october 2011

A Supro S6420 with volume issues.

Hi Jeff,
I have a Supro S6420 that I recently had serviced by a local tech. I brought in the amp because the bottom end sounded like mush. It broke up at low volume, and for a 35-watt amp, it was not very loud. In fact, my ’66 Princeton Reverb has a lot more volume and headroom.

So, my tech replaced the filter caps, removed a non-original output cathode bypass cap, checked all power and plate-load resistors, checked the transformers, and adjusted the bias (VP 425, 45 mA bias). All preamp and output tubes are good and the speaker is good. Still, it has no volume. My Princeton blows it away. What’s causing this lack of power?

—Bob,
Townsend, Tennessee



Hi Bob,
Thanks for writing. Well, it sounds like your very cool Supro S6420 is not living up to your expectations, and from your explanation of the servicing— most particularly removing a non-stock output cathode bypass cap—it seems likely that its previous owner may have felt the same way. However, I’m not necessarily agreeing there is something “wrong” with the amp. If you plug in a Tele, crank it up, and you suddenly sound like the first Led Zeppelin album, the amp may indeed be functioning properly! Comparing it to a Fender Princeton Reverb is quite honestly an apples-to-oranges scenario, but let’s see if there may be an explanation for your findings and better yet, a fix.

Regarding the Fender having a better bottom end, this may be due to a couple of factors. First: Most Fender amps, due to the type and position of the tone-stack circuitry, have a substantially reduced mid content. This results in the amps’ top and bottom end being more pronounced. Coupled with the substantially more closed-back design of the Fender cabinet, this may very well explain why the Princeton’s bottom end sounds fuller.

Even though the Supro has a 15" speaker, the fact that the baffle board and cabinet are not much larger than the speaker itself and the rear of the cabinet is virtually wide open, the enclosure is basically a baffle with sides. There’s not much low end being developed by the enclosure itself. If you’ve ever compared a 10" speaker mounted in a nice enclosure with a 15" speaker mounted on a piece of plywood, you’d know which one would win the battle of the bass.

Another consideration would be the speaker itself. Just because it “works” or doesn’t have any buzzing or tears in the cone, doesn’t necessarily mean it sounds good. Speakers get old and tired, and I can’t tell you how much a new speaker can completely change an amp for the better. A worn-out speaker along with the cabinet design could easily be responsible for the fact that the amp is not as loud as you believe a 35-watt amp should be.

You mentioned that the tubes are “good,” but didn’t explain how you determined this. Just because a tube checks well, does not necessarily mean that it sounds good. Substitution is always the best way to confirm how the tubes are holding up, so if you haven’t already done so, swap out the tubes for a quick check.

You also mentioned that the amp breaks up early and does not have much headroom. This too could be the result of a tired speaker, but it could also be design-related. Looking at the schematic, I see the final stage of the power supply— which supplies power for the first preamp stage—is fed by a 100k resistor. This is a far larger resistor than is typically used in such basic amps, and it would substantially reduce the voltage supplied to this stage. Though this is great for producing compression and overdrive, it reduces headroom. Decreasing the value of this resistor to 27k, or even 10k, should increase the headroom in this stage.

There is one other aspect of the comparison to your Princeton that I’ve not yet addressed: The Princeton has an extra gain stage in its basic amplification section. By contrast, this could easily make the Supro sound anemic. The good news is there’s an unused half of a 12AX7 in this Supro that you could press into service. Your tech would know how to turn this into a working gain stage. Placing this extra stage ahead of the existing first stage would yield more gain and saturation, while placing it after the volume control would yield more sensitivity.

If you’re not willing to go that far at this point, there are a couple of quick changes you can implement to make the amp more “full sounding.” Just be aware that the fuller the amp becomes, the more difficult it may be for it to stay clean at higher volumes. If you’re willing to give it a try, I’d suggest changing the input cap and resistor of the first gain stage. These are located immediately after the input-jack resistors. Currently, the capacitor has a .005 μF value and the resistor measures 270k Ω. Try changing the cap to a .01 μF or even a .047 μF, and the resistor to a 1M Ω. This should give the amp a fuller response, and the mod is easily reversible if you don’t like the result.

If all this fails to bring the amp to what you believe is an acceptable performance level, I’d recommend a quick substitution of a similar output transformer. Again, something may appear to be good, but a quick, temporary substitution is the best way to know for sure. I hope you get your Thunderbolt thundering!

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Ready to explore the technical aspects of the 5-way super switch?

Last month, we started discussing 5-way switches that offer more wiring possibilities than the standard-issue Strat unit we all know so well [“Introducing Fender's 5-Way Super Switch," September 2011]. Ready to explore the technical aspects of this beautiful beast? This is where the fun really starts!

I'll keep it as simple as possible because the switching matrix of this device is much more complex than it seems. I'll guide you step-by-step through all five switching positions and show you which lugs are active and connected to each other in each position.

Let's recap the essential points: The 5-way super switch is not simply two standard 5-way switches in one package. Nope, there's more—lots more: Instead of two stages of the normal 5-way switch, this switch offers four completely independent stages with six terminals each, so we have a total of 24 terminals. Here's another way to look at this: We have six lugs on each stage, rather than the five that are on the standard 5-way switch.

To simplify things, we can say that each stage has five input lugs and one output lug (aka “common"). Keep in mind that all four stages are completely independent from each other, but they all switch at the same time! So if you want to combine two or more stages for a special wiring, you need to connect them with a short jumper wire.

Fig. 1
illustrates what this super switch is doing in position #1. This would correspond to the bridge pickup alone on a normal 5-way switch. In these diagrams, “C" is short for “common." You can see that in this position, input lug #1 is connected to the corresponding output lug—C—of all four stages.



Look at Fig. 2 to see what's happening at position #2, which corresponds to the bridge and middle pickups connected together in parallel on a normal 5-way switch. We're following the same principle as before: On all four stages, input lug #2 is now connected to the C output lug.



I think you've got the working principle of the switch now. Fig. 3 shows position #3, which would be the middle pickup alone on a standard 5-way switch.



And position #4 (middle plus neck pickup wired in parallel on a normal 5-way switch) is shown in Fig. 4.



And finally, Fig. 5 reveals how position #5 is configured. On a normal 5-way switch, this would select the neck pickup alone.



All right! That's the basic principle of the 5-way super switch. You can spend hours developing your own custom wiring using this device to implement all kinds of gimmicks like out-of-phase, series/parallel, and countless other options.

To get a sense of how complex wiring schemes can get with a super switch, go to the Fender website and look at the various “Fat Strat" wiring Exploring Fender's 5-way Super Switch BY DIRK Wacker diagrams. These schemes can make a great starting point for your own custom variations.

Thanks to our friend Bartek from the Guitar Wiring Blog (guitarwiring.blogspot.com) for granting us permission to use his great diagrams in this column.

Stay tuned for more Strat mods coming next month, when we'll return to more practical mods after all the switching theory we've covered in the last few columns. Next up: Wiring a “tone switch" for your guitar. Until then, keep on modding!

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It’s much easier to capture a fat-sounding track if you have a good, thick guitar tone coming out of your amplifier.

If you’re into cooking, you probably know about roux. It’s a simple mixture of fat—usually butter—combined with flour and used for thickening up sauces. Also, many sauce recipes (especially Chinese styles), will call for adding cornstarch near the end of the cooking process to help thicken things up. I certainly don’t recommend pouring roux or cornstarch into your computer or hard drive to thicken your guitar tracks, but there are some things we can do to add mass to our tones in the studio.

Get it right from the start.
It’s much easier to capture a fat-sounding track if you have a good, thick guitar tone coming out of your amplifier. It’s worth taking the time to find the right combination of guitar, amp, and pedals to ensure you have a thick tone at the source. For me, “thick” usually means humbuckers or P-90s, but you can also get fat sounds out of a guitar equipped with Fender-style single-coils, if the amp provides some additional girth. I find that a resonant instrument, such as a lightweight solidbody, a semi-hollow or chambered axe, or a hollowbody gets a fatter sound than a heavier, more inert solidbody.

Capture the sound well.
Revisit the past few installments of Guitar Tracks at premierguitar.com, and you’ll find many suggestions for mic’ing techniques and mic selection. Jimmy Page got huge tones out of a tiny Supro amp and a Tele, and one trick he used was distant mic’ing. On those early Zep albums, he lived by the old studio maxim, “distant makes depth.” So go ahead and try mixing in room mics along with the close mics on your amps.

Compare and contrast.
When you’re arranging your songs, mix in some thinner, lighter sounds along with your fat and heavy sounds, giving each one its time in the spotlight. If you follow a thin, light tone with a fat, heavy one, the latter will sound even fatter and thicker than it would if it was surrounded by other fat tones.

Split it.
Create a stereo version of a mono sound. A common way to do this is to use a very short delay—try between 12 ms and 24 ms with no feedback or regeneration. Just place the dry sound on one side of the stereo field and pan the delayed sound to the opposite side.

There are some cool variations of this technique, including the old Van Halen trick of panning the dry sound to one side, and its reverbed counterpart to the other. You could also go dry on one side, with chorusing, flanging, or phasing on the other. Try copying your track and placing the original on one side, with the copy on the other, nudged a few milliseconds earlier or later. Then cut some frequencies and boost others slightly with a multi-band equalizer on the original track. On the copied track, cut and boost the opposite frequencies by the same amounts.

Double up.
Record a pass of your guitar to one track and then record a second pass to a second track. Playing the second track as close to the first as possible will result in a big sound. Varying the second track slightly (not going for an exact duplicate of the first) can give you a different, bigger sound. Then pan the two tracks opposite one another in the stereo field. Instant thickener!

A variation on this technique is using a completely different guitar voicing for the second track—maybe a totally different guitar and amp. You could even play an octave up or down on the second track. Fine-tune the two sounds so that they complement each other in the mix.

Layer it.
Why stop with one double? You can add more doubled tracks on top of the original, be it multiple octave-up versions, octave-down versions, and so on. Keep in mind that there is a point of diminishing returns—the sound can get so washy that you lose presence.

The trick is careful mixing. I once created a huge guitar tone by playing the basic track and panning it to the center, and following that by playing a double for each side. These were mixed back in volume from the original. Then I played an octave double—using an inverted chord voicing—twice. These two tracks were also mixed way back in volume compared to the original. The result? A full, thick sound that didn’t come across as overly processed or multi-tracked—the original track carried the day. The doubles were there for added heft, harmonic content, depth, and width.

Pitch it.
Place your dry guitar track in the center of the stereo field and use an effects send to run it to a stereo pitch-shifting plug-in. Set the left side of the pitch shifter 3 to 5 cents sharp and the right side 3 to 5 cents flat, and then mix in behind the dry signal to taste. This creates a cool, chorus-like sound, but thicker and sans the warbly motion. Don’t shift too far, or the effected tracks will sound out of tune. Another option is to place your dry signal on one side of the stereo field, and both pitch-shifted versions (sharp and flat) on the other side of the stereo field.

Combine and conquer.
There is no rule that says you can only use one of these techniques to craft your massive guitar track. Try combining effects processing with doubling (and varied inversions and voicings) to achieve a huge tone. Using these techniques, your tracks will bulk up nicely and fill out your songs more than ever before. There is simply no reason to suffer with thin, wimpy guitar tracks in your productions!

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