Nuvistor Valves
r-type.org
r-type.org
As an adult Thomas J. Lindsay’s books on building small regenerative vacuum tube receivers caught my attention — I also got caught up in building both tube-based guitar amplifiers and hi-fi audio amplifiers. These allowed me to dive into tubes and finally learn about them — you know, posthumously as it were.
After initially thinking that tubes were probably inferior in all ways to solid-state, I came to find them to still be very capable and even arguably better — at least with regard to sound amplification. Somehow too I had imagined in my mind they were outrageously dangerous to work with — thousands of volts — and assume,ed they were fragile and quick to "burn out".
The circuit I used however never went over 300 or so volts (to be sure, you still need to be careful with these circuits in a way you may not be familiar with if Arduino circuits are all you know).
The tubes have never seemed to burn out for me — even after one or two amps have been my "daily drivers" for well over a decade (two decades?) now. Perhaps other circuits used less capable tubes or pushed them to their limits? Perhaps other enclosures like TV's did not allow adequate ventilation? I don't know.
And as for fragility — I mean they are glass, but the tubes I used in my hi-fi amps were NOS from WWII bomber radios. They seem to hold up to a good deal of bouncing around.
I've used the same tubes at a very reasonable 325V in a SET (actually single-ended pentode) hifi amp (built myself).
TV tubes would often burn out because of transients from switching channels.
[0]https://schematicheaven.net/fenderamps/twin_reverb_ab763_sch...
When you plug your guitar into input 1, its arrow contact with ground is disconnected, leaving the 1 Megohm resistor as the only direct path.
That's a million ohms "isolating" or "connecting" your sensitive guitar signal with ground, whichever way you want to look at it.
So your high-impedance guitar signal will not be all lost by finding an "easy" path to ground, and a nice strong signal appears at the input grid of one of the triode structures found within the dual-triode 7025 tube.
The 7025 was simply a 12AX7 that had been built for lower noise & microphonics, or selected for it, and given the 4-digit military-industrial tube number instead of the 12AX7 nomenclature recognized by consumers. You need low noise in the input stage more than anywhere else.
Follow your signal carefully and you will see that it has to pass through some resistance before it hits the grid, from input 1 both 68K resistors are in parallel, resulting in a 34Kohm working connection between your guitar and the grid.
Well that 34K is almost nothing compared to the megohms of internal impedance of your magnetic pickup. Once some waveforms have made it out of that highly-coiled super-long thin-ass copper wire inside the pickup, which makes your pickup have orders of magnitude more impedance ohms than its measurable DC resistance ohms, 34K is not even a speed bump.
Might as well be zero ohms and it sounds about the same but tubes kind of like a little something on that pin for stability.
Pedals can be good for more than one reason, like simple boosters that are perfectly clean and put out no distortion of their own but overdrive that input triode to complete metal mayhem, or sophisticated pre-processors with no additional gain at all. However, your signal will usually have a lot lower impedance coming out of a pedal into the amp, and so the interaction with the grid will be quite dissimilar in some ways, for better or worse.
Plus that's usually adding many more components between your instrument and that grid, and some of these solid-state components can be so tasty you don't want to do without them completely, but when you think about it, sometimes a single pedal will have more components between your guitar and that sensitive grid, than there is between that grid and the speakers. This is a legendary top pro Twin Reverb, and look how simple the rest of the vintage circuit is, before your music goes through that monster high-voltage audio transformer and hits the speakers. The tubes are doing a lot of work here.
Remember the Twin Reverb became the choice of pros and stayed that way for years before pedals were even invented.
IOW when you plug straight in is the only time you are "magnetically coupled" with the first grid, aye to the vacuum of space.
There is no further contact with what most people call "matter".
And from that point on your music is being sprayed through space, directly from your fingertips like no other way ;)
Certainly it is very wonderful to observe the relative simplicity of tube amplifier schematics, which rely on a mere handful of passives for bias and tonestack, compared to solid-state amplifiers, that require complicated arrangements to provide negative feedback necessary to stabilize the transistors.
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 ;)
I'm a musician, an an electronics expert, though I don't use a tube amp myself. There's a lot of chatter on web forums about replacing tubes and capacitors, suggesting that it's done much more often than necessary. Someone's amp will get crackly, so the first thing they'll try is new tubes. Then capacitors. Finally the flaky pot or connector that's the actual root cause.
Gingerly repaired it served me many years in the late 1960's and still sits in the shack. Sporatic-E was strong in those days, to the extent you could easily DX low channel TV channels cross country. I turn it on every few months for 10-15 minutes in an attempt to keep the electrolytic capacitors polarized.
that is one of the few domains where valve technology still holds a superior position, in terms of product lines
Any fool knows a dog needs a home.