Trap Valve Amplifier

[PaulM]

Not sure that this is the 'right' section to post this in, but probably the best bet!

Whilst working on our 'Project Vivat' TV OB truck www.projectvivat.co.uk we found the need for a sound distribution amplifier in the system to feed lines, amplifiers and monitoring. Being a project based in 1953/4, it has to be valve and the only likely candidate that I have is a Marconi Trap Valve amplifier module type 3088. This is a two valve unit with balanced audio in and out with a low output impedance meaning that multiple 600 ohm outputs using resistors can be created.

It's a general purpose module and the example is 'new' and never used so it will almost certainly work OK.

Now, that's all wonderful, but why 'Trap Valve Amplifier'? I've seen the term on many Marconi manuals and just marked it down as a general purpose amplifier, but what's the origin of 'trap'? What's it a trap for (!), where's the term come from (GPO?) and when did it cease to be used? I ought to know the answers to these questions, but I don't and none of our Vivat team do either!

Anybody know?

Best regards,

Paul M

[beamcurrent]

Edward Pawley, the BBC historian, writing about the BBC at Wood Norton in 1939-45 says ""a trap valve amplifier was simply an amplifier with a high impedance input, several of which could be paralleled across the output of the main amplifier to feed individual distribution lines without risk of interaction between them."

The Encyclopaedia of Radio and Television (1950) mentions a "Trap Amplifier" as a way of branching off from the main circuit to avoid affecting the main circuit.

So the term "Trap" in these descriptions implies that it traps the loading or fault condition away from the main circuit.

Does not help much with the from to-dates, but the use of trap amplifiers may have fallen out of use with the development of amplifiers with effectively zero output impedance + line matching resistances which would have removed the need for Trap amplifiers as isolation devices.

[PaulM]

Yes, Brian, we did find all of that, but the word 'trap' is still not fully explained - I don't buy the 'trap a fault' idea - that's an isolation amplifier. Pawley is not always 100% reliable - found several errors over the years. He's really only a secondary source, not a primary and he seems to ignore a lot of non-BBC information.

The earliest contemporary primary source reference that we have is 1938:

http://www.bbceng.info/ti/early/TV_1...0Amplifier.pdf

That's another BBC example. Is this just another example of weird BBC terminology, or is it older, perhaps GPO or Marconi, or 'imported' from the USA?

Still looking . . .

Paul M

www.projectvivat.co.uk
www.becg.org.uk
www.golden-agetv.co.uk

[PaulM]

For the record, there's a couple more from the BBC in 1938, but the one below is interesting named for driving headsets and loudspeaker amplifiers - essentially the distribution role that we're now using one for:

http://www.bbceng.info/ti/early/TV_1...0Amplifier.pdf

Best regards,

Paul M

[m0cemdave]

The term "Trap Valve" , an early name for an isolating or buffer amplifier, comes from an analogy to a water trap. It ensures that the load following the amplifier has no effect on the source.

Somewhere I have a vintage BBC training manual that explains this, I will see if I can find it...

[beamcurrent]

I had a look in my BBC 1942 training manual, It mentions "Trap Valves" a few times. It does not say the origin of the term, but in the description it does say that valves (thermionic) are inherently one way devices and can be used to isolate circuits, as mentioned before.

However this does sort of imply that maybe we are thinking of the wrong meaning of the word valve, in this context, ie not thermionic valve but a one way device. Would this make sense?

[m0cemdave]

Yes, exactly.

[Bufo Bill]

Wow, that truck is beautiful. Very commendable project. Thanks for the link.
Bill.

[PaulM]

Thanks, Bill. It's taken a huge amount of time (and money). Despite being a relatively small black and white truck, it has taken more time, effort and money than the restoration of our other two large units, 'Yorks Tyne Tees' and 'Southern'. Their write-ups are on our (still under construction) BECG website ('Projects' pages): www.becg.org.uk

I've attached some pictures of the Marconi Trap Valve Amplifier. It's actually one that seems to have been made for Turkey - the annotations are all in Turkish. The word for 'gain' is interesting, it's 'Kazanç', which by a literal translation actually means 'winnings' which is sort of understandable! I also have some very similar Pye modules.

I have found something that may explain 'Trap Valve' in the form of steam trap valves which offer a means of sampling high pressure steam, which is sort of what these electronic ones are doing - sampling the signal for various monitoring needs. See: https://www.pbmvalve.com/valves/clean-steam-trap-valves

It's not quite right, I feel, but neither are the 'one way analogy' or the 'water trap' ideas.

This needs a primary source reference as subsequent secondary sources could well be wrong. The 1938 BBC references are the best so far, but I suspect that it's rather older and possibly not BBC in origin. It's not hugely important (a working Trap Valve Amplifier is much more to the point!), but it would be good to know the real source of the term.

Best regards,

Paul M