Are You Sure Leo Done It This Way?

Leo Fender was a tireless innovator, and his tube guitar amps set the standard for the industry that continues today.  Many companies make clones or clone kits of classic 1950s and ’60s Fender amps; many try to be exact copies.  Most current production tube amps use circuits based on, or very similar to, those that Leo Fender tweaked over half a century ago.  Of course, tube circuits predate Leo’s amps, and he paid license fees to Western Electric on many of those 1950s amps for using their circuits.  A triode preamp circuit is a known quantity, and there are only two or three different variations of the basic stage one preamp.  The layout is one thing, it’s how you voice those tube preamp and power circuits that shape the tone or sound of the amp.  But what about the components of the amp build?  Do you need “vintage” components in a vintage-style amp?

Carbon Composition Resistors

Many boutique amps and vintage amps and kits use carbon comp resistors. Why?  Well, that’s what Leo used, and those amps have that long sought after sound.  Modern resistors are carbon film or metal film, and most modern amps don’t sound like a tweed Deluxe or Bassman.  Leo didn’t use those, and many people want the look and sound of a carbon comp resistor, because that’s what the vintage amps had.  But, wait a minute, there’s more to it than that.  Do carbon or metal film resistors sound the same as carbon comps?

What does a resistor sound like?  Nothing, unless you drop it on a hard floor in a quiet room.  Tube amps are built on basic math- multiplication and division.  Current equals Voltage divided by resistance.  Put a 100,000 ohm resistor on that 300 volt DC supply and it works out to 0.003 Amps, or 3 milliamps (300 / 100,000 = 0.003).  So, what’s the difference between a carbon comp 100K 1/2 Watt resistor and a 100K 1/2 Watt carbon film resistor or metal film resistor, and what are the assets and liabilities of each one?

Carbon comp resistors look cool.  They’re brown with colored stripes around them, and they look (and perform) just about exactly like those ones in Leo’s 5E3 tweed Deluxe or 5F6A Bassman, so they must be good.  Leo used them (so did other amp builders of that era).  OK, so why did Leo use them?  Here’s a real shocker.  He used them because it was what was available, and the price was right.

Carbon comp resistors have know qualities.  Among these are poor stability, with at best a +/- 5% tolerance, and it’s usually wider than that.  They will change value when stressed with over-voltages, and if internal moisture content (from exposure for some length of time to a humid environment) is significant, soldering heat will create a non-reversible change in resistance value.  Outside of guitar amps, they’re rarely used because modern resistors have better specifications, such as tolerance, voltage dependence, and stress. Carbon comp resistors also cost more today than carbon or metal film.

On the other hand, carbon film resistors have a working voltage of up to 600 volts, and have operating temperatures of -55C to 155C (-67F to 311F).  If your amp ever gets that hot or cold you will have other issues besides resistor performance!  Metal film resistors typically have a 1% tolerance, and possess good noise characteristics and high linearity due to a low voltage coefficient, and they’re very stable.

I like to use rugged MIL-SPEC, low noise, 1% tolerance, high temperature, 500V metal film resistors.  I also use commercial metal film and carbon film resistors. I use some carbon comps, but mostly to satisfy the desires of vintage amp buyers, not for any performance reasons.  The way I see it, if Leo Fender was alive today and building amps, he would use 1/2W carbon film resistors in his amps, because they are the lowest price resistors that have the right electrical and mechanical characteristics.

Cloth Wire

Old Fender amps and guitars (and most old amps and guitars) used this cool cloth covered wire.  Why?  Pretty much for the same reason that they used carbon comp resistors. It was cheap, it had the right characteristics, and it was what was available.

Today, we have much better wire. We have silver-plated, MIL-SPEC aerospace grade, Teflon coated, 600V, 200C wire.  It’s easy to work.  It’s flexible yet holds its shape, it takes solder well, and is small diameter, which is helpful in a cramped chassis.  It’s also pricey.

Less costly is the MIL-SPEC Tefzel coated, tin plated wire. It has similar ease of solder and work characteristics of the Teflon wire, is rated 600V, 150C, and is about half the cost of the above named Teflon wire.  We also have high-temperature MIL-SPEC PVC wire, which has many characteristics of the Teflon wire – takes solder easy, easy to work, small diameter, and is rated 600V, 105C, for about 1/3 the cost of the MIL-SPEC Teflon wire.

There is a new cloth wire out for those who prefer the vintage look.  After all, you may want to show off the wiring in your amp.  This has a cotton cloth braid over a 600V, 105C PVC insulated wire. It’s much easier to work with that the old-style cloth wire, and has superior performance characteristics to the good PVC wire, although it’s still not as easy to work with as the Teflon or standard PVC wire.

Use the new cloth-covered PVC if you absolutely must have the look of cloth wire in your amp, otherwise use a high temp PVC, Teflon, or Tefzel wire.  It’s also the same price as the aero-grade Teflon wire!  Don’t use any PVC or other wire that’s rated less than 600V or 105C.  85C wire has no place in a guitar amp.

Capacitors

I’m not sure I want to open this hornet’s nest, but here goes.  Capacitors are used as power supply filters (to reduce AC ripple in the DC power supply voltage), to block that same DC voltage from the AC signal path and couple tube gain stages together, and to bypass tube cathodes so that the amp gets the desired sound and gain.

Many of the capacitors Leo used aren’t available today.  Electrolytic capacitors are used as filter and bypass caps, and the ones we have today are very good. I use high quality European made electrolytics.  The controversy comes in when people talk about coupling caps.  The ones that Leo used aren’t made anymore.  No surprise.  There are lots of things from the 1950s that aren’t made anymore, and in spite of nostalgic dreams, much of what we have today is better. Televisions today not only outperform the old tube sets, they cost less.

Some companies today sell very expensive vintage style capacitors, and lots of people buy them.  You can easily find $120 .022 tone caps for you guitar, but a $0.79 Radio Shack ceramic disc will give you the same sound. The only signals that go through tone caps is shunted to ground!  You never “hear” any of the signal that goes through the tone cap.  One company makes vintage size electrolytic caps that are nothing more than a modern small size cap placed inside a larger container! They cost more, of course.

I like to use high quality Mallory 150 coupling capacitors.  Mallory 150 has always set the standard for top quality metalized polyester film, high voltage axial coupling capacitors, and that’s what I like to use.  I use high quality ceramic disc capacitors in tone and oscillator circuits.  I tend to stay away from orange drop polypropylene types.  These caps came into guitar amps in the 1960s, and are the heart and soul of the 1970s CBS Fender silver faced amplifiers.  They’re very stable, that’s the good part.  They were cheap in the ‘60s and ‘70s.  They’re stable and cheap and that’s why amp builders used them, not because they had superior sonic characteristics.  If you can find one, look up an old TV repairman and ask him if orange drops give a TV a better picture that a metallized polyester cap.  He’ll probably look at you as if you’re totally nuts.

In Summary

I build vintage-style amps.  To my ears and to the ears of those that play and hear them, they sound very good.  They have that vintage amp sound- the breakup, the tone, the dynamics, and the touch-responsiveness that make those 1950s tweed amps so much revered.  I get that sound by tweaking the component values while using modern high-quality components that offer better characteristics than the old-style components.  I don’t get that sound using vintage style carbon comps, orange drops, and cloth wire.

If you want the old-school components, I’ll make it that way.  It will certainly make it more “vintage”… and it will make it more expensive.

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