Ragpicker
Wholenote
I'm playing it in a different key
Posts: 336
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Post by Ragpicker on Sept 11, 2020 22:22:11 GMT -5
I picked up a Princeton RRI . Nice amp . My question is, can I use an 8ohm extension speaker AND the 8ohm internal speaker ? There is verbiage on the output jacks that says 8 ohm MINIMUM . Of course we know that running two 8s means the transformer is seeing 4. Im told that Fender XFMRs will be ok with 4ohms. Help !
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Sept 12, 2020 4:16:36 GMT -5
I suggest that if you’re going to be playing it loud, make sure that the total load the amp ‘sees’ is 8 ohms.
At low power outputs it doesn’t really matter.
If the load is higher than 8 ohms, then at high power outputs, the power tube screen grids will dissipate more than the designer intended.
The output transformer can’t get damaged directly by use of a load impedance below the amp’s intended nominal - if the tubes and amp are functional then they can only pass so much current, which will be way below the transformer’s limit.
But if the amp’s used at high power output level into a ‘low’ load, the power tube plates may overheat. If that happens their service life will be somewhat reduced, if the overheating is significant then they might wear out in minutes or fail catastrophically, eg short out. It’s the latter that can cause transformer damage.
Of course that’s a worst case scenario, you’d have to be unlucky and trying hard for that to happen.
Whatever, my take is that it’s the somewhat lower resilience of modern tubes that’s the reason Fender put that warning on the back panel. I think that the warning should advise that the total load should be 8 ohms, rather than ‘8 ohms minimum’.
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Post by Vibroluxer on Sept 12, 2020 15:41:48 GMT -5
So does that mean you need a 16 ohm speaker in the amp if you plan on running an 8 ohm load with an 8 ohm cab?
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Sept 12, 2020 17:41:28 GMT -5
If you’re looking to crank it with an ext cab, that would be one way around it. Another would be to fit an OT with the output impedances you need, and a switch to select the one appropriate. Here’s a Classictone Princeton Reverb OT, $40 with 4 and 8 ohms outputs. www.classictone.net/40-18045.html |
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Post by Vibroluxer on Sept 12, 2020 18:46:02 GMT -5
I was just curious, thanks for the education!
Dang, I meant a 16 ohm speaker in the cab and another in the amp.
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Post by Peegoo 🏁 on Sept 12, 2020 20:51:54 GMT -5
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Post by Vibroluxer on Sept 12, 2020 21:10:22 GMT -5
That's interesting. I'm guessing most guitarists don't do that and just plug in the cab.
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Sept 13, 2020 4:45:24 GMT -5
Yes, and by and large, unless they absolutely crank it / get unlucky (perhaps due to a tube not being as resilient as a type bogey), they won’t notice an immediate problem.
Haha, re the 16ohm speaker thing, I read it how you meant it.
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Post by rdr on Sept 14, 2020 9:07:09 GMT -5
One solution can be use of a Weber Z-matcher. It also includes a volume controlled line out. I have the 50-watt version.
Tedweber.com/z-matcher/
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Post by Seldom Seen on Sept 14, 2020 15:31:59 GMT -5
I'd run the two 8-ohm platforms from a Radial CabLink in series at 16-ohms and play on though it'll take an adaptor to connect the on-board speaker to the CabLink.
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pdf64
Wholenote
Posts: 556
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Post by pdf64 on Sept 15, 2020 8:39:51 GMT -5
A ‘higher than nominal’ loading (16 ohms if a Radial Cablink was used as above, compared to the amp’s 8 ohm nominal) pushes additional dissipation on to the power tubes’ screen grids. So I can’t see an advantage over a ‘lower than nominal’ loading, eg plugging the two 8 ohm speakers into the regular jacks on the back of the amp, thereby resulting in a 4 ohm total load? As whichever way a mismatch goes, the power tubes get put under additional stress. Consider that if connecting ext cabs in series was ‘better’ than them being in parallel, then why wouldn’t the manufacturer arrange the jacks on the back of the amp to be in series (rather than in parallel)? As any consequent improvement in tube life or amp reliability would reflect well on them. Note that some late 70s Fenders did indeed have their speaker jacks wired in series; the ext jack included a switch that changed to a higher impedance tap on the OT secondary when that jack was used. el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf |
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Post by LTB on Sept 16, 2020 8:08:08 GMT -5
One solution can be use of a Weber Z-matcher. It also includes a volume controlled line out. I have the 50-watt version. Tedweber.com/z-matcher/ I am using the Weber Z-Matcher to run an external cab with 8 ohm Eminence Lil Texas Neo speaker in conjunction with my Waza Speaker in my Katana Artist (rated output 8 or 16 ohms). It is rated at 100 watts RMS and no perceivable coloration of sound. I did this because I do not want to change the Waza that came in my amp and do not want to wire them in series. |
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