Feeling like a teenager, with a hard to please English teacher, I did much fretting over my assignment, “The 18 watt Marshall: Then and Now.” Usually this led to heavy eyes, followed by a sound sleep. And in typical fashion, reminiscent of my imprisonment in the NY public schools back in ‘76, I waited until the 11th hour to get serious. We’re talking about a severe case of writer’s block compounded by every possible distraction.

The 18 watt amp must be the most documented amp in existence today. There are users groups, an online community dedicating to building them, hosts of kits offered, and every possible tweak explored and beaten to death. The latest topic of conversation in those circles is “Should we buy into the 18 watt PCB kit?” Honestly, that’s where it’s gone. They’re practically raining from the skies, washing up from the churned gulf, or getting hauled out with the Friday trash. The perceived rarity led to a flood of clones. Oh well.

So what is it that keeps Mickey wide awake, burning the midnight oil? Octal tubes. Big, fat, powerful, liquid or crunchy, power amplifier tubes, of the types 6L6, KT66, EL34, 5881, 6550, and the diminutive and understated 6V6. But hey, you might say, “Talk about overdone, Mick – you’re going from over-documented to l’typicato!”

Perhaps… but what has me excited is what is fundamentally the same about all these tubes, and what has been hitherto under-documented. The fine art of tube mismatching, both from a push-pull pairing all the way to output transformer primary mismatching. Sound dangerous? You’d love to just plug those sweet little 6V6 tubes in your Twin, but are scared to death? Feeling wide awake now?

Surfing the internet tides, we find scads of information, much of it from the tube manufacturer literature of the day (the proverbial horse’s mouth), discussing power bandwidth, and impedance matching of output pairs. Much is said about “Reflected Loads” from speakers, and both tubes and x-former hardware being unable to take the current changes. So what are they talking about? Every output type of tube is rated for nominal impedance matching of the output transformers.

In the world of linear electronic devices, reducing the loading of an operational amplifier by 50%, would result in the transistors attempting to deliver twice the current. Meltdown is imminent. By applying the same logic to the tube, we assume we just fried a perfectly good tube. But in truth, it’s not the case. Tubes are simply not linear devices; so what does the tube do? The answer is partially why we prefer the tube to the transistor in the first place: Distortion.

Distortion of the amplifier stage, while desired by many guitarists, is undesired by virtually every other application of the tube. Distortion is the product of clipping occurring in the output section, where portions of the signal, as measured on a scope, would exhibit DC like characteristics. The signals appear to be clipped – kind of like your lawn! When the tube literature of the day was written, the manufacturers were unanimous in their pursuit of squelching any distortion in the circuit. Hi-fi stereos, communications devices, broadcast radio – all of these applications are diminished by distortion. So nominal loads were calculated to reduce THD (total harmonic distortion).

I keep saying “Nominal Loads”. This is because audio amplifiers do not see a fixed load. In truth, your speakers have impedance anywhere from 6 ohms to 100 ohms, depending upon the frequencies. They are a reactive load. As such, the load reflected back through the transformer to the output amplifier is always changing! And THD is calculated, in a well documented hi-fi power amp, against frequency. Because the tubes, against, different reflected loads, are delivering varying THD. More or less of the signal component power is in the form of distorted signal.

By swapping tubes, and not matching to the nominal impedance is nothing more than what may be happening in normal operation – other than shifting the power bandwidth (frequency response at specific power) and THD. Not to mention this nominal impedance mismatch at the output transformer is being done every time somebody yanks a couple of the output tubes! Furthermore, the output transformer is a device which takes high-voltage at low current swings, and converts it to low-voltage at high-current swings. So the portion of the amp which really wants to be matched as closely as possible is the speaker impedance.

In closing what may “suck” for every other user of the vacuum tube, becomes the artistic palette for today’s guitar virtuoso.

So what considerations should we give when swapping tubes?

First and foremost, be sure that the tube you are swapping does not exceed the filament (heater) supply of the OEM tube. This will fry your power transformer, the single most expensive piece of equipment in your amplifier.

That your amplifier has enough bias voltage to properly bias the tube in question. Improperly biased tubes will themselves have a reduced lifespan.

That we do not exceed the maximum voltages of the specified tubes

So, back to that late night dream of plugging the wonderful little 6v6 into my Twin. Can we do it? The answer, in a nutshell is yes. The plate voltage and bias of my Fender Deluxe is the same as the Twin reverb (V+ of 420V and –37 bias), and the 6v6 has a lower filament current draw. Be sure to bias, and let me know what you think!

NOTE: I would never plug the mighty 6L6 into my Fender deluxe, because I then WOULD fry the transformer.

Enjoy,

Mickey C.

Soultone Amplification, Inc.