Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on October 18, 2012, 07:29:10 am
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Hello gangsta's !
My client asked me yesterday what speaker load had to be connected to the speaker out jack of a Fender Tremolux blackface guitar amplifier( AB763 blackface ).
I didn't know :w2:.......... My super reverb (AB763 blackface too) needs a 2 OHM load. The back panel schem doesn't show, neither does the schem ( attached ).
Regards
Colas
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Standard Fender OT/speaker ohms outputs are 8 ohm if amp has 1 speaker and 4 ohm if the amp has 2 speakers?
Brad :icon_biggrin:
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Fender almost always used 8Ω speakers, except for the Champ/early Princeton amps.
The Tremolux AB763 schematic (http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_AB763.pdf) shows 2 speakers in parallel. I'd take that as a 4Ω load expected at the speaker jack.
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so 8 ohm if only one speaker cab ( 8 ohm ) is used ?
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Fender almost always used 8Ω speakers, except for the Champ/early Princeton amps.
The Tremolux AB763 schematic (http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_AB763.pdf) shows 2 speakers in parallel. I'd take that as a 4Ω load expected at the speaker jack.
Depends on how many speakers Fender had in the amps cab.
Brad :icon_biggrin:
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4 ohms load , but Fender Blackface can take 100% mistmach = 2 to 8 ohms
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so 8 ohm if only one speaker cab ( 8 ohm ) is used ?
No, the correct load for an AB763 Tremolux is 4Ω, just like a Twin Reverb.
I'm showing you how to figure this on your own, using the schematic. The Twin Reverb (http://www.el34world.com/charts/Schematics/files/fender/TWIN_REVERB_AB763.pdf) shows two (presumably 8Ω) speakers in parallel for 4Ω, while the Super Reverb (http://www.el34world.com/charts/Schematics/files/fender/super_reverb_ab763_schem.pdf) shows four 8Ω speakers in parallel for 2Ω. The 5E7 Bandmaster (http://www.el34world.com/charts/Schematics/files/fender/bandmaster_5e7_schem.pdf) shows three 8Ω speakers in parallel for 2.67Ω, and the Deluxe Reverb (http://www.el34world.com/charts/Schematics/files/fender/DELUXE_REVERB_AB763.pdf) shows one 8Ω speaker for 8Ω.
Yes, in the real world, you can cheat the load higher and lower. But the question was what is the intended load for a given Fender amp. Now you know how to answer that for yourself.
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HBP,
I tried to figure this out from the schem but it doesn't show. I am not familiar quite well with Fender amps ( nor with any other I can say :laugh:).
Best regards
Colas
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I believe that there was an option for a single 10" cabinet for an AB 763 Tremolux, using the same head.
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HBP,
I tried to figure this out from the schem but it doesn't show.
The schematic items circled are the symbols for a speaker. Two are shown, and wired in parallel.
From there, you have to apply your own knowledge, knowing that Fender mostly used 8Ω speakers, except in the Champ/VibroChamp/Bronco (4Ω speaker) and in some odd bass models (with a 32Ω speaker).
I believe that there was an option for a single 10" cabinet for an AB 763 Tremolux, using the same head.
We're making this harder than it has to be. The tweed 5E9 Tremolux (http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_5E9A.pdf) used a single 12" for an 8Ω load, 5G9 Tremolux (http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_5G9.pdf) used a single 12" for an 8Ω load, the blonde 6G9 Tremolux (http://www.el34world.com/charts/Schematics/files/fender/TREMOLUX_6G9.pdf) used a single 10" for an 8Ω load (same with 6G9-A), and the 6G9-B Tremolux (http://www.el34world.com/charts/Schematics/files/fender/tremolux_6g9b_schem.pdf) used two 10" speakers for a 4Ω load.
The schematics don't tell you the speaker size; use something like the Fender Amp Field Guide (http://www.thevintagesound.com/ffg/) (or your fave resource) for that info. But the schematics do indicate number of speakers and the wiring. You should be able to figure proper load impedance from that info.
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HBP, knowing that Fender used only 8 ohm speakers, the math is not a prob to find out the total load. But I did not know that, thank you for the short tutorial.
Regarding the tubes, we often see that 7025's and 12ax7's are just different names for the same tubes, in tube data charts as a matter of fact. Why, especially in Fender amps, we see both tubes bearing different names in a same amplifier ? There must be a difference between both tubes then, even a slight difference, no?
Regards
Colas
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( sorry for the red color, it is hard to read )
HBP, knowing that Fender used only 8 ohm speakers, the math is not a prob to find out the total load. But I did not know that, thank you for the short tutorial.
Regarding the tubes, we often see that 7025's and 12ax7's are just different names for the same tubes, in tube data charts as a matter of fact. Why, especially in Fender amps, we see both tubes bearing different names in a same amplifier ? There must be a difference between both tubes then, even a slight difference, no?
Regards
Colas
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Regarding the tubes, we often see that 7025's and 12ax7's are just different names for the same tubes, in tube data charts as a matter of fact. Why, especially in Fender amps, we see both tubes bearing different names in a same amplifier ? There must be a difference between both tubes then, even a slight difference, no?
We just have to separate what is true today from what was true when Fender made these amps.
If you look at a 7025 data sheet (http://www.mif.pg.gda.pl/homepages/frank/sheets/168/7/7025.pdf) from the old days, they almost all claim the 7025 is identical to the 12AX7 but has less hum/noise.
The first stages of the preamp have the most amplification following them, so a little hum/noise there has a big impact. Later stages have less amplification following them, so hum/noise in that stage could be slightly higher while still being largely undetectable in the speaker. The 12AY7 was also originally designed as a low-noise tube for the first stages of an audio amp. Therefore, Fender began specifying the 12AY7 for input stages in the 50's, then switched to the higher-gain 7025 for inputs once that type became available.
The higher gain of the 7025 most likely offset the increased loss of the new blonde/brown and blackface tone circuits.
Eventually, most 12AX7's probably converged to include features to reduce hum and microphonics. Such changes might be how the heater is physically arranged and placed in the cathode sleeve, shortening of plate (to stiffen the structure and reduce microphonics), altering plate geometry, etc. A late-60's RCA 12AX7A might have been just as good as a mid-50's 7025 with regard to these issues, as the manufacturer improved their processes.
And there used to be differences in other similar types in the past as well (just look up all the types that supposedly substitute for a 6L6).
Modern tube vendors will put any label on a tube that sells the tube. They may use/abuse substitution charts to apply various labels to types that they have which are similar to each other, but include some kind of difference. Groove Tubes would do that to denote different country-of-origin, or maybe slap a "7025" label on an otherwise normal 12AX7 on which they had performed some kind of extra testing.
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Cristal clear explanation.
Best regards,
Colas