Typical speaker for 1931 Marconi 246?

[shawthing]

Hello all, I have a Marconiphone 246 (Super Power II) from I believe 1931. I am fairly new to sets like this, that require external speakers rather than having them built in, so I would like to ask if anyone could please suggest the kind of loudspeaker that might typically have been used with this set? Would it have been common to see horn speakers that seem more typical of the 1920s still in use in the early 30s, or had other things come along?
Thank you, Chas

[Edward Huggins]

1931 does seem a bit late for an external speaker. Looking at the circuit diagram, it is definately going to be a high-impedance type, almost coming straight from the anode of the O/P valve. Edward

[Mike. Watterson]

Cone speakers with moving iron common from 1928/1929. Usually a wooden cabinet. 10" to 12" cones. About 2,000 Ohms to 4,000 Ohms.
Some moving coil units by 1931, they would need a transformer.
Using 2K + 2K (in series) headphones common too.

This has only two valves, so headphone operation MIGHT have been more common.
http://www.radiomuseum.org/r/marconi...per_power.html

It's easier to get a pair of 2K + 2K phones than a cone speaker. You can also use a pair of piezo disks with 2.2K in parallel fitted into cheap over the ear headphones or 1920s/1930s replica.

A horn would be rare after 1927/1928.

[shawthing]

Thank you Edward and Mike for your replies, that gives me something to go on - I'll hunt for something suitable. In the meantime I have a small panel speaker which looks utterly wrong but which will enable some initial testing...
There doesn't seem to be much info around on this model - is that because it's rare, or because it's not of much interest?

Chas

[Edward Huggins]

What do you mean by a small panel speaker? Most older speakers from about 1935 onwards will have an impedence of 3/5/15 ohms. Whatever you do don't use this!! Edward

[peter_sol]

Does the speaker you plan to use for testing have a transformer?

[ms660]

The Marconi Valve Data Book lists an MPT4 as the output valve for a Marconiphone 246, a loudspeaker would have to be reasonably matched to that valve.

MPT4 valve data:http://www.r-type.org/pdfs/mpt4-1.pdf

The other two valves are listed as MH4 and U10.

Lawrence.

[peter_sol]

Well it does not give the load impedance so not too critical it seems.

[ms660]

The Osram data sheet gives the optimum load as 8,000 ohms (8k) for their MPT4:

http://tubedata.itchurch.org/sheets/060/m/MPT4.pdf

Lawrence.

[PJL]

http://www.radiomuseum.org/r/marconi...el_no_140.html or something similar. Speakers from this era come up quite regularly but you need to check the original high impedance speaker is still in the box as they are commonly replaced with modern low impedance types.

[SeanStevens]

I have one of these:

http://www.radiomuseum.org/r/marconi...diogram_q.html

it is a little later than your 1931 model, but still uses the MPT4. It has a GIGANTIC mains energised speaker. I think if you could find the schematic for this radio you would possibly find a suitable speaker referenced??

SEAN

[ms660]

Anyone know which receivers used a loudspeaker energized by the mains as opposed to DC?

EDIT: Or a separate loudspeaker unit energized by the mains.

Lawrence.

[shawthing]

Hello all, thank you for the various very informative replies!
To Edward, I'm not at home right now so can't check, but the speaker I plan to use came with the unit and had been used with it before, so although I will check, I think it will be compatible enough for test purposes. It's quite small, about 4-5" and is mounted in a metal panel with a small grille and front terminals - I'd guess it's been taken out of some larger piece of test kit or workshop installation.
Re. the output valve, the chassis is marked MPT4 but there's an MKT4 in the socket. Comparing datasheets for the two, that also gives 8k Ohms as optimum output load so I'm assuming that won't change things?
On a different note, the chassis is labelled for a U10 rectifier, but there's an FC116 solid-state rectifier sitting in its place. Would that have been done simply because someone didn't have a suitable valve, or is there some advantage to using it? Would there be any advantage to replacing a U10 there - apart of course from the obvious one of greater authenticity?
To PJL, that Marconi speaker Model 140 looks rather elegant and as if it would be a decent match visually - thank you.
Chas

[Herald1360]

According to R-type the MKT4 was a direct replacement beam tetrode for the MPT4 pentode. It should happily drive any reasonable 4R loudspeaker via a suitable 45:1 output transformer.

The U10 rectifier is directly heated so using a solid state sub shouldn't be a problem. Check on load HT and add a suitable resistor if needed.

[Mike. Watterson]

Originally Posted by pwdrive

Anyone know which receivers used a loudspeaker energized by the mains as opposed to DC?

Usually "mains energised" is mains derived HT current replacing permanent magnet. The common arrangement was that all the HT current of receiver went through the "energiser" coil. Some where higher impedance (more turns, less current) and shunted across the HT rail. Some had a third "humbucking" coil in addition to voice coil.
One technique was to ensure "hum" on output stage cancelled "hum" on energising coil, which also acted as HT smoothing choke, so polarity of coil connections dramatically affects hum, esp. if there is a humbucking coil.

I don't think ANY used actual AC mains.
My Ever Ready and HMV mid 1930s models both have speakers with energising coils instead of magnets. Both perform well. Apart from cost saving, they may have stronger magnetic field and there is no magnet to age. Certainly very loud.

[ms660]

Yes I'm ok with the DC energized ones (series & shunt)

Chas Miller mentioned mains AC energized ones in one of his books, he said they weren't popular and were soon superseded by the more familiar types, it was that that prompted me to ask the question:

https://books.google.co.uk/books?id=...peaker&f=false

Lawrence.

[Mike. Watterson]

Originally Posted by pwdrive

Chas Miller mentioned mains AC energized ones in one of his books

Nothing is showing up on that. "You have reached your viewing limit for this book."

I'm baffled as how an AC electromagnet doesn't just result in massive 50Hz hum!

I wonder did he ever have one?

Here is description of how a speaker with no magnet works:
http://www.radioremembered.org/edspeaker.htm
(basically as I said above)

A diagram of it (approximate)
http://www.radioremembered.org/dynspkr.htm


I'd love an explanation of how raw AC "excitation" can possibly work. Perhaps the usually expert Chas Miller took a wobbler on this one. I've been doing radio stuff and reading stuff since 1960s and I have never heard of it, nor can I formulate a theory.

You are a pretty expert guru. Have you ever seen an actually AC energised speaker or schematic?

[ms660]

It starts near the end of page 36, quoted text is given below:

"A short-lived development was the field coil which could be energised from the mains and thus gave rise to the name 'mains energised loud-speaker'. This term is all to often misapplied to later types with fields powered by the HT supply of mains operated receivers, properly known as 'HT energised speakers'. Always be suspicious of any speaker described as 'mains energised' because there is a 99% chance that it won't be!"

Lawrence.

[ms660]

Mystery solved, DC mains not AC mains

Lawrence.

[ms660]

Maybe this is what Chas Miller was on about, in this article it gives home brew details for a "mains energised" loudspeaker that can also be used on AC but via it's own dedicated rectifier, WW Sept. 28th 1927, bottom of magazine page number 391:

http://www.americanradiohistory.com/...ld-1927-09.pdf

So...Were field coil loudspeakers, powered via a dedicated rectifier and AC, manufactured and sold in a separate cabinet? If so, who made them?

Lawrence.