Substation Onyx

The Peavey 5150/6505 product line is a legendary series of products that have shaped the sound of heavy metal ever since Andy Sneap used the original 5150 on Machine Head’s Burn My Eyes in 1994. The amplifiers’ circuit topology has influenced a multitude of high-gain successors, and it’s still considered to be imperative to the sound of modern metal today. Their low cost and widespread professional use simultaneously makes them the poor man’s high-gain tube amp and the golden standard.

Of course, I didn’t know any of that when I purchased my 6505+112 combo amp as an undergraduate student. I was simply looking to find a great-sounding tube amp that didn’t break the bank. Prior to this I had been using Line 6 modeler amplifiers or borrowed Vox ACs, which stranded me between all the options in the world, and no options at all. The 6505+ seemed to be the most inexpensive way to get a digestable amount of tonal options with a promise of nailing my favorite metal sounds.

For many years, this amp was perfect and suited my needs. As is with all tools, within time the shortcomings and limitations of my 6505+ became evident the more I used it. So, this past summer, I did some research, found a schematic, took my 6505+112 apart, and modified it. I couldn’t be happier with the results. The modifications implemented are as follows:

• Speaker Replacement – As is always the case, a large portion of an amplifier’s sound comes from the speaker cabinet itself! The 6505+112 combinational amplifier is impressively resonant due to its closed back, and comes with a notoriously divisive Peavey Sheffield speaker. I like this speaker. It doesn’t have a very strong low end (mine had a gradual roll-off below 200 Hz), but the upper mids are impressively present without having any irritating, whistly peaks (anyone who has had to do surgical post-EQ on a distorted guitar recording knows EXACTLY what I’m talking about). However, for the punchy, thick 90s-to-early-2000s sound I was wanting from this combo, the Sheffield wasn’t cutting it. I replaced it with a period-correct UK-made 16Ω Celestion Vintage 30.
• FX Loop Clarity – The FX loop within the 5150/6505 product line is far from transparent. I used to run a 4-cable method wiring scheme with my 6505+, and I always noticed that I had to bump the lows and some low mids significantly in a global EQ setting to compensate for the loop. Placing a 22 nF capacitor in parallel with R86 on the back panel breakoff PCB took care of this problem.
• Lower Resonance – The resonance controls on the amp are powerful, but the resonance frequency corner overlaps significantly with the Lows control from both preamps’ tone stacks. I replaced C37 and C74 with 10 nF capacitors, which shifted the resonance controls for both channels down in the frequency spectrum, so now I could freely adjust the low-end rumble coming off the speaker without having to compensate by dialing back a preamp bass control.
• Rhythm Channel Organic Gain – This mod is more complex, and it involves revoicing the gain structure. The idea is to get a smoother taper of distortion off of the Pre gain control, similar to a vintage amp without a master volume. I did this because while the Rhythm channel in a stock 6505+ is awesome when the Crunch switch is engaged, there isn’t much that it can do that the lead channel can’t… and with the Crunch switch out, the amp verges on brittle unless the gain is up fairly high. This mod serves to separate the Rhythm channel’s identity from the Lead channel a little and give it a lot more versatility. Upon completion, the Rhythm channel is now fully capable of good, useable clean tones through breakup tones, all the way to vintage high-gain tones, even with the Crunch switch out. I replaced C36 with 100 pF, R91 with 1 MΩ, R69 with 10 kΩ, and C25 with 47 nF.
• Fuller Cleans – For those who have tried coaxing a clean tone out of the stock 6505+ Rhythm channel, you can all attest that as you pull back the Pre knob, there’s a significant loss in bass as you lose gain, so you either have to settle for a brittle clean tone or a fuller tone that’s pushing a hair of distortion. Removing R13 entirely doesn’t change the amp into a Class A jazz amp, but it does restore enough bass for you to find a sweet spot for clean tones. I strongly recommend pairing this mod with the Rhythm Organic Gain mod.
• British Tone Stack – This one’s just for fun. Remove C47 and replace C34 with a 1 nF capacitor to move the Rhythm channel’s tone stack closer to Marshall values. I found that this gave each of the controls a lot more usefulness AND further separated the Rhythm channel’s identity from the Lead channel. I recommend pairing this mod with the Rhythm Organic Gain mod if you want to have something in the vein of a Marshall in the Rhythm channel, rather than the stock sound.

The video below includes some samples I took of the amp before modification, after replacing the speaker, and after finishing the modifications.

If you plan on modifying your own amplifier, of course you do so at your own risk. I am not liable for any damage sustained to your person or your equipment. I am not claiming ownership of these mods; most are well-known mods commonly performed on the 5150/6505 family of products. I have listed parts numbers specific to my 6505+112’s PCBA and schematics, which are easily searchable. These mods can be performed on any 5150/6505, but you will need to cross-reference the schematic of your amplifier with the 6505+112 schematic to see what parts need modifying. Happy tone hunting!