It’s a compound (true) Mallet. Exhaust from the rear cylinders dumps into the intake of the front cylinders (which is why they’re larger; the steam’s expanded once already, and you need more to do the same work) … the exhaust from the low pressure cylinders goes up the blast pipe and stack. SO… it sounds just like a regular two cylinder rod engine.
Compare that to the Simple Articulateds, which had steam to all four cylinders from the throttle, and both sets exhausted up the stack independantly… this gave you the syncopated “In and Out” chuff … and the Uintah/SV mallets are on the Simple Articulated list.
BUT …
Just in case you “really” wanted to hear the double chuff on this one …
A Simple articulated locomotive had an “emergency valve” in the cab that gave it extra help starting, and at slow speeds could provide extra power on grades. When this valve was engaged, the engine “became” a simple articulated engine, and would sound just like one…making the double chuff happen even on an engine where it normally would not. So, you’ve got an excuse.
And, you can read about it for real with all the pictures, diagrams, and official instructions here: http://www.catskillarchive.com/rrextra/mallet.Html
On mine, I want to figure a way to rig two chuff timers (The Stanamatic Myopic Optichuff being among the first things to go,) one on the front engine and one on the rear … and then use the aux function on the RCS to be able to function as an operating “emergency valve.” Guess it’s a good thing I’m planning to chop the cab and add a tender forthwith…
Matthew (OV)
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