Choice of capacitor

[Al (astral highway)]

Originally Posted by SurreyNick

Thanks Dave. I will keep that in mind
Above what value would you say is best to avoid?

Hi Nick, Dave's basically agreeing with the earlier advice - stick to any of the film types (metallised polypropylene or polyester).

At the risk of putting my neck out here (I'll take a chance and back down appropriately if I'm talking nonsense) it's not clear how your circuit is going to work... I haven't got my calculator out yet, but I feel that only a tiny weeny AC current is going to flow through the anode at the signal frequency - call that 10Khz, the mid-point of our hearing.

Imagine your circuit like this. You've got an R (say it's set at 200R), in series with an inductor (say we're using even just the 15H partition) and a capacitor (4uF). That's a conventional LCR circuit.

There's another inductor (the loudspeaker) in parallel with the top section of the choke but it has a relatively low value and so the impedance of that branch is overall quite low.

BUT...the inductive reactance of the left branch is *enormous.* The capacitative reactance is negligible in comparison, but because of the inductive reactance alone, the impedance at the signal frequency of (say) 10,000KHz is huge.

Since the current flowing is determined by the source voltage divided by the impedance, you're going to get a tiny current flowing, too low to power your loud speaker. It's virtually a small signal again now...

I haven't worked this out yet either, but I think your choke may well be self-resonant at some signal frequencies...

Totally prepared to put my hands up if my instincts are awry, but tbh, there's no point in fussing about this capacitor if the circuit isn't going to work for other reasons. It's an elephant in the room situation!!

[SurreyNick]

OK. I was planning on using Suntan 400v Polyester Caps for the 0.05uf and 0.1uf values, and this thread began with a request for a recommendation for the one of 2uf - 4uf in the output stage. For the two 100pf caps I will use ceramic. The Suntan are dead cheap far-eastern caps which was a bit of a worry but I am led to believe they are OK. Thoughts?
N.

[Al (astral highway)]

Just a quick P.S, Nick.

Have you already acquired the horn speaker? What's its impedance?

Also, if you wanted to rethink, you could do one of two things:

1) Get a good looking output transformer and build a beautiful cabinet to house a low-impedance speaker. It may not be contemporary with your valves, but it will tick the design/ aesthetics values box

2) Source a high impedance speaker (maybe there's a sad, scrappable Phillips set out there somewhere) and put that directly in your triode's anode circuit. I don't have the data for your output triode but someone else will be able to comment...

At the moment, this is all about impedance. Once we've sorted this, the secondary and at the moment trivial tone-control issue can be dealt with...

[SurreyNick]

Mmm. Well that's a bit of a worry Al and it has rather taken the wind out of my sails!

Here I was happily following Camm's advice in which he states quite clearly that a choke-coupled output arrangement using any standard L.F. smoothing component can be employed in his three-valve theoretical circuit, and....

So, if I'm going to drive a horn speaker I need an output transformer then. Yes?

merlinmaxwell has offered me this one https://www.vintage-radio.net/forum/s...ad.php?t=65905. It's a 1925 S G Brown H1 horn loudspeaker and it is so beautiful I really would like to find a way of matching it to my circuit. From radiomuseum (http://www.radiomuseum.org/r/brown_h_1.html) it has an aluminium horn, wooden base, impedance 120 or 2000 or 4000 ohms, brown reed driver, aluminium diaphragm. "Designed to reproduce the human voice or music without trace of distortion..." (Brown Bros. catalog).

I can't find the valve data for the Triotron SP2, which is what I am using in the output stage, but likely it will be around about the range of the YD2 in this 1934 table of Triotron valves. By all accounts Triotron made a series of output valves with various power output ratings. ZD2 had the lowest power rising to SP2 with the greatest. The series was: ZD2, UD2, YD2 & SP2.

F.Y.I. I am using the HD2 as the detector valve.

Nick

[Al (astral highway)]

Originally Posted by SurreyNick

... it has an aluminium horn, wooden base, impedance 120 or 2000 or 4000 ohms, brown reed driver, aluminium diaphragm. "

Wow, that's a lovely offer from MerlinM.

And ah, good, if these are selectable setting for each speaker, then maybe we can do something with the 2K setting directly in the anode circuit of the output triode...

[SurreyNick]

I know Al. He is a gentleman's gentleman I am sure you can understand why I want to find a way to adapt the circuit to make it match. Fabulous news too if you think it can be done.
Do you want me to upload the whole circuit diagram just in case any other adaptations need to be incorporated?
BTW. I really appreciate all this help from you and others.
Nick.

Ceramics are quite good for audio coupling where there is very little AC across them (as there wouldn't* be in a good design). In this application I doubt (guarantee in an ABX http://en.wikipedia.org/wiki/ABX_test test) you could tell the difference twixt a ceramic and a film type.

*or do I mean would, I mean low AC voltage across.

[Al (astral highway)]

Originally Posted by SurreyNick

Do you want me to upload the whole circuit diagram just in case any other adaptations need to be incorporated?

Hi Nick,

I think that would be for another thread, could get a bit messy if it's tied-up with this one.

Let's find a suitable load for your triode to operate efficiently/ in conjunction with a way for your speaker coil to be energised properly, and take it from there...

[SurreyNick]

Sound advice Al
No pun intended, I promise!
Nick

I really would like to find a way of matching it to my circuit.

Just put it in the anode circuit, it'll match near enough. Engineering is about being good enough, otherwise nothing would happen.

[Al (astral highway)]

All's well that ends well, then. Tone control is simple now...

[Al (astral highway)]

Just a quick addition, Nick.

Back in the day when Camm was writing, the emphasis with these sorts of sets was being able to cut down shrill treble sounds, whistles and so on, and that's what tone control in these sets was mostly about, cutting treble rather than boosting bass.

So I think a simple treble-cut ability is still what you're looking for, as this isn't a hi-fi project, and the horn speaker has a limited bass response anyway.

Is this in line with your expectations?

Cheers,

[SurreyNick]

Ah, well that is simple and straightforward then
As to the question of tone control, yes Al, that is all my expectation is.
Nick

[julie_m]

It's usual to return an L/C-coupled speaker to HT- rather than HT+. Though it probably will work either way around; as the AC impedance between the two points is very low, and the capacitor is seeing to it that there is no DC component present.

If the capacitor you use as the speaker coupling capacitor is polarised, you will need to be careful which way around you wire it; the terminal nearest the speaker should be the same polarity as the end of the HT supply to which the speaker is returned.

[Telleadict]

Hello

This might come too late to be of help but it seemed of interest.

I don't have the technical knowledge of most on here but I have a coppy of F J Camm's "Practical Wireless Encyclopaedia" - mine's the 11th edition of 1946 although the first edition was 1932 so it covers a lot of the period that Nick is interested in.

Under the entry for "Output Stage" he discusses matching the speaker to the output valve and on the second page mentions "choke-capacity" output as an alternative to a transformer. The last scan is of how an output choke, presumably something like Nick has, would be wired.

I hope these scans are legible, if not I could e-mail better coppies direct to anyone who needs them.

Bye for now
William

[Al (astral highway)]

Originally Posted by Telleadict

Under the entry for "Output Stage" [Camm] discusses matching the speaker to the output valve and on the second page mentions "choke-capacity" output as an alternative to a transformer. The last scan is of how an output choke, presumably something like Nick has, would be wired.

hi William, thanks for this.

Just to say, no-one is doubting that Camm's configuration could work in theory, with the right component. However, in practice, the choke that Nick has to hand has a far higher value (two to three times higher) even than most vintage chokes that were designed specifically for use as smoothing components in power supplies. It would do a great job in a power supply filter choke capacity (within its current rating) in any of the standard arrangements.

The choke he has definitely would not work in Camm's configuration as the signal frequencies would not get through -- far too much inductive reactance.
This is a clear, mathematical property of the component.

This is now of minimal significance, however, in light of Nick's acquisition of a high-impedance speaker that he can place in the anode circuit of his triode, keeping it perfectly happy. And he will also have a good response from his high-impedance horn speaker, so happy days!