# Using lower plate/screen vacuum tube voltage

I recently got a few antique '1T4' type vacuum tubes from a store. Digging around on the internet, I found its datasheet. From this, I can see the following:

The heater(filament presumably) voltage is around 1.4V, which is relatively easy to obtain. However, in the Characteristics section for a class-A amplifier, it lists a plate and screen voltage of 90 and 67.5V, something which is not exactly 'safe' by any means(requires a transformer).

Is it possible to apply a lower voltage(say 12 or 24V) to the plate/screen while still retaining functionality? By functionality, I mean it is still able to operate as an amplifier.

Additionally, why is the voltage for Grid 1 listed as negative?

• In the days of vacuum-tube, many tubes (or valves, if you prefer) were operated using a 12 VDC battery for plate voltage! Certainly, that's sufficient for RF amplifier, mixer, detector and audio preamplifier. However, to get enough current to power a loudspeaker, higher voltage is needed. Here's a schematic for a receiver with 18 VDC plate supply: tr.pinterest.com/pin/144889312992226572 Commented Jul 7 at 4:53

It's a pentode (with the suppressor tied to the cathode voltage.) And I'd like to avoid discussing it and instead stick with operating it as a triode -- grid 1 only. The datasheet has a simple chart for that (admittedly with the screen set to a nominal voltage) and, besides, your questions make it clear this is a good place to start.

Suppose you were expecting to operate this with a $$\B^+\$$ voltage of $$\150\:\text{V}\$$ and were using a plate resistor of $$\20\:\text{k}\Omega\$$. I've placed that load-line in blue:

I've selected a quiescent plate voltage of $$\100\:\text{V}\$$, allowing a swing of $$\\pm 10\:\text{V}\$$, as shown above with the thick green line along the x-axis. The quiescent plate current is $$\2.5\:\text{mA}\$$.

In rough terms, it appears that the grid voltage swings from $$\-600\:\text{mV}\$$ to about $$\-1.9\:\text{V}\$$, with a quiescent point of about $$\-1.2\:\text{V}\$$. The quiescent point isn't exactly in the middle so this suggests some distortion over the range. But not a lot.

The gain will be about $$\\frac{90\:\text{V}-110\:\text{V}}{-600\:\text{mV}-\left(-1.9\:\text{V}\right)}\approx -15\frac13\$$ and $$\g_m\approx 770\:\mu\text{S}\$$.

These are often set up with a grid leak resistor galvanically tying the grid to ground and a cathode resistor that, with the quiescent current applied to it, jacks up the cathode a little bit to achieve the relatively negative grid voltage. Here, it suggests that $$\\frac{1.2\:\text{V}}{2.5\:\text{mA}}=480\:\Omega\$$. So a $$\470\:\Omega\$$ resistor would likely be selected. But keep in mind that different tubes of the same marking may settle differently.

The Characteristics and Typical Operation specifies a plate voltage $$\90\:\text{V}\$$ and a quiescent current of $$\3.5\:\text{mA}\$$, saying $$\g_m\approx 900\:\mu\text{S}\$$ (using the same screen voltage indicated in the chart.) This is consistent with what was just computed for a slightly different operating point.

With all that out of the way, the reason that the grid voltage is usually (almost always) operated negative with respect to the cathode is for about the same reason that a JFET's gate is operated so that it is reverse-biased with respect to its source. If the grid were positive, it would attract electrons from the cathode and capture many more than if it repelled them. Such a grid current is not desired.

Just think of it as a JFET, more or less, except for the power supply rails.

If you look back at the above curves and imagine a small plate voltage of, say, $$\15\:\text{V}\$$, you'd wind up in all that mess with the curves diving back towards the origin. There's little good to be had there in the Ohmic region.

• "you'd wind up in all that mess with the curves diving back towards the origin" -- ah, but screen voltage wouldn't be 67V either -- they'll all be shrunken, roughly by the mu_g2g1 ratio (which says that reducing Vg2 is like reducing Vg1 by a proportionate amount), and Ia even for Vg1 a little positive (1-2V say) will be quite small (<1mA?). The curves will still be orderly though. Unfortunately they don't actually give mu, but an operating point can be chosen from the triode curves, where Va = Vg2 is plotted. Commented Jul 7 at 10:41
• @TimWilliams To add, I didn't use the curves where the screen is tied to the plate voltage. I probably should have done do, but it would have complicated the writing and muddied the explanation. I'll hold short except to note that the plate impedance drops precipitously. If the questioner wants to try these devices at low B+ voltages, I can only imagine much logbook work ahead. Commented Jul 7 at 10:48

You can run tubes with low screen and plate voltages. Transconductance and dynamic range will be lower than nominal, but they work.

The 1T4 is designed for maximum transconductance at 0V grid bias relative to the negative cathode pin. Negative bias reduces plate current and transconductance.