Vacuum Tubes running on low voltage - workshop experiment
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- Опубліковано 15 кві 2019
- A Valve / Vacuum tube preamp circuit on the breadboard being demonstrated running on very low DC voltages.
Please be kind I am much better at designing and building electronics stuff than at making UA-cam videos
(As predicted, I did get a copyright strike but I'll leave it since I'm not monetising my channel so Lorde can feel flattered that I chose her out of all of Spotify!) - Наука та технологія
Before the transistor came into its own, portable radios were tube types with small glass envelope tubes. I recall that one such tube was 1T4, which required a filament (heater) voltage of 1.5 volts from a single dry cell. The B+ voltage was 36 volts I seem to recall or, perhaps it was 48 volts. These batteries were expensive, for me at least at that time, and I made do with a crystal set.
In the late 1950s there were automobile radios that had vacuum tubes designed to operate in their optimum gain and linearity from only the battery 12VDC. Their filaments were 12VDC operated also. The audio output stage was transistorized. This allowed the radio to operate without the need for a HV DC power supply using vibrators, power transformers and rectifiers for the HVDC usually seen in automobile vacuum tube operated radios only a few years earlier. Of course this approach did not last long as transistors became more reliable and could survive in automobile temperatures etc. Progress moves on!
This is in fact an interesting topic. Made some LTSpice simulations with JCM800 preamp schematics with low voltage ( down to 12 V) and this confirms that the tubes actually work with low voltages too in reality.
Thanks for sharing your observations and your curiosity.
Thanks there, I did enjoy
Very cool. Thanks. I expected that certain components would have to be adjusted to make it work at low voltages (bias, etc.) so this was surprising. I might have to experiment with this now.
This is really interesting, over the last few months I’ve been making low voltage tube sound making things, oscillators and filters, and getting really exciting results. It was particularly of note that you mentioned that they work at their most linear at the high voltage; I’m trying to make strange / novel sounds rather than high fidelity, so perhaps I’m actually better (as well as safer - total amateur here) in my low voltage world…
I tried an experiment yesterday with an old 6v6g running on a 30 volt bench power supply (and another for 6.3), old output transformer and a speaker. Actually got decent audio. A comfortable listening level was as loud as it would go but nice that it worked. Nice safe way to show people how valves work. This morning I added a couple of cheap buck boosters, one set at 6.3 the other at its maximum of 28 volts. Input 5 volts, draws 700ma. So a phone charger could run it. Might build it into a transparent case as a novelty amp
Presumably they needed that sort of voltage a century back when the vacuum was so poor. We've presumably improved the getters now too - but I'd be interested to see the linearity argument put to the test. If they were sufficiently linear at 20V that makes them usable for soft-clipped input stages.
I would like to see more of this type of experimentation.
Why tubes need high voltage?? i dont find the answer yet
In case you didn't (from memory of HS physics) the voltage is required to set up a large differential between the plates. The heater really only gets the electrons bouncing about off the surface, the vacuum allows them to fly between the two plates (in a diode) or be controled in a triode or greater. You can do this in free air if you have enough voltage - per a Tesla coil although that's actually moving through a cloud of ionized particles. Again (from memory) as I recall the electrons jump from one atom to the next making the first one slightly positive which attracts a nearby electron.
Is there any papers or other video that go into this in further detail? One of my uni projects is to create this scenario for a piece of audio gear. Is it literally just because you're keeping the filaments hot/on. Thanks
Just that particular tube - the 6N6 - seems to function down to very low voltages. Others don't. No tube would work without the filament on, but the usual thinking is that the anode requires high voltage to function. The observation in this video proves that isn't always the case.
Would like to see that on an output pentode
Do you have the whole schematic for this preamp ?? would be nice to have one, to get some ideas myself for a preamp im thinking to build for my guitar
Yep - here it is atrad-audio.co.nz/index.php/3ch-line-level-preamp/
It’d better to apply a THD+N meter to compare the low voltage to high one .
My only nitpick is that the 6N6P seems to be a special case made exclusively in Russia. The datasheet (though it is in Russian) has operational curves going all the way down to 20v (but obviously reduced amplification per se). So the question is what design features made these tubes so low voltage? Interestingly they seem so plentiful that they are affordable imo.
Be interesting to try this with an ECC99 which is more-or-less equivalent, see if you get the same result... I don't have any ECC99s handy else I would.
@@A_RosnerNZ "more or less is right." They are similar but not identical. There low price represents (evidently) that they aren't swappable in the same socket because of heater differences imo. What would need to be done is put each of them on a curve tracer and note their differences. Evidently the 6n6p has a specific cathode material which makes it easier to boil off electrons and hence can operate at lower voltages. The ECC99 is more ruggedized version not necessarily that electrically different.
Have seen many tube preamp,s etc run on low voltage.
I have a valve/tube intercom that works off a 1289 4.5v torch/lantern battery and a 67.5v battery (BT101). Want to build a psu so the intercom works off 4 × 1.2v rechargeable high capacity D cells (4000mAh to 7000mAh).
I like the idea of using a small power tube in a line amp. I imagine that even better results could be had if choosing R values specifically for the lower B+.
Using Fb helps as it compensates for the higher distortion (like transistors). I have toyed with the idea of cascaded 6n1p or 12au7 with plate to cathode Fb. Might even give enough gain for tone controls.