surfguy13 Posted February 22, 2013 Share Posted February 22, 2013 Can anyone tell me what the resistance is for a AB165 circuit Bassman head from 1967? I was told by an amp tech that I could use a 4,8 or 16 ohm cab. Is this correct? I can't find any indication as to what the resistance is on the internet so maybe the tech was correct? Any info would be MOST welcome!! Thanks. Quote Link to comment Share on other sites More sharing options...
6v6 Posted February 22, 2013 Share Posted February 22, 2013 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: http://www.turretboards.com/layouts_schematics/fender/schematics/fender_bassman_ab165.gif 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: http://www.tubesandmore.com/products/P-T1750L 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. Quote Link to comment Share on other sites More sharing options...
Bill Fitzmaurice Posted February 22, 2013 Share Posted February 22, 2013 [quote name='surfguy13' timestamp='1361545599' post='1987322'] Can anyone tell me what the resistance is for a AB165 circuit Bassman head from 1967? I was told by an amp tech that I could use a 4,8 or 16 ohm cab. Is this correct? [/quote]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 Link to comment Share on other sites More sharing options...
surfguy13 Posted February 22, 2013 Author Share Posted February 22, 2013 [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. [/quote] [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..... Quote Link to comment Share on other sites More sharing options...
3below Posted February 22, 2013 Share Posted February 22, 2013 UK mains transformer should not make any difference providing it is right specification - giving correct HT (B+) etc. Quote Link to comment Share on other sites More sharing options...
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