Over Enthusiastic Vacuum Tube Copilot Project

Now the cheerful madness

All old hackers eventually must make a vacuum tube radio project, it's in the rules. But I decided to combine vintage radio tech with Microsoft Copilot, and may God have mercy on my soul. As for why I am using "Overly Enthusiastic" as part of the title, it will become obvious. Alternate title would begin with "Thank you for correcting me". WARNING: This is a build and not a radio/electromagnetic wave theory and application tutorial. Many things are done without explanation, just like life.

First Contact

Human

I want to make a 1920's style radio with vintage tech.

Copilot

Building a 1920s‑style radio is one of those projects that hits the sweet spot between engineering, aesthetics, and pure nostalgia. And since you’ve got that LVL1 “Over Enthusiastic Vacuum Tube Copilot Project” page open, you’re already swimming in the right waters. Let’s take that inspiration and turn it into something you can actually build.

Editor

Copilot is a spy. Need to keep it in a barren browser window for the future. The Copilot then lists details about radio types, topologies and components. Since I am in-charge, I sort the options:

Copilot

**TRF (Tuned Radio Frequency) Receiver**
 * Multiple RF amplifier stages, each tuned to the same frequency  
 * Very authentic to the early 1920s  
 * More challenging to align
**Regenerative Receiver**
 * Uses positive feedback to boost gain  
 * Extremely sensitive  
 * Requires careful control to avoid oscillation  
 * Very “hands‑on” listening experience
**Superheterodyne (Late 1920s)**
 * More stable and selective  
 * More complex  
 * Historically accurate for the *late* 1920s **

If you want the most “1920s experience,” a regenerative set is the sweet spot.

Topology is Everything

Human

Regenerative reciever path.

Copilot

Love that choice—regens are wonderfully interactive and very 1920s.
Here’s a focused path to a buildable regenerative receiver.
Overall game plan
 Band: AM broadcast band first (easier coils, easy to test with local stations).
 Tubes: 1–2 small triodes (e.g., 12AU7/12AX7, or a single dual triode) for detector + audio preamp, plus an audio output stage if you want a speaker.
 Layout style: Breadboard with exposed tubes, big tuning cap, and visible coils.
1. Basic regenerative receiver block diagram ....

Editor

Lots more detail, but I have twists in mind. We will do the block diagram thing. And in all honesty, I do block diagrams for my unassisted projects. But I am not that chipper.

Here is the first twist, simply saying coil or coils, does not convey the spectrum of coil types. As an example, here are two eBay coil listings for era appropriate coils.

There are coils, coils with taps, coils next to coils, coils in coils and many other combinations.

Coils

There is an option with coils referred to as "taps", which simply means that between the ends of the coil are points where the coil wire has a connection point available. These allow a coil to be used while varying its function by using the extra connections. The value of the coil is expressed as inductance as measured by Henrys. By choosing what end and tap or tap and tap you get a different value of Henrys. More technical details can be purchased from any American university as funds provide.

When you have multiple coils in proximity, they interact. This interaction is called "coupling". These interactions depend on relative location, orientation, winding direction, shape and size. More technical details can be purchased from any American university as funds provide.

Two devices utilizing coil interaction are Variometers and Variocouplers. Simply put, a variocoupler consists of two coils where there positioning is adjusted while they are independent and not interconnected, think transformer primary and secondary coils that are moved about themselves but not shorted. Variometers can be described as two coils in series where there positioning is adjusted relative to one another thus affecting the inductance value of the total chain. More technical details can be purchased from any American university as funds provide.

Copilot hasn't mention Variocouplers or Variometers yet but let us see its response.

Human

what about variometers and variocouplers?

Copilot

Great question—that’s exactly the kind of 1920s detail that makes a regen feel right.

Quick definitions
Variometer:  
What it is: Two coils, one inside the other, on the same axis. One rotates inside the other.
What it does: Changes the total inductance by changing how much the magnetic fields reinforce or oppose each other.
Use: Acts like a variable inductor—you can tune the set by turning a knob instead of switching taps.

Variocoupler:  
What it is: Two separate coils that can rotate relative to each other.
What it does: Changes the coupling between coils (how tightly they’re magnetically linked).
Use: Controls how strongly the antenna (or feedback coil) is coupled to the tuned circuit—key for regen behavior and selectivity.

Editor

Yeah, yeah, I didn't mention the whole "one inside the other" part or how it applies here. I shall endeavor to do better.

Antenna Assembly

Copilot has already spilled the beans on using a Variocoupler for the antenna assembly and it made reference to "taps" in the variometer, but we will use "taps" on one of the variocoupler coils to adjust the tuning on the variocoupler. Here is a mini schematic of the antenna assembly being built.

And as fate would have it, I just so happen to have this:

Human

I happen to have a 1920's radio front panel with a variocoupler, with taps on the primary coil, will that help.

Copilot

Yeah—that absolutely helps, and it’s kind of a gem to have.

Here’s what that front panel/variocoupler gives you in practical terms:
Built‑in antenna/coupling system:  
The primary with taps was almost certainly meant for the antenna side. You can:
Connect your antenna and ground to that primary.
Use the taps to match different antenna lengths/impedances.
Use the rotating secondary (variocoupler) as your coupling control into the tuned circuit.
Perfect role in a modern regen:
You can treat that whole assembly as the front‑end:
Primary (tapped): Antenna input + matching.
Secondary (rotating): Feeds your tuned circuit + detector grid.

etc..

Human

I am thinking of using it as a antenna tuning type setup. The primary for antenna and ground connection while using the secondary to feed the signal/frequency tuning circuit prior to the detector.

Copilot

That’s exactly how I’d use it—and it fits the 1920s spirit really well.

Here’s the clean way to think about it:
Primary (tapped) = antenna tuner / matcher
ANT → top of primary.
GND → bottom of primary.
Taps → you pick the tap that gives you the best combo of signal strength and selectivity for your antenna length.
Functionally, this is your antenna tuning + coupling network.
Secondary (rotating) = RF feed to your tuned circuit
One end of the secondary goes to your tuned circuit input.
The other end usually goes to ground (or the cold end of the tuned circuit).
The rotation of the variocoupler adjusts how tightly the antenna system is coupled into the tuned circuit—so you can “back off” strong stations and sharpen selectivity.

etc...

Editor

It is amazing how it would use the items exactly how I would use the items. Is Copilot reading my mind as well as open browser pages?

Tuned Circuit

Human

Lets talk simple tuned circuits.

Copilot

Perfect—if you want to build a regenerative receiver with a simple, clean tuned circuit, we can absolutely do that. And your 1920s variocoupler panel becomes the front end, while the tuned circuit becomes the heart of the regen.

Let’s walk through the essentials in a way that keeps the build approachable and very vintage in feel.
🎛️ What a “Simple Tuned Circuit” Really Means
A classic tuned circuit is just:
𝐿 + 𝐶
in parallel (or series), resonating at a chosen frequency
For the AM broadcast band, the usual target is:
L ≈ 200–250 µH
C ≈ 20–365 pF (variable capacitor)
That gives you roughly 500–1700 kHz tuning range.

etc...

Human

I don't have a variable capacitor in that range handy, but i have a variometer and some fixed value capacitors.

Copilot

Nice—that’s a very 1920s constraint in the best way: no big tuning cap, but you’ve got coils and tricks.
Let’s keep it simple and concrete.
1. Using the variometer as your tuning element
You can treat the variometer as a variable inductor 
𝐿,and use fixed capacitors for 𝐶. 
The resonance is still:
𝑓=1/2𝜋sqrt(𝐿𝐶)
So:
Variometer: gives you a range of inductance (say, roughly 𝐿 min → 𝐿 max).
Fixed capacitor: sets the band you’re in.
Rotation of the variometer: tunes within that band.
Practically:
Pick one fixed capacitor for the AM band to start.
Later, you can switch in different capacitors for different bands or “bandspread.”

etc...

Editor

From a schematic standpoint, Copilot suggested circuit A and I suggested circuit B. And everything is constrained in the best way for 1920's radio design.

Detector with Regeneration

Human

ok, simple triode detector setup.

Copilot

Nice—let’s lock in one clean, buildable triode regen around your vintage front end.

etc..

2. Simple triode detector connections
Assume a generic small triode (e.g., half of a 12AU7, 12AX7, or similar).
Grid circuit:
Grid:
Connect to the tuned node through a small capacitor
Grid coupling cap: ~10–100 pF
From grid to ground: grid leak resistor
Grid leak: ~1–5 MΩ (start with 2.2 MΩ)
Plate and tickler (regeneration):
Wind a tickler coil on/near the variometer or on a small separate form:
About 1/4–1/3 the turns of the main tuned inductance is a good start.
Plate circuit:
B+ (say 90–150 V) → regen control → tickler coil → plate
Plate → audio load (headphones or transformer primary) → ground

etc...

Editor

To simplify, again Circuit A from Copilot and Circuit B is what I am going to build.

But What I Actually Built Was This ...

So, I built the following circuit, Copilot be damned.

Now, I have no doubt that Copilot would recognize my design as keeping with the spirit of 1920's radio design. It might even praise my can-do attitude that exemplifies the radio builder spirit of the era.

Human

<<link to the schematic>>

Copilot

Great — since you’re looking at that JAC OEVTC Basic‑Built Schematic on the LVL1 wiki and you want a simple triode detector, let’s build a clean, reliable regenerative detector that works perfectly with:
your 1920s variocoupler (antenna + coupling)
your variometer (tuning)
your fixed capacitors
a single triode as the detector
Below is a complete, practical design that stays true to 1920s practice but is easy to build today.

etc...

Editor

Copilot basically repeats its earlier design.

Debris