This output transformer is mentioned several times
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=retrieveTfg&binCount=1&Ne=4294957561&Ntt=oiutput+transformer&Ntk=I18NAll&Nr=AND(avl:uk,searchDiscon_uk:N)&Ntx=mode%2Bmatchallpartial&N=4294955308&Nty=1
Can this be dropped into the superfly replacing the 125a wihtout any mods to the circuit?
I would say yes. I used one for my original Firefly and for testing my sub-mini version, but reverted to the 125A for cosmetic reasons. In fact, IMHO it's a better sounding transformer than the 125A (YMMV!), but is physically a bit bigger. I've got one in my current 4-4-0 build, and it sounds grand.
You will need to choose the correct taps for the output impedance you want.
For info, I measured the turns ratios of the RS transformer as follows:
Primary connections 1 and 4 (2 is the centre tap)
Secondary
D-C 67.2:1
C-A 57.6:1
D-B 44.8:1
D-A 31.0:1
Primary connections 2 and 4 (3 is the centre tap)
Secondary
D-A 15.6:1
Primary tap 2 forms a centre tap between 1 and 4, and tap 3 forms a centre tap between 2 and 4. the 1-3 primary tap is no use for the firefly, because there is no corresponding centre tap, but the ratios are :
Primary connection 1 and 3 (N.B. No centre tap)
Secondary
D-C 50.0:1
D-B 33.3:1
D-A 23.1:1
To calculate the output impedance (in case you don't know) multiply your speaker impedance by (turns ratio) squared.
For example taps 1 & 4 on the primary with C & A on the secondary gives a turns ratio of 57.6:1 , so an output impedance for an 8 ohm speaker of:
8 x 57.6 x 57.6 = 26,542 ~ 26.5 k ohm
With this transformer in the original Firefly I used primary taps 1, 2 and 4, with D-C, C-A and D-B for 4, 8 and 16 ohm outputs respectively.
Going for lower impedances made it louder but harsher (again, YMMV!)
To clarify: You would connect B+ to terminal 2, and the anodes of the "power" valves to terminals 1 and 4 respectively, then choose the secondary terminals depending on your speaker impedance. You won't break anything by going for ~20 - 25k output impedance to start with (i.e 8 ohm speaker to terminals C and A).
Andy
