by fuzzfactor | 2025-08-19 06:02:11+00:00
Thank you for your observations too, I might be able to make it a bit clearer.

Mainly those are my observations about the tube input (preamp) stage in general, while the Twin schematic was present as a good example.

Those little preamp tubes are the ones that can last decades since they don't usually carry much current and the cathodes & anodes don't wear out very fast. Never throw them away from mere age, and always pick the tastiest one from your collection to sit in the socket that you'll be plugging your guitar into.

It's the very-hot-running output power tubes that need more frequent replacement, but they can still last years and even if way overdue for replacement still perform quite well anyway.

Now you got me started on that ;)

You probably know that the higher voltage delivered to the big power tubes, the more power output that can be achieved.

The original 6V6 and 6L6 of 1936 were metal tubes, not glass, the 6V6 steel envelope is the same diameter as the octal base & socket, the 6L6 metal base & tube is slightly larger diameter, and overlaps the socket a bit, to accommodate the more sizable inner elements.

If you look carefully at their characteristic curves you see great similarities except the smaller 6V6 is limited in both voltage rating and power output.

These tubes will arc if you try and overvoltage them, mainly on spikes, plus people didn't actually like metal tubes better so glass versions were soon available, having about the same inner structures as the metal originals. People kept pushing them and the 6V6 is physically too small to handle much more than it did to begin with, but the 6L6 had further revisions over the years at many companies, and ideally the 6L6GC could handle much higher voltages and currents than originals.

The ratings actually remained quite conservative so that every tube in the box is supposed to be able to handle the specs, so engineers didn't return whole boxes unless they were really sub-par.

For guitars you don't need to stay within the most linear low-distortion operation points, so design voltage is often maximized until a ceiling is hit.

With good tubes it's often the socket which became the limiting factor, and an arced socket can persist for years if not identified, casting repeated doubt on new replacement tubes. All it takes is one slight arcing event between pins and the socket and tube base will get a small burned spot then arc more easily from then on.

The real culprit is the EMF from big speakers pushing back through the audio transformer, and 10 or 20 percent distortion is not what does it, but square waves can find the weak points all too often. The transformer steps down the high-voltage tube winding to the low-voltage speaker winding so drastically, that a return spike from the speaker box goes in reverse and can be stepped up to where a 450V spike appears on top of 450V that's already there at the tube pins, and a 900V spark jumps. A lot of times the initial damage is from blowing a speaker at high power and the instant removal of a big load sparks between tube pins, cooks the socket bad enough to make everything else suspect from then on. Plus after that the tube can test good even though its got a carbon trace in its base, if the inner elements were not damaged, since the tube tester is not near max voltage, nor realistically dynamic. Probably still just fine for 250V of hifi.

Better to have a socket arc than a transformer short internally though. The hotter the transformer gets, the less effective the thin enamel coating on the copper wire works as insulation between those tightly wound coils too.

Another problem with guitars is the hot tubes are right there where the loud speakers are and it rattles the daylights out of them, and when they're hot they can be more delicate.

With some of the "space-age" power tubes for hifi I could get plenty of life at even higher voltages than for guitars because the cathode, anode, and screen wear comes from the current carried, not the voltage, as long as the voltages don't arc or misbehave.

Regardless of voltage, it's the current carried by the electron beam from cathode to anode that sucks the cathode for all its got, and impacts the anode until it gets as hot as it can stand. Screens can light up pretty bright sometimes when its cranking. Maybe even some glowing spots sometimes on the anode itself. You may not want any glowing at all, but it's still your "maximum electron beam" for the situation. Once you reach that current limit, the "simple" way to get more power is to increase the voltage carried by that current through the transformer.

That requires special capacitors and special transformers, then when you get the anode voltage high enough you need to cut back on the screen voltage so much that the screen can arc to the anode if you're not careful.

You've just got to know your limits, sometimes take peoples' word for it, other times find out for yourself.

I like the curves they've got, they continue to serve me well, but I don't always stay on the chart :)

Anyway, that's all I know about Nuvistors, obviously not squat but would probably use one in an audio preamp anyway, nowhere near a vibrating speaker box though ;)