Anybody try a 12U7 in a starved plate pre?

[jfkelliher]

I did try to search and I apologize if I missed it...But I'm just wondering how one of those cheap starved plate mic preamps that use 12A_7 family tubes might sound with a 30v max car radio tube.

 

[ruffrecords]

I think the Valve Wizard did an article on staved plate 12A_7 series tubes. Google is your friend.

Cheers

Ian

 

[rock soderstrom]

Here are extensive info about a low voltage (not starving) mic pre amp with the car radio tubes ECC86 (aka 6GM8) and EF98 in german.

https://www.soundandrecording.de/tutorials/roehren-preamp-im-eigenbau/
In total there are three parts, note the linking in the report.

 

[jfkelliher]

Thanks guys. I think I'll pick up a tube mp and pop one in it. I will also see if I can track down a German cohort or two to help me make sense of the stuff. I am in the America and English is the only foreign language I know.

 

[rock soderstrom]

Thanks guys. I think I'll pick up a tube mp and pop one in it. I will also see if I can track down a German cohort or two to help me make sense of the stuff. I am in the America and English is the only foreign language I know.

Null problemo

The good thing is that the articles are available as a website. This means you can translate them into English in your browser. This works quite well, as both languages are pretty similar.

What also works very well is translating with ChatGPT, an AI-based universal language talent.

The following excerpt is translated with ChatGBT, which I think is really well done. What do you think?

Tube Preamp in DIY by Dieter Leckschat, December 18, 2016

Tubes are in! In this multi-part series, we want to show that even those who are not very experienced can successfully build a high-quality tube preamp.

Part 1-1: Prototype with EF98 Tube
Tube preamps have always been highly regarded in the field of recording and provide a welcome addition to the analytical sound of good digital technology. In this first part of the series, we lay the foundation for understanding the circuit and create the opportunity to experiment with variations of the amplifier circuit, which will be introduced in the second part.

Technical Basics
The DIY project starts with low-voltage electron tubes, which means that there are no dangerously high voltages in the entire device. This allows us to propose such a project with confidence because we want to keep our readers around for a long time! Low-voltage tubes were produced for a while for various applications in battery-operated devices before transistors took over the market (in the 1960s). However, the circuits we describe will also be designed to work with slightly higher voltages suitable for tube types that are still being manufactured today. At this point, we can already mention a major advantage of tubes over semiconductors: tubes are much more robust against temporary faults in a circuit. Unless you burn out the tube heater, there is hardly any risk of damage, even with wiring errors. So let's delve into some technology.

Basic Concepts: Diode Tubes
The very first electron tubes, called diode tubes, had the function and property of allowing current to flow primarily in one direction while almost no current flowed in the other direction. These tubes can be used as rectifiers, which is still practiced in the power supplies of some guitar amps to this day - known as a "rectifier." The diode tube, in its original form, has two terminals called the anode (positive terminal) and the cathode (negative terminal). The effect is achieved by directly or indirectly heating the cathode, which subsequently emits electrons. The interior of the tube is evacuated ("vacuum tube"), allowing the emitted electrons to move freely. They are attracted by the anode, which is essentially a nearby metal plate when a positive voltage is applied to it. When the electrons flow towards the anode, an electric current is generated since current is nothing more than moving charge carriers (electrons). This does not work in the opposite direction because like charges repel each other, and a negative voltage applied to the anode prevents electrons from flowing. Instead, the anode repels the charged particles flying around in the vacuum.

ECC86 Double Triode EF98 Pentode
An improvement is the indirect cathode heating, where a heating filament heats the cathode without being electrically connected to it. This technique is used in most tubes still in use today. A further development of the diode, which first allowed amplification of an electrical signal, is the triode tube or simply triode. Another electrode in the tube, called the "grid," influences the current flow between the cathode and the anode. It is positioned between these two electrodes and, being an actual grid, allows the majority of electrons to pass through. However, if a negative voltage is applied to the grid, it repels the electrons on their way from the cathode to the anode. With careful design of the geometry, a relatively small voltage at the grid is sufficient to significantly alter the relatively large current from the cathode to the anode. It is important to note that the primary effect is that a voltage change at the grid leads to a current change at the output. Thus, we are not dealing directly with a voltage-amplifying component (see box).

In the example mentioned, we used a 10-kilohm anode resistor. This leads us to an important point because high voltage amplification comes at the expense of high output impedance of the tube stage. If a low-impedance load is demanded at the output, the voltage will unintentionally drop, which must be prevented. More on this in the topic of circuit design later on. An example of a low-voltage triode is the ECC86, a double triode. This means that two separately usable triodes are housed in one glass bulb, which is convenient for multi-stage amplifier circuits. The 12AX7, which is used in almost all guitar amps, is also a double triode (but designed for high anode voltages).

(Excerpt from the first article)

Not bad, eh? Brave New World!