5150 III heads have an amazing blue channel. Fender put in a fail-safe to fix this thankfully, and I'm not the first to do this simple and great mod. Simply snipping a small capacitor fixes this.
This is the easiest mod one can make to this amp. However, if you're not tech savvy at least a bit, hire a tech to do this. Innards of amplifiers have enough current to kill, so DO THIS AT YOUR OWN RISK as always. What I did is just snip out capacitor 137 and the blue channel comes to life, making it just as great as the blue channel on the other 5150 III amps.
Now that I've modded that blue channel, I screwed the amp back into the chassis and kept the front off so I could mess with tubes to modify the sound of the red channel.
Red Channel Gain kick-down through tube selection
This will be way easier, as it's just tube preference, they're preamp tubes so there's no worry about biasing, these tube changes will:
1) Make the gain knob more of a usable sweep so it doesn't hit full saturation at 10 o'clock, and
2) Get rid of the squeaky midrange the red channel can sometimes have.
I wound up with this configuration that accomplished both:
V1: First driver for all 3 channels. I used Mesa SPAX7-A (a decently high-end 12AX7) originally and now have a Tung Sol 12ax7 in there. I usually use Tung Sols in V1 of all my amps, just my preference, I can hear a difference even if Glenn Fricker can't. Lower noise floor and it just punches harder IMO.
Red channel specific tubes, V2-4 are all the additional gain stages for the red channel:
V2: JJ 12au7, much lower gain tube, makes the gain knob sweep come to life. 40 is the new 30, right? Well 2 or 3 o'clock is the new 10 o'clock on the dial.
V3: Sovtek 12ax7LPS, to sweeten the midrange and make the channel sound more balanced
V4: Mesa 12ax7A, just a good tube! A Tung Sol would do nicely here too.
Conclusion: made an already awesome amp a lot better with a few small tweaks. Slightly lower noise all around, tighter/thrashier blue channel, and a singing lead red channel that now has some variation.
