# Compound Mallet HP trigger?



## wigginsn (Jan 9, 2008)

I'm building a compound mallet, and got to wondering if I need to add a sound trigger to the high pressure driving wheel set.


At first I was thinking of course I need to, but then if the HP steam exhausts into the LP cyls then there wouldn't be an audible chuff..

Or would there? 


All input appreciated.


Cheers
Neil


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## Semper Vaporo (Jan 2, 2008)

When running in compound mode there might be some hissing associated with the passage of steam from the HP to the LP cylinders, but it would be easily drowned out by the chuff normally extant from the LP cylinders going up the stack. However, many compound locos had a valve known as the "Simpling valve" (actually two different valves, one to redirect HP steam to the LP cylinders and the other to direct the exhaust from the HP cylinders to the atmosphere) that at start up could be set to apply HP steam to both sets of cylinders and thus the HP cylinders would exhaust to the atmosphere, sometimes through the blast pipe with the LP steam exhaust up the stack, but sometimes just through an opening near the cylinders, or below, beside or above the boiler which would produce a loud noise. If it went up the stack then it would be the typical chuff, but if exhausted someplace else the noise would be 'different', but still a type of chuff. This noise would continue until the engineer would change the simpling valve back to compound at his discretion, which usually occurred somewhere between 4 or 5 MPH to maybe 10, 15, or 20, depending on the load being started and what grade it was on. Down grade and it might not be needed to put it in simple, but upgrade with a heavy load and it might stay in simple mode for many miles.


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## wigginsn (Jan 9, 2008)

Thanks Semper. 

Makes sense. I'll do a bit of digging and see if this loco had one of those valves. 

Cheers 
Neil


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