David Kronemyer

Amplifying the Archtop Guitar with the Marshall JTM45

October 29th, 2006 by David Kronemyer · 3 Comments


A. Archtop Guitars

It was 1968 and the band I was in played covers of current pop songs. This was the high watermark of the swingin’ 60s, so our repertoire was eclectic. I knew only one style of playing, which was intricate finger-picking arrangements. Basically, this still is all I can do! I prefer heavier gauge, flat-wound strings (including a wound “B”), and metal picks. You really can dig in, attach the string, and feel it vibrate in response. No plastic plectra, here. Flat-wound strings, like those made by Pyramid (which is all I’ve used for about a quarter of a century) actually have a kind of compression effect on the sound.

I never did like solid body guitars, and still don’t. I got a Gibson ES355 from an old country-western guitar player in Las Vegas. I remember paying $325 for it. I loved the way the sound curled up around the pickups, then leapt out like some kind of jungle cat – taut, spry, convoluted, and wound around upon itself. Not at all like the thin, wiry, slinky sound of a Fender Stratocaster. I played it through a Fender Twin Reverb amplifier. It wasn’t until the mid-1970s that I got the urge to drill holes in it, and install active electronics (since removed, of course). That’s about the same time I started selling off a lot of what we now call “vintage” electronic equipment, to buy over-priced digital synthesizers. Just to show you what happens, the cheap-sounding digital equipment started to get valuable, just after I had sold it, in order to re-acquire all of the analog stuff. An illustration of the principle, “you can’t win for losing,” that’s for sure.

Along the way, somebody told me I might enjoy trying out a “jazz” guitar. Although I suppose there were some jazz influences (to the extent they were reflected in current pop songs), what we played definitely wasn’t “jazz,” per se. More like “weird rock.” But, after checking out the bigger body, I was hooked. Before too long, I had acquired a modest collection of Gibson archtops, see Figure 1. Yes, it’s probably true that blondes have more fun! Now, the notes could linger in the body of the guitar itself, before propelling themselves through the pickups, and out into the amp. I kept using Pyramid flat-wounds, and the finger picks.

Figure 1

Did I mention we played kind of loud? With a big archtop guitar, I was able to achieve a level of resonance, and sustain, that was starting to get pretty close to the way I thought things ought to sound. I also discovered that by selectively damping strings, and positioning oneself in relationship to the amplifiers, one could create very harmonic feedback – almost like you were accompanying yourself, with another instrument.

B. Marshall JTM45s

Over time, I started becoming vaguely dissatisfied with the Fender Twin Reverbs. I needed an amplifier with a higher level of precision, and resolution. Something, in short, that would complement the great new sound I was getting from my guitars. That’s when I started playing the Marshall JTM45.

Now, when I mention the name “Marshall,” there’s no need to get upset, or frightened. Yes, it’s the same manufacturer of those refrigerator-size stacks of amplifiers, that have propped up the back-line of countless touring acts through the years, many of a decidedly non-jazz persuasion. Some of their models have about as much subtlety and sophistication, as a sledge hammer.

The JTM45, however, is different. It’s the earliest model Marshall made, maybe only a few a week, before they got to be a big company, and went into full production. “JTM” stands for “Jim and Terry Marshall” (I guess he named it after his wife, how sweet!); “45” is because it’s a 45-watt amplifier. There are a lot of theories about the JTM45, and how essentially it’s a clone of the Fender Bassman amplifier. Analysts of circuit schematics may have a difficult time refuting this charge.

The missing point, though, is that the JTM45 sounds a whole lot different than your typical Bassman – a sound, for that matter, quite unlike that of any other amplifier. When properly set up, playing the JTM45 is the audio equivalent to driving a Ferrari. Just like a good argument could be made that only American companies (such as Gibson and Fender) knew how to make guitars, so a good argument also could be made that only British companies (such as Marshall, Hiwatt and Vox) knew how to make guitar amplifiers. The great trinity of British guitar amps.

These early JTM45s come in several different varieties. The later models, probably dating from around 1965, have the familiar Marshall “script” logo, see Figure 2.

Figure 2

From about 1964 – 1965, Marshall used a somewhat rarer “black & gold” block logo, see Figure 3.

Figure 3

Although it’s hard to tell from the pictures, in addition to a different logo, the cabinets had a different type of finish – smoother, and not crinkly, like the skin of some kind of weird reptile. Before then, mainly around 1964, it used the even more rare “maroon & silver” block logo. This looked just like the “black & gold” block logo, except it was “maroon & silver;” see Figure 4.

Figure 4

Note also the different style of knobs. The earliest Marshalls of all, dating from about 1963 – 1964, had a “red metal badge” logo. They too came in different styles, see Figure 5 – Figure 8.

Figure 5

Figure 6

Figure 7

Figure 8

Because of their low production, there was a lot of mixing-and-matching of components. For this reason, it’s difficult to associate a single style with a single logo type. It’s not hard to see how this happened; a stock of components simply was used, until the supply was exhausted, without regard to what belongs where. For example, Figure 5 has an aluminum panel, without grounding switches. Figure 6 has an aluminum panel, with grounding switches. Figure 7 is an “offset chassis” with an all-white front panel; together with an unusual model without the black tolex covering at the top. Figure 8 is a later-model 50-watt head, together with an all-white front panel. [It’s 50 watts only because it has a slightly larger output transformer; otherwise, it’s identical to its colleagues]. Figure 9 shows several different variations, together.

Figure 9

C. Setting Up the Amp

Let’s face it, these amps are getting old. In order for them to work right, they must be fixed up properly. The problem of restoring the JTM45 is exacerbated by the fact that most British guitar players took notoriously poor care of their amplifiers. They were rode hard and put away wet. Bloody limeys! So, the first thing you have to do is get rid of the stupid modifications that well-meaning, but misinformed, guitarists have installed over the years.

Once the head’s back to stock, capacitors often have to be replaced. Occasionally – but only as a last resort – it’s necessary to re-wind the output transformer. The reason why I say “only as a last resort” is because the output transformer is a critical part of the amplifier’s sound. I have a policy of not touching the output transformer unless it’s patently out of commission, and then I always rewind it just like it was. Nothing wrecks a good JTM45 faster than a bad output transformer replacement.

Then comes the matter of the tubes. The JTM45 requires six of them – two power tubes, three preamp tubes, and a rectifier, see Figure 10.

Figure 10

The choice of rectifier is easy, they only can be used with a 5AR4, and the best ones were made by a company called Mullard, see Figure 11.

Figure 11

Preamp tubes are model ECC83, the British equivalent of today’s common 12AX7. Mullard, Amperex, Brimar or Telefunken made the best ECC83s, see Figure 12.

Figure 12

It’s highly amusing to rotate preamp tubes of different manufacture through the same amplifier, thereby discerning their various performance characteristics. I have a variety of subjective impressions about what each brand sounds like, as opposed to the others, but these tend to fluctuate with the amplifier being used (and other factors, like the time of day).

As far as power tubes are concerned, back in the old days there was a tube called the KT66. It’s not my intention here to get into the technical aspects of tubes, or theories of tube audio design. Suffice it to say for now that British General Electric Co. (“GEC”) invented the KT66 in the late 1930s. A GEC division called the “Marconi-Osram Valve Co.” (or “MOV,” which sometimes appears on the packaging) sold it. It was marketed under the brand names “Genalex” and “Gold Lion.” The “KT” in KT66 is an abbreviation for “kinkless tetrode,” referring to electronic characteristics of the tube’s design. Makes you wonder what a “kinky tetrode” would do.

There are several different iterations of the KT66, some of which have clear glass, some opaque, see Figure 13.

Figure 13

While the JTM45 can be used with other output tubes, like a KT77 or an EL34 (later models only), the KT66 is by far and away the best. The notes resonate and linger in the body of the archtop guitar. Then they burst out of the pickups, charge into the amplifier, where they once again have the opportunity to languish within the vast sonic space of the KT66. When they emerge through the speaker, they are smooth, sculpted, contoured – dulcet, and creamy.

D. Use of the Variac

You can’t just plug a JTM45 into the wall, and expect it to work. To begin with, the power transformer only has tapes for UK power, i.e., 220 – 240 volts. The issue, however, is more complex, because the KT66 tubes must be biased, in order to run at optimal strength. This requires a carefully-auditioned combination of power tap selection, and input voltage to the amplifier. Because power transformer voltages must be chosen, and the amplifier biased, so as best to accommodate the individual tubes with which it has been fitted, it only can be run off a custom-designed variac and transformer rig, see Figure 14.

Figure 14

Please note, the input voltage meter isn’t strictly necessary, but the output one is, in order to insure correct AC voltage going into the amp, adjusted with the variac.

E. The Rotation Method

Each of these amplifiers sounds different! This is due to a combination of factors, the main ones of which are: choice of preamp tubes, aging of components (with resulting drift in values and tolerances), choice and characteristics of output tubes, power to the output tubes, and winding of the output transformer. This is not a bad thing. One learns to appreciate, and indeed anticipate, these sonic variances – rather like what the famous Danish philosopher Soren Kierkegaard called the “rotation method.”

The JTM45 is like a notch filter – admittedly narrow, but deep. It is not an “all purpose” amplifier, but rather one designed for specialized applications, at which it truly excels. Once you start playing, it’s hard to acclimate to anything else; whatever ordinary, decent amplifier you formerly used, somehow ends up sounding deficient.

This phenomenon in turn attenuates you point of view. There may be Eskimo tribes, for example, that have fifty different concepts of “cold,” simply because they have more gradations of experience that must be parsed out into what we subsume under a more general heading. Analogously, the JTM45 exposes the player to filigrees and nuances of sound, that other amplifiers can’t match. In this respect, with its sonorous timbre and mellifluous tonality, the JTM45 is the “best of the best.” While it’s unlikely Jim Marshall and Ken Bran designed the JTM45 specifically for use with an archtop guitar, I haven’t heard an amplifier yet to match it (provided the foregoing instructions are followed carefully).

F. Compression

One of the first things you notice when playing an archtop guitar through a JTM45 is that it could use a little “push” at the front end. The amplifier has so much headroom, that even slight variations in pickup strength, can profoundly affect the sound. It helps the overall presentation, if the sound receives a modest boost somewhere between the guitar, and the amplifier. The best way to do this is with a device called a compressor, which essentially evens out the guitar’s dynamics, by making softer notes louder. Not only does this supply the required emphasis, but it also “tightens up” the amplifier’s sound – analogous to taking an otherwise-flabby note, and putting it on an exercise program. Because of the way I play – metal picks and finger-picking – a compressor is particularly necessary.

There are tons of cheesy compressors around. Many of them are small boxes that live on the floor, and are designed to be turned on and off, by stomping on them with your foot. For this reason, they often are called “stomp boxes,” see Figure 15.

Figure 15

Please try to stay away from these. It’s senseless to have a cheap little stomp box (or even a more-expensive, relatively-good stomp box) act as a regulator, or a governor, on the great sound going from the archtop guitar to the JTM45.

Instead, the best compressor around for this type of work is something called an LA-2A, made by a late, oft-lamented company called Teletronix (actually, if I’m not mistaken, the LA-2A now has been re-issued by Universal Audio). The sound comes in through a photo-optical attenuator, that actually glows brighter, or dimmer, depending upon the intensity of the note. It’s then picked up by a receiver, which sends the note on its way, only slightly processed.

Like everything else, this can be too much of a good thing; the trick to using an LA-2A is to give the note just enough compression to expand the sound, but not enough to where you really can hear it. Put differently, it should be adjusted so you just start hearing compression, and then back off so it’s not apparent. When done properly, this will give the note just enough room to “breathe” (like a bottle of fine wine), so it emerges fortified and energized, before heading off into the amplifier. The note will have an edge to it – a militancy, a stridency. It’s not limpid, or flaccid, but rather, charged up, energized, and ready to go.

The output impedance of any guitar’s pickups isn’t matched to the input impedance of any studio processing device. “Impedance” is the opposition of the input device to the current coming from the output device. With impedance mis-match, power won’t get transferred efficiently, and there will be other adverse consequences, such as signal attenuation and noise. To get them to correspond, it’s necessary to use something called a “direct box” – direct, as in, the guitar can go directly into the studio processing device. The direct box makes both devices correspond, by drastically lowering the output impedance of the guitar, so it’s more-or-less the same as the input impedance of the processor.

I ended up with a set-up that goes into the direct box, then to a tube microphone preamplifier, then into the LA-2A. In theory, it would be possible to take the line output from the direct box straight into the LA-2A, but it sounds a whole lot better to use the microphone output of the direct box (which has even lower impedance than the line output), then go through the microphone preamplifier, and then into the LA-2A.

Not being as heavily into antique tube studio processing equipment as I am into JTM45s, I decided to use modern interpretations of old designs for this preliminary processing phase, see Figure 16.

Figure 16

To some, it may be hard to defend this decision; about all I can say, is that I wanted to focus on the sound of the guitar and the amplifier. The fact of the matter is that the original Teletronix LA-2A actually may not be as well built as its successors. Furthermore, even after you’ve found a good one, it creates its own indeterminacy (based on component drift, signal-to-noise ratio, other similar factors), which wasn’t the point of the exercise.

The combination of Avalon direct box, Fearn microphone preamplifier, to Éclair LA-2A clone, is more-or-less high end. I particularly like the Éclair, because it has a grittiness, and a graininess to it, that other LA-2A clones (e.g., one made by Manley) don’t have. Don’t get me wrong, I like the Manley, too, it’s just a little more “hi-fi” – which is a good thing, right?

G. Other Performance Factors

I also add a dash of reverb, just to taste. The only type of reverb that is permissible for use with any type of guitar (especially archtops) is a spring reverb. It has a “boinginess” to it that no digital reverb, however accurately modeled, ever could hope to emulate. The Surfbox, made by Soldano, is perfect for this. It looks way cool, thus complementing my nascent yearnings for a simpler time when it was just a player and his guitar (without all of this rigmarole). California Dreamin’, who could go wrong with a device called the “Surfbox.” It also sounds terrific – a little darker than other units (like a Fender), but that’s OK. It also helps that it’s impedance-matched to the other components in the signal chain, facilitating hook-up of the amplifiers, see Figure 17, Figure 18. The Sony processors are for a different application; more on that, later.

Figure 17

Figure 18

The JTM45 has two separate channels – a “normal” channel, and a “high treble” channel. It also has separate equalization controls for bass, mid and treble, and something called a “presence” control. Which channel to use, and how to set the equalization and presence controls, depends entirely upon the sonic interaction between the guitar and the amplifier. They vary not only in frequency, but also width of the surrounding frequencies that are affected (the “Q” factor), and gain.

There are several factors that influence the determination of which channel to use, in addition to the amp. These include the guitar, string gauge, how heavy you press on the strings, angle of pick attack, and how heavy you hit the strings. Strictly as a matter of preference, I find the high treble channel typically provides better articulation. It’s more refined, supplying more detail. The EQ is simple to adjust to achieve even frequency response, or whatever other tonal effects are desired. I typically start with all controls “straight up,” but then start boosting the bass and treble, and cutting the middle (to eliminate “boominess” in those frequencies). The bottom input has slightly less gain, which is better for archtop guitars. It’s necessary to let everything warm up for at least four hours before playing, in order for the electronics to settle in. For some reason, it doesn’t sound as good to me after it’s been on for over 24 hours; the sound dissimulates and becomes spongy, less sure of itself. So, there’s a window.

The end result of this combination is a pleasing euphony. Notes cascade from the guitar like rain, each blending with its immediate predecessor, which still lingers in the electronics. They are fully saturated; you can clearly distinguish the leading edge of the note’s envelope, from the air space it displaces. The gain stages can be balanced for maximum clarity between the guitar, direct box, microphone preamplifier, compressor and amplifier. If the amplifier is just slightly overdriven by the compressor, the notes crenellate and acquire a sharper edge – just a hint of breaking up in the treble. The bass becomes more plangent. This isn’t distortion, rather, more of a deconstruction, or disassembly of the note, into its constituent harmonic layers.

Of yes, I almost forgot, you’ll notice from the pictures that this particular set-up is in stereo, splitting the pickups to different amplifiers. Something about “double your pleasure, double your fun.” Having learned my lesson not to modify old Gibsons, it became apparent I’d need to get some more guitars, specially constructed for this purpose. Darn, I hate it when this happens. I don’t know if this is some kind of secret I’m not supposed to reveal, but if the truth were known, a new custom-built archtop actually sounds (or, I should say, has the potential to sound, depending upon execution) considerably better than an original; easier to get around, better neck, more resonance, less “clunky.”

H. Speakers

A final word about speaker cabinets. By and large the original speaker cabinets supplied with JTM45s sound terrible. They were just speakers screwed into enclosures, with little thought given to proper acoustic design. I’m sure the speakers were nice when they started out. But old speakers have endured years of abuse in bars and clubs, cigarette smoke, things being spilled on them, etc. My philosophy is that the sound comes from the guitar and the amplifier; the speaker’s job is to interpret and project that truthfully, without excessive coloration. I know there is room for disagreement on this point, as with all others, but it’s my preference.

The choice combination for me thus is “old head – new speaker.” There are getting to be some good custom-made cabinets out there, based on sound acoustic principles, and fitted with modern speakers, themselves based on earlier designs. To me, this sounds a lot better than any tired old speaker ever would. The only restriction on using speakers with the JTM45 is that the cabinet must be 16 ohms, which isn’t as common as the typical 8-ohm cabinet. While I’ve never tried it, I believe that using a JTM45 with an 8-ohm cabinet would cause the JTM45 to explode. As a matter of policy, I always test an unknown cabinet with a multi-meter before plugging in. I prefer 2×12” cabinets; in my view, the 4×12” is just too many speakers for this size head.

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