Mode Zero

Here we have a 3 valve circuit I found on eBay....

I have a few questions:

1 - Are the inverted triangles that end some lines Earth/Ground markers? If so, I assume that the earth is LT- ?
2 - Why is HT- not also connected directly to Earth?
3 - Does 'Audio Out' on the right hand side feed tones out and is it positive with respect to LT-?
4 - If so, would it therefore be possible to connect one end of a reed bank coil to the Audio Out and the other end direcly to LT-?
5 - If so, what resistance should that coil be?

I can feel a coil winding exercise coming on....

DG

1) Yes!

2) HT- is connected to ground via a 1K resistor and 25uF decoupling capacitor. This is probably to ensure the grid (pin 3) of the output valve is properly biased with respect to ground. (Should be a small negative voltage.)

3) Yes, but there is a blocking capacitor in series. Capacitors pass AC and block DC. There should be no DC component after the capacitor.

4) Yes. But always check with a voltmeter first! (you don't want to short the HT to ground because of a leaky capacitor!)

5) (Holding a wet finger in the air!) I would have thought that a few K should be OK, but you may need to experiment. The DL92 is regarded as a low output device, hence the need for a high impedance load. However, it was designed for portable radio use, so their definition of "low-output" may not tie in with ours!

Here is a sample circuit for a DL92, showing a speaker impedance of 8K for a quarter watt output, but it is DC coupled, so the speaker actually forms the anode load. Not a good idea, I would have thought, but cheap!


TRF receivers are notorious for marginal stability, and I'm guessing the strange connection between the HT- and ground is an attempt to improve this, as well as providing the necessary bias for the output device.

Best of luck!

--
Pete

Thanks for the comprehensive and infornative answer! Looking at a valve circuit must remind you of the old days...

One thing looks a little odd, though. You talk about a speaker with a 8k impedence? Most speakers I see come with 4 or 8 ohm resistance. I understand that impedence is dependent on frequency, but would an 8 ohm speaker really hit 8k impedence at audio frequencies? If not, where would you find an 8k impedence speaker? And the inductance calculators on the net seem to be very complex...

Ha! I think i need disturb the assembled experts here no more - at least not about reeds. Ths Feb 1963 RCM&E has a full article about building your own, including coil winding. They recommend 6000 turns of 44swg for a typical valve output.

It's easy when someone else does all the work....

mmmmm...a bit over 3 thou " ...without an appropriate winding machine, that will certainly try your patience and skill.

Dodgy Geezer wrote: 23 Jun 2024, 16:54 One thing looks a little odd, though. You talk about a speaker with a 8k impedence? Most speakers I see come with 4 or 8 ohm resistance.

Yes, but an 8ohm speaker would usually be coupled to a valve output stage via a transformer. Remember, that circuit is for a portable radio, and only 1/4 watt output.

Having said that, I thought 8K sounded a bit high, but I do have some 2K headphones that I used for crystal sets back in the day...

And saying that makes me feel very old, for some reason!

bluejets wrote: 24 Jun 2024, 00:26 mmmmm...a bit over 3 thou " ...without an appropriate winding machine, that will certainly try your patience and skill.

It will certainly need some kind of a winding jig - /Meccano will be my friend here, I would guess... here's a link to the relevant publication...

https://rcbookcase.com/details.php?publ ... ode=search

Dear RCM&E mag, when the "&E" was a significant proportion of content when first published. That and ELECTOR is where I gained most of my soldering &E skills. Big Meccano set as well for Engineering intuition.

I remember an unexpected meeting back in '82; I had just walked from the RAE(F) Library after some design research and out into a monsoon. Dived into the nearest door for shelter to be confronted by an interesting old chap in a tiny workshop wearing glasses like the bottom of milk bottles. He was surrounded by reels of copper wire of all sizes and some antique but very functional winding gear. It turned out that he was the last RAE Armature Winder (from the days of Magnetic Amplifiers etc.) and about to retire himself. We had a very interesting chat while the heavens opened. Happy days !

Good luck with your winding if you get there; close after patience, tension is the key. Do protect your pads if you run the wire through your finger tips.
[Now I too am feeling v. old]

Spike S wrote: 24 Jun 2024, 10:09 I remember an unexpected meeting back in '82; I had just walked from the RAE(F) Library after some design research and out into a monsoon. Dived into the nearest door for shelter to be confronted by an interesting old chap in a tiny workshop wearing glasses like the bottom of milk bottles. He was surrounded by reels of copper wire of all sizes and some antique but very functional winding gear. It turned out that he was the last RAE Armature Winder (from the days of Magnetic Amplifiers etc.) and about to retire himself. We had a very interesting chat while the heavens opened. Happy days !

Good luck with your winding if you get there; close after patience, tension is the key. Do protect your pads if you run the wire through your finger tips.
[Now I too am feeling v. old]

Spike, RAE? I was an apprentice at RAE Farnborough from 73 to 77 eventually leaving base workshops in early 78. I worked with a Spike in the flight simulator at one point. He drove a pale coloured (green) Is it you?

Jim

Jim
Off topic so PM sent.