[quote name='6v6' timestamp='1361547181' post='1987359']
Well on an amp that old, it's a bit hard to be certain, since the output transformer may not be original - the only way to be 100% sure would be to measure it, but looking at the schmatic and part numbers can give information on what would have been fitted originally:
[url="http://www.turretboards.com/layouts_schematics/fender/schematics/fender_bassman_ab165.gif"]http://www.turretboa...ssman_ab165.gif[/url]
This shows that the output transformer (TR3) has a single secondary winding, and that the part number is 125A13A
A bit more googling finds various replacement transformers claiming to be equivalent to a 125A13A, and all of them I looked at have a single 4ohm secondary, e.g:
[url="http://www.tubesandmore.com/products/P-T1750L"]http://www.tubesandm...oducts/P-T1750L[/url]
So assuming your amp is original, I'd say it will work best into a 4ohm load.
What your amp tech told you is somewhat true, although I personally would not make that recomendation, particularly if you plan to use the amp for bass. If you have a high quality output transformer, you can mismatch the load a bit on tube amps and get away with it, *but* doing so risks sounding bad, reducing headroom and possibly reducing the life of your power tubes, so I would definitely stick to a 4ohm load.
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[quote name='Bill Fitzmaurice' timestamp='1361549374' post='1987405']
The [i]impedance [/i]of the original cab was 4 ohms. It was designed to handle two, making it 2 ohms. We used to run them with anywhere between 1 and 8 ohms with never any complaint. I doubt it would be all that happy at 16 ohms, though.
[/quote]
Can't thank you enough for all that really useful information!! Really is amazingly helpful and to know that a 4 ohm cab is best suited - and yes, a 16 ohm cab is NOT going to right at all. I guess you could get away with 8 ohms at a pinch but 16 is out.
The OT is original to the amp but a UK spec mains transformer has been fitted so that may make a difference? I have the 110v transformer so that could easily be re-installed.
Thanks again guys.....