Nuvistor Valves
14 comments
·August 17, 2025JKCalhoun
i_am_proteus
Guitar amps that routinely blow tubes usually operate past maximum rated plate voltage. As an example, the Fender Twin Reverb[0] runs 6L6GCs[1] at 460V plate voltage, above the stated class-AB pentode max of 450V.
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...
fuzzfactor
Look closer at that Twin Reverb schematic.
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 ;)
analog31
There's a chance that in the heyday of tube electronics (mostly TVs), tubes were being stressed to within an inch of their life for cost reduction.
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.
tibbon
Tubes burn out a lot less than most people think. I’ve got working tubes that are 60 years old and in active use. I have a tube tester and they do fail, but not ever 1-2 years like some think is a needed replacement cycle.
ycui1986
tube’s failure is such that if they survive the infant fatality, they will be more robust and reliable than transistor. The transmitters on Voyager probes have been going for 47 years. Those tube were pre-screened ones that survived from infant fatality. It’s like sea turtle, if they survive first few years in the wild, they will a very long lifespan.
wa2flq
The Lafayette HA460 6M AM transmitter used a 6CW4 Nuvistor as a preamp. Mine survived a lightning strike to my 6M beam, which melted the Gamma Match and destroyed the front end harmonics filter. All tube, expect for a set of DC inverter transistors that allowed for mobile operation. Relatively good RF sensitivity for an AM rig of the era.
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.
ringeryless
could this be used in a guitar amplifier circuit?
that is one of the few domains where valve technology still holds a superior position, in terms of product lines
JKCalhoun
I expect so. Pull up a spec sheet for the Nuvistor in question and see if meets the needs. Triode or pentode ... there are guitar amps and topologies for either. Depending on their specs, they might not work in the power amp stage of a guitar amp but perhaps the pre-amp — so you could get a hybrid.
fuzzfactor
I put my electronics lab in mothballs over 10 years ago but up until the end my main audio scope was an HP from the 1970's that was all solid-state, except for the Nuvistor on each channel's input.
"Valves" had always been a mystery to me. When I began to really get into electronics, vacuum tubes were a kind of distant echo from my childhood that I barely remembered. As a kid I remember staring into the back of the small B&W television we had when I was young — the insides looking to me like some kind of Things to Come cityscape in miniature — all lit with orange neon. And there too was the ubiquitous "tube tester" in the Rexall Drug Store (looking somewhat like a prop from perhaps Lost in Space).
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.