Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dinkotom on August 01, 2016, 10:07:27 am
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Does anyone have this amp? I'd like to know the individual speaker impedance for this amp, schematic says 16 Ohms:http://schematicheaven.net/fenderamps/cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf (http://schematicheaven.net/fenderamps/cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf) But I have four 8 Ohm speakers (they look stock, blue fender label, CTS I guess) in my amp, paralleled total load measured is 2.3 Ohms. The back of the amp says 4 Ohm minimum load. My best guess is that the previous owner replaced the speakers with incorrect impedance. Or Fender screwed up...
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I agree.
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Check if your jacks exactly match the schematic.
As I read it:
Grn-Blk is 4 Ohms. Grn/Yel-Blk is 2 Ohms.
No plugs, the amp is shorted.
Plug to left (on schem) jack: connect 4 Ohms
No plug to left jack and plug to right jack: connect 2 Ohms
Plugs to both jacks: connect two 4 Ohm loads
Personally I'd incline to ONE jack and a 2/4 Ohm selector switch. Trust the user to do the right thing.
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I am wondering if one of your 8 ohm speakers has failed. 2.3/0.75 = 3 ohms, which is very close to the 3.2 ohms expected. I would verify that I did not lose a speaker.
FYI One amp manufacturer would consider your 2.3 ohm load, if dcr, to be a 4 ohm nominal load. (Sound Electronics, the current company that once made the sound city 120.) I do agree with Sluckey.
As PRR has pointed out, the schematic shows both a 2 ohm and 4 ohm tap. It won't hurt to plug into the 2 ohm tap.
I would also verify that the o/t is also correct. Look for part numbers on the o/t, then cross reference on the hammond transformer website, for specifications. Fender didn't always put the correct parts in their amps. I found reference to the 013897 part number on the bustedgear website. I did not find that part number on the hammond website. found reference to same part number on Mercury Magnetics website, provides additional basis for PRR's 2 and 4 ohm tap info.
You did not say whether or not this was dcr (direct current resistance) of the load. It is my understanding testing for nominal load is a lot trickier than checking dcr.
I would feel more comfortable wiring the speakers for an 8 ohm resistance.
The same manufacturer also shows a diagram on their website, on how to wire the four 8 ohm speakers to have an 8 ohm resistance. (The website shows the wiring for 32 ohm using four 32 ohm speakers) it is a form of series-parallel wiring.