Very simple & cheap High Impedance 4K Phones.

A pair of vintage 4K phones suitable for Radios before 1932 or possibly Crystal sets are perhaps €20 to €40 ($30+).

A simple solution to test a radio requiring 4K headphones (2K + 2K) or a Reed/Armature speaker direct to Anode (Plate) is a pair of Piezo ceramic "Beeper" discs. As well as bare disks there are even cheap plastic cased models for PCB.

Check for a + and - symbol on cased ones. If polarity is marked connect +5V and see does it "beep". If so it's unsuitable as it has a built in transistor or IC.

A wide variety were measured and found to be between 9.5nF and 100nF. The 100nF part was very large.

I wired two identical plastic case ones in series with 2.2 K Ohm resistor across each. Thus the DC load is 4.4K and AC load is 4.7nF approximately. I plugged it into the standard built in phone jack socket on my Super Range Portable four and found that the audio was quite poor in Bass, but otherwise stations much louder and clearer with several inaudiable stations quite readible. As there is poor Bass and no lack of Treble I will also try some larger ones that are 22nF to 47nF each.

I'll leave it to the reader to experiment with a crystal set (try 2 x 10k, 22k and 47K resistors and no capacitor as the 4.7nF of the "earphones" will be enough on a diode detector!).

You could fit the bare ceramic disks or encapsulated ones in cheap 32 Ohm or 8 Ohm phones or make old style headset, perhaps out of cut up coffee tin and coat hanger. Do take care with insulation as the voltages are 45V to 240V depending on the type of radio.

Later I will try it with my 1929 approx. "Kit" radio which has no built in speaker.

I suspect the "crystal" earpieces with 3.5mm jack still sold by Maplin and others are actually nothing like 1960s models despite the identical appearance. Peering in the opening they do look like a flat metal disk rather than the foil with crystal glued in.

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Though the measured capacitance is nearly 100nF, a large 35mm approx piezo beeper sounds excellent.  I used 6.8k ohms (6 800 R) across it for the DC bias. So I will look for cheap ones on eBay and usual suppliers such as Conrad, Maplin, Farnell etc to make a pair of "cans".

Very, very much more volume and also not "tinny". Sounds good on 198kHz BBC R4 LW from Droitwich here west of Limerick City (Ireland's Mid West) at lunch time. Just the 3 valve 1929 TRF SG + 2 triodes with central heating pipe as earth and a wire on curtain rail (less than 3m) as aerial.

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The main point of this experiment is to listen to the output of old valve sets that have Triode (or Pentode on later models) and no transformer. These assumed a high impedance (greater than 3K) reed, amature  (with horn or cone) or 2k per Ear (4k) headphones.

Maplin has the YU85G 35mm bare piezo ceramic disk at €2.15 which isn't cheap. It needs clamped or stuck at edges to something for best output. Note that it will be at 120V typically so insulation is needed. You can experiment with a 3.9K to 15K load resistor depending on the DC bais and HT voltage. A 12K resistor with the  YU85G gives a pleasant tone. The capacitance was measured at 35.5nF on a cheap chinese Digital LC meter.

Maplin has just been re-organised with a huge shift back to Electronics, kits, construction, Computer parts, with some tools, Radio, Satellite and Disco gear. All the more "junky" stuff is gone. Prices still a bit eye watering. But no postage and immediate.

Perhaps someone will find a Chinese / Asian source at 50c?

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The bare 35mm piezo disks are on eBay about 50c to 65c each for 10 off (Under €7 for 10) from various UK and Chinese sources.

Warning: This device is potentially lethal unless in an insulating case!

Previous experience suggests the size and hole of the ready made loud "bleepers" is not by accident. Also I found mounting the disk at 3 points is louder than glue all the way around.

So I cut a disk with hole and soldered a band almost all points around it. This assembly was soldered just at 4 points to the brass disk.

Pierce can with a knife at top and bottom, then with sissors cut off the top rim and base. Take care to avoid slashing your fingers with the jagged edges!

 

Trim off the jagged edges!

Curl the can inside out to flatten the sheet. This material can be soldered through the lacquer print on the outside. The inside seems to be pre-tinned. It's a wonderful source of material to make screening cans (but some applications need Aluminium, this is tinned steel), connection pins and sockets etc.

 

I used sissors to cut outer and punched centre with a knife. Drilling is a bad idea!

 

Make a ring/band to fit.

Pre tin the two inside ends and then curl it holding the open ends against the bench (or card) to butt solder them.

Then pre-tin a good quantity of spots on the lid:

Now solder the band/ring to this.

 

Solder the brass disk to the band (lid with hole is underneath)

Be careful not to overheat the brass disk and only solder at three or four points.

Finished earpiece unit without insulation. Ear side.

 

For safety it needs to be in an outer case. I'll make a pair of 1920s style phones (without terminals) out of the tin sheet with the earpiece unit insulated. Or a single earpiece type. The difference in volume and bass compared with a loose disk is amazing!

You always need a parallel resistor. The value depends on application and may be best on the plug or end of cable. For the PM2 triode on my old "Kit" 6.8K and 12K both seem fine. In theory if the source is high impedance the resistor value not only affects DC voltage but also "tone". On a Triode the bais current will also vary somewhat with resistance. I will try a crystal set. I suspect about 470K may be best for diode DC load. Any case might benefit from a few holes and sound adsorbing / damping material.

If you have two for full "both ears" headphones they can be series and single parallel resistor on the cable or plug if mono. I can't think why you would want a stereo version

These are less ideal for a low impedance output designed for 4 Ohm to 16 Ohm speakers or 32 Ohm headphones. The idea is for direct drive from Triode anode (plate) to HT. Hence the importance of insulation. The can in the photo is either at HT or Anode (Plate) voltage!

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