Part 1: Adventures in building a Tube based Mic Preamp

Found some low voltage tubes on eBay, and that started a curiosity in researching and learning more about the history of these “space charge” tubes.


Basically, Space Charge tubes are designed to operate using low voltages, instead of using high voltages (150V to 450V) typically used in normal tubes.

Since these space charged tubes operate in such low voltages AND low current, we can’t expect to get a lot of power out of these tubes.

Thinking of driving a big speaker? Forget about it. It’s not going to happen at these low voltages and low current. Driving a headphone transducer? Maybe possible with some additional current boost circuitry. That’s it.

But for my own purpose, I’m not interested in driving a guitar speaker or headphones. Instead, I’m more interested in capturing the “tube essence”, i.e. the unique and special characteristics of vacuum tube sound… and to try doing that without HIGH VOLTAGES.

So I’m primarily interested in voltage gain amplification — increasing microphone-level signal to a useable line level signal — just enough (with some extra oomph!) to drive the inputs of an AD converter or audio interface. As I said, this is a journey of adventure!

ASIDE: Now, whether you’re a tube purists or not, I am not going to argue with you. If you think you need 300Volts – 450Volts to truly have the sound of a tube preamp, you’re entitled to that opinion. But don’t forget, at the end of the day, the voltage level output of your “real” tube preamp isn’t 300Volts or some high voltage DC. The output of your tube preamp going to your audio interface, or to the next equipment in your signal chain is just tens of volts rms, for example 13Volts rms is equivalent to +24dBu output signal. What I’m saying in this article is, if it’s possible to have the same kind of tube sonic signature without the danger of working with high voltages, and still be able to achieve the same output dBu levels, won’t you go for that? Or at least give it a try and listen?

So, what’s so special about tubes. They say it’s the “warmth”. But what does it really mean?
Some say it’s the harmonics? Again… what does it really mean.

So for me to truly “understand” these things, and the best way to learn about tubes, is to actually play with them on a breadboard. I want to see waveforms generated by a vacuum tube and see how they behave — and see if it’s possible to use them in some kind of circuit for our benefit.



Above is a photo of the first circuit I tried, built on a breadboard as an experiment. Just your basic tube single-ended circuit you’ll find in any tube handbook or datasheet. I applied some voltage to the pins for the heater, and add some plate and cathode resistors and see where it takes us next. It started glowing, and got hot to the touch. Cool! At least, the tubes I got from eBay isn’t a dud or dead. Our first good news!

Then I started applying some signal to the grid, and monitoring for any output signal. Hmmm… I was getting some output signal, but it’s not pretty. Doesn’t even resemble the input signal. I need to do more reading.

I start reading about biasing, and selecting plate current and grid voltages. It’s like a trifecta of parameters, with many possible solutions. Looking at the datasheet, I decided to start somewhere in the “middle” of the range, and I choose a plate current and a plate resistor so my plate voltage sits at roughly 1/2 of my supply voltage.

By setting my “starting” position at the midway point, theoretically I can have maximum signal swing without clipping too early at the positive or the negative portion of the waveform. You see… if I position my midway point too close to my supply voltage, the waveform will have it’s positive half chopped off too early. And conversely, if I position my midway point with the voltage too low and close to 0 volts, then my waveform will have it’s negative half chopped off too. So the best solution is 1/2 of my supply voltage for maximum signal excursion on both the positive and negative cycle.


Finally, after some tweaking, we achieve AMPLIFICATION! To be more exact, we have VOLTAGE GAIN!!! From the scope screenshot below, you can see we fed it a small input signal (YELLOW TRACE), and we got a larger output signal (BLUE TRACE). Yes! We’re done here… pack up, and let’s go. (just kidding)


For me to see this tube circuit working and showing voltage gain is pretty cool. 🙂 The values are not optimized yet, but voltage gain amplification is working. That’s a baby first step.

Turning ON the MATH FUNCTIONS of my scope and doing an FFT (Fast Fourier Transform) on the output signal, I can see good 2nd harmonics, right after the fundamental frequency. And very little 3rd harmonics.


It’s noted that EVEN harmonics (2nd, 4th) enhance the signal… makes it sound more thicker/musical. While ODD harmonics (3rd, 5th) are the unwanted kind, sounding harsh and nasty. Happy to see we have good 2nd harmonics with this setup!

ASIDE: Harmonics are by-products of the non-linearity of the amplifier device being used. If the input frequency is 1Khz, known as the fundamental frequency, 2Khz will be the second harmonic, and 3Khz will be the 3rd harmonic. Note, these 2Khz, and 3Khz were not in our original input signal. But on our output, they show up… not by a large amount, but it’s there.

Harmonics by itself are not bad, since in the real world, all musical instruments (and even our voices) produce complex sounds having several harmonics. (Harmonics are also called overtones.) We’ll talk more about harmonics in a future article as this can become a complex subject as our ears have the amazing ability to distinguish harmonics depending on the frequency and volume level.

After tweaking the resistor values to extract the maximum signal out of this tube, I measured my output voltage and divided it by the input voltage. We have achieved about 4.21Vrms output voltage for an input voltage of 577mV. A voltage gain of 7x, or about a +17dB gain. Also, 4.21Vrms is equivalent to about +15dBu output level.

Referring back to the datasheet for this medium-mu tube, I can see we’re in the right ball park. That’s the most this particular tube can do.

But +17dB gain, and +15dBu output level isn’t enough for use as a mic preamplifier. We need more gain, and a larger output level than that.

I also tried hooking up the tube output directly to the inputs of a powered speaker so I can hear it. Hmmmm… the voltages collapsed. The low impedance input of the powered speaker monitor is loading down the tube output too much. As I said earlier, at these low voltages and low current, there ain’t much power we can extract from these tubes.

So yeah, we need to solve these 2 issues first if we’re going to get somewhere from here. Happy to say, we did but that will be a new topic for another day.

[to be continued…. please follow our blog/like our facebook page to get the latest updates]

See Part 2:

Leave a Reply

Your email address will not be published. Required fields are marked *