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MODS for 5150 II / 6505+

These are my combined mods for the II/+ version. The lead channel is the same as the first 5150/6505 except for three capacitors and a resistor. The clean channel is completely different, using a separate 12AX7 rather than swapping out circuit values inside the same gain structure.

Disclaimer: DO THESE MODIFICATIONS AT YOUR OWN RISK. YOU SHOULD KNOW HOW TO WORK SAFELY ON HIGH-VOLTAGE CIRCUITS BEFORE ATTEMPTING THESE. I will NOT provide support for DIYers doing these mods, so please do not ask. I developed them myself (they are not a knockoff of Voodoo or FJA, which I have never heard nor seen in person), and I’m posting them for informational purposes. If you need help, I can do these mods for you affordably.

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3-way comparison between Soldano SLO, Peavey 5150, and Fender 5150-III.

I don’t normally post stuff other than my own work, because I try to stay out of “internet gear culture” — but this video demonstrates the RIGHT way to do a comparison. Notice that the same DI’ed guitar track has been reamped through three different amps, using the same cab and mic setup, so that they’re controlling for as many external factors as possible. Switching is instantaneous. You can hide the video window to get a truly blind comparison.

It also demonstrates how incredibly similar these amps are. I’m about to have the opportunity to finally implement some 5150 mod ideas that I’ve been working on for a long time, one of which includes a full conversion of the drive channel to SLO values/topology (including converting the EQ driver to a DC coupled cathode follower).

Obviously the first 5150 was conceived as a knockoff of the SLO, though at this point it’s probably an even more iconic sound (due to the amp’s ubiquity, low cost, and reliability). Where it deviates are MOSTLY things like pot tapers, which means that “6 on the mids knob” on a 5150 is more like “8 on the mids knob” on a SLO100. That fact tends to greatly exaggerate the difference in sound between the two amps, because people almost always visually match the settings when they compare. But the total range of control is basically identical in most cases. (The SLO also has a nicer and more traditional power stage, but it’s common and very easy just to add a choke and some 470-ohm screen resistors to the 5150.)

The other major difference is that there are no cathode-follower stages in the 5150, which instead uses a triode with 1-to-1 local negative feedback to get lower impedance for a plate-driven tone stack. The Soldano’s use of DC-coupled cathode followers for both the FX loop send and the tone stack, a la Marshall, yields a ‘harder’ response and significantly more 2nd harmonic distortion (likely responsible for the perception that the SLO has ‘thicker mids’).

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24 watt power amplifier. This uses the Tiny Giant schematic, which is a pretty straight datasheet-recommended circuit for the TDA7240 chipamp (smaller cousin of the TDA7293 chip used in the power stages of Marshall Valvestate amps). No tone controls, just a volume knob. Runs on an old 20V/3.5A laptop power supply that my wife gave me after the dog chewed up the mains cord. I put a 2.1mm DC barrel plug on it for this amp.

As just a power amp, it has flat frequency response so it sounds awful without either a guitar-centric preamp or an EQ in front of it to dip out some mids. The idea is that you can use a POD or similar in front of it, and use this to get the output to a guitar cabinet with minimum fuss and weight.

If I could find a compact multichannel tube preamp that I actually liked, having something like this might even convince me to sell some of my home-use amps.

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Anonymous said: in regards to the last question, if circuitry has more of a role in the amps tone than the tubes, then why do certain amps use different tubes? In example, i've seen that Sunn model T reissues have 6l6/12ax7 tubes, ive seen them with 6550 tubes, and i have seen clones of this amp using kt66 tubes. Is there any particular reason or advantage behind the same amp, or a similarly designed amp having different tube configurations?

Again, a really complicated question. Most things in the musical instrument industry are done because “we’ve always done them that way” (guitar players hate change) or because “Leo Fender did it that way” or “Jim Marshall did it that way.” We associate 6L6s with Fender and EL34s with Marshall, but those tubes were NOT chosen for their sound, they were chosen because they were readily available, suited the requirements of the design, and were relatively cheap.

In the heyday of vacuum tubes, musical instrument amps were a vanishingly tiny portion of the market, and one of the least-precise and least-demanding applications. Tube manufacturers made thousands and thousands of different tube types to suit the equally numerous industrial, military, and consumer applications of ever-evolving technology. They also patented the types they designed.

6L6s were made in the USA. Fender used them simply because they were incredibly common (like the 6V6 and 5881) at the time. Easy to find cheap output transformers to match them, easy to design for because the Radiotron Designer’s Handbook basically gave you the circuit to copy. Jim Marshall copied the Fender Bassman but it was expensive to put American 5881s in it, so he used KT66s, which were made in England and therefore cheaper and less subject to shipping damage. Mullard, a British company, designed and patented the EL34. Marshall switched to them because they were cheaper and more reliable than KT66s, and nearby and plentiful. That’s really it. Their contribution to the “Marshall tone” was accidental and probably not nearly as significant as everyone thinks, compared with the design of say, the tone stack (among other things).

When you’re talking about amps designed/made today, tube selection is more a matter of marketing than functionality or sound. If you want the consumer to associate your amp with Marshall roar, you put EL34s in it. Vox chime? EL84s. SVT thunder? 6550s or KT88s. And on and on. You design the object to allude to classic, canonical sounds. It helps greatly if the consumer believes that the pretty glass bottles are the sine qua non of the amp’s tone.

As far as the Model T reissue, it uses 6L6GCs, full-stop. Fender designed this amp’s power stage, and it is IDENTICAL to many other Fender-made amps from the 90s, including its use of 6L6 power tubes. The 70s Sunns used 6550s, because they were Dynaco knockoffs and Dynaco amps used 6550s because they were what you used with ultralinear output transformers, which were better for hi-fi. You can’t put 6550s in a reissue Model T because it’s not even close to the same amp, you’ll stress the heater winding in the power transformer. KT66s are possible to use but will also stress the heater winding and produce less power. Why do they work? Because they’re very similar to a 6L6. They were made by British companies to compete with the American 6L6 without infringing on someone else’s patent.

Notice the complete absence of sound character or quality in any of this. The tubes that have been enshrined as musically magical are basically a weird historical/economic accident.

Recommended reading:The Ultimate Tone - Kevin O’Connor
Radiotron Designer’s Handbook 4

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
http://www.john-a-harper.com/tubes201/

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Anonymous said: what do you think of 5881 tubes? I heard an amp using them recently and really liked the tone. I've had kt88s in my amp for a while and am not too pleased. I like the sound of 6550s and kt66 tubes as well, but havent used them.

It’s a more complicated question than you might realize.

In my experience, power tubes don’t have a “sound” of their own. They might impart certain subtle characteristics to a particular circuit, but they definitely DON’T have “EQ” in any real sense. Tubes’ non-linearities show up in different ways depending on circuit parameters like plate voltage, screen voltage, output transformer primary impedance, supply stiffness, etc. In most cases, I can’t tell the difference between different power tube types in the same circuit until the power stage of the amp is clipping, and even then, it’s very difficult.

If you’re comparing two different amps, then power tube comparisons are largely meaningless. You’re more likely hearing the difference between the two circuits and not the tube types. An obvious example is the fact that the prototypical Fender Bassman used 5881s. In the 90s, Marshall started installing 5881s in JCM900s. It didn’t make the JCM900s sound like a Bassman, they still sounded just like a JCM900.

Moreover, the case of 5881s is an interesting one in itself. What is marketed today as a 5881 (any that are “made in Russia”) is not in fact one. It is a Soviet military surplus tube that happens to have characteristics similar to a 6L6, and also happens to be extremely reliable and resistant to microphonics. It’s cheap because there are so many of them laying around. It works in basically any 6L6 circuit at the cost of a small amount of power output, so you see it all over the place.

If you liked the sound of an amp with 5881s, think about acquiring THAT amp rather than changing tube types in another amp to match it.

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Not really a pedal, since there’s nothing to step on. This is a completely passive Varitone-style control designed to extend a guitar’s tonal range without modifying the instrument. It goes first in the effects chain, making it part of the guitar’s resonant circuit (placed after a buffer or used with active pickups, it does nothing — it relies on interacting with the LCR network formed by a passive pickup and controls). The knob is a 5-position rotary switch that selects a capacitance value to place in parallel with the guitar’s output. The 3-way toggle switches between high-range, low-range, or bypassed.
Guitar tone controls are a fixed capacitance in series with a variable resistance. They allow you to lower the height of the pickups’ resonant peak. This control allows you to lower the frequency of the resonant peak instead. Kind of like an X-axis to the tone control’s Y-axis. It’s pretty much the same as changing your cable length to alter the guitar’s high end, but far less kludgy and with much greater range and precision. You just stick to a reliable, low-capacitance cable of a convenient length, then dial in the peak from there.
I wouldn’t use it with most humbuckers, since it can only lower the resonant frequency, not increase it. But it’s very useful with bright pickups, particularly when trying to match them with different amps to eliminate harshness in the clean sound, or increase clarity/attack in overdrive.

Not really a pedal, since there’s nothing to step on. This is a completely passive Varitone-style control designed to extend a guitar’s tonal range without modifying the instrument. It goes first in the effects chain, making it part of the guitar’s resonant circuit (placed after a buffer or used with active pickups, it does nothing — it relies on interacting with the LCR network formed by a passive pickup and controls). The knob is a 5-position rotary switch that selects a capacitance value to place in parallel with the guitar’s output. The 3-way toggle switches between high-range, low-range, or bypassed.

Guitar tone controls are a fixed capacitance in series with a variable resistance. They allow you to lower the height of the pickups’ resonant peak. This control allows you to lower the frequency of the resonant peak instead. Kind of like an X-axis to the tone control’s Y-axis. It’s pretty much the same as changing your cable length to alter the guitar’s high end, but far less kludgy and with much greater range and precision. You just stick to a reliable, low-capacitance cable of a convenient length, then dial in the peak from there.

I wouldn’t use it with most humbuckers, since it can only lower the resonant frequency, not increase it. But it’s very useful with bright pickups, particularly when trying to match them with different amps to eliminate harshness in the clean sound, or increase clarity/attack in overdrive.

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I’m back from tour and taking new work. Get in touch.

Fender Twin Reverb, model AA769. This might be my favorite Twin. It’s the late-60s SF version that has most of the older circuit values from the BF version, with no master volume. You get all of the tone of a $6000 museum amp, often for less than $1000 on the used market. If I had a regular use for one of these (I’m sure I could invent one if I tried hard enough), I would buy one in a heartbeat. The last two Rosetta records have used these amps as part of the clean-tone mix, along with my Model A and my TSL.
This amp just had a couple bad bypass electrolytics in channel 2, so that there was a nasty fuzzy overtone when using the reverb. No big deal to replace. I’ve worked on this particular unit before, which belongs to a family friend.
The AA769 does 80W from four 6L6GCs. On a scope you can literally see why people love to crank these things up. There’s a lot of 2nd-order harmonic distortion (compression of the positive voltage swings relative to the negative) well before the power stage clips, so the clean tone is noticeably thicker at high volumes than lower volumes, yet without what we would normally call “distortion.” You don’t hear as much of this effect in the later models that tweak the LTP for a more hi-fi sound.
Bias circuit is the later ‘balance’ control and is not actually adjustable.

Fender Twin Reverb, model AA769. This might be my favorite Twin. It’s the late-60s SF version that has most of the older circuit values from the BF version, with no master volume. You get all of the tone of a $6000 museum amp, often for less than $1000 on the used market. If I had a regular use for one of these (I’m sure I could invent one if I tried hard enough), I would buy one in a heartbeat. The last two Rosetta records have used these amps as part of the clean-tone mix, along with my Model A and my TSL.

This amp just had a couple bad bypass electrolytics in channel 2, so that there was a nasty fuzzy overtone when using the reverb. No big deal to replace. I’ve worked on this particular unit before, which belongs to a family friend.

The AA769 does 80W from four 6L6GCs. On a scope you can literally see why people love to crank these things up. There’s a lot of 2nd-order harmonic distortion (compression of the positive voltage swings relative to the negative) well before the power stage clips, so the clean tone is noticeably thicker at high volumes than lower volumes, yet without what we would normally call “distortion.” You don’t hear as much of this effect in the later models that tweak the LTP for a more hi-fi sound.

Bias circuit is the later ‘balance’ control and is not actually adjustable.

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Peavey VTM-60. Pretty common problem of cracked solder on the control PCB, since the only thing supporting it are the pots and jacks on the front panel, which themselves are only connected to the board by their solder pads. Easy enough to fix, and otherwise very reliable amps. Precursor to the 5150. You would be surprised at the number of important metal and hard rock recordings these amps were (and still are) used on. They’re the “Mississippi Marshall.”

Peavey VTM-60. Pretty common problem of cracked solder on the control PCB, since the only thing supporting it are the pots and jacks on the front panel, which themselves are only connected to the board by their solder pads. Easy enough to fix, and otherwise very reliable amps. Precursor to the 5150. You would be surprised at the number of important metal and hard rock recordings these amps were (and still are) used on. They’re the “Mississippi Marshall.”

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Hybrid compressor that I keep wanting to call the Atomsmasher (after James Plotkin’s seminal 2000 recording of the same name, a big influence on me). Controls from right to left: threshold, ratio (big knob), attack, level (big knob), release. As with all my pedals, the controls are arranged in their order in the circuit.
It’s not really my design, since it’s basically a hybrid of the Engineer’s Thumb and the Diamond Comp (the yellow one). Full-range, suitable for guitar or bass. It’ll do the Ross/Dyna Comp ‘click’ sound that everybody likes but it can get a lot more hi-fi than that. Works as a limiter too.
I had a stash of vintage phenolic knobs that seemed perfect for it.

Hybrid compressor that I keep wanting to call the Atomsmasher (after James Plotkin’s seminal 2000 recording of the same name, a big influence on me). Controls from right to left: threshold, ratio (big knob), attack, level (big knob), release. As with all my pedals, the controls are arranged in their order in the circuit.

It’s not really my design, since it’s basically a hybrid of the Engineer’s Thumb and the Diamond Comp (the yellow one). Full-range, suitable for guitar or bass. It’ll do the Ross/Dyna Comp ‘click’ sound that everybody likes but it can get a lot more hi-fi than that. Works as a limiter too.

I had a stash of vintage phenolic knobs that seemed perfect for it.

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