Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frpax on September 06, 2023, 03:41:29 pm
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So, I'm building a 5E3.
There are 2 jacks for speaker(s). One goes to the speaker in the combo, and the another one for an extension speaker (or cabinet, or whatever).
The speaker in the Combo should be 8 ohms (it has an 8 ohm tap from the OT).
Question(s) are:
Can I run an extension cabinet (or speaker) at the same time, as long as it's 8 ohms? In other words, if I plug in an extension, sound comes out of both?
Or do both speakers have to come out to 8 ohms combined??
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Combined speaker impedance. 8 ohm tap = 8 ohm speaker load.
Dave
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Combined speaker impedance. 8 ohm tap = 8 ohm speaker load.
Dave
Thank you.
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Or do both speakers have to come out to 8 ohms combined??
For maximum power transfer your total speaker load should match the OT impedance. However, you will not hurt that amp to run a 4Ω combined speaker load connected to an 8Ω OT.
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Or do both speakers have to come out to 8 ohms combined??
For maximum power transfer your total speaker load should match the OT impedance. However, you will not hurt that amp to run a 4Ω combined speaker load connected to an 8Ω OT.
Good info. Thank you!
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... The speaker in the Combo should be 8 ohms (it has an 8 ohm tap from the OT).
Question(s) are:
Can I run an extension cabinet (or speaker) at the same time, as long as it's 8 ohms? ...
Or do both speakers have to come out to 8 ohms combined??
When Fender sold that 5E3 Deluxe, they also sold an Extension Speaker that was just an 8Ω speaker in a box.
When the player plugged their extension speaker in, they got a 4Ω net load to their tweed Deluxe. Power transfer was slightly reduced, but the net result seemed louder.
People warning you to "match the tap impedance" are advising you in good faith. But the amp manufacturers didn't sweat the details as much as folks do on forums.
(https://i.imgur.com/nXe1497.png)
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amp manufacturers didn't sweat the details
:laugh: they sell amps, blowing up amps is good for sales
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I agree with what I have been reading here. Personally, I would not hesitate to operate an amp at a 100 percent mismatch, but advising someone else to do it is a whole other can of worms. Maybe I should have added the caveat "Operating an amp at a 100 percent mismatch is not optimum, but it is unlikely to cause a hardware failure in the amp."
Dave
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amp manufacturers didn't sweat the details
:laugh: they sell amps, blowing up amps is good for sales
You're joking, but Fender sold those extension cabinets in the 1950s, 1960s, and 1970s (at least). Yet, no one ever heard of "blown up Fender amps" from using the extension speaker, and users mostly accepted that the amp-maker knew best if they were providing a product.
(https://images.reverb.com/image/upload/s--eibw_4gs--/a_exif,c_limit,e_unsharp_mask:80,f_auto,fl_progressive,g_south,h_1600,q_80,w_1600/v1432413618/lpbzrupvjbcfbtzl2lqx.jpg) (https://images.reverb.com/image/upload/s--PlVJ9om4--/a_exif,c_limit,e_unsharp_mask:80,f_auto,fl_progressive,g_south,h_1600,q_80,w_1600/v1432413787/arivltogphqf309a5bkl.jpg) (https://images.reverb.com/image/upload/s--6b4E0YSq--/a_exif,c_limit,e_unsharp_mask:80,f_auto,fl_progressive,g_south,h_1600,q_80,w_1600/v1432415510/vd7urnervc9c4ecyrdpx.jpg)
It's kind of important people know they're not going to blow stuff up using an extension speaker. RCA published the effect of changing loading of an output tube way back in 1936 (https://www.one-electron.com/Archives/RCA/RCA-AppNotes/RCA%201936%20AN-60%20Operation%20of%20the%206L6.pdf) (scroll to the last page for the graph). They mostly showed loading moving downward, because moving upward increases distortion of all types while also reducing power output, and was therefore likely considered "not desirable."
We don't need "optimum load" all the time. We also mostly don't get it anyway, because the speaker is almost always an impedance higher than marked on the frame (https://eminence.com/cdn/shop/products/Red_White_and_Blues_a27168d6-7636-4636-980e-a478ff3ce9f1_1024x1024@2x.png?v=1622744939) ("Impedance" is plotted in a Peach-color, using the scale on the right edge of the graph).
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A cathode-bias power amp won't blow-up in a dead short. (Unlike early transistor amps.)
It can't make any power in zero ohms, but it won't die.
Half impedance is a perfectly sane approach, if (as in the Fender sales-lit) it lets you double the coverage; wide room or 2nd room.
Double impedance usually survives, but this way you are flirting with over-voltage arcing in the OT. (Note that, on HBP's graph, an "8 ohm" speaker is 60 ohms on your lowest notes, and the OT was papered to stand that much, but maybe not a whole lot more.)