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Ohm drop when speaker is hooked up ???

fishinjim

Member
Messages
101
Amp is a music man rd112. Circa 79.
Two 6l6gc tubes. One 12" speaker. Switchable between 4 ohm or 8 ohm.

When I hook up a speaker (currently 8 ohm eminence) to the amp, I get a speaker reading of .9 to 1.1 ohms.
Disconnect the speaker, and it reads as it should (roughly 7 ohms).
Checked with three other 8 ohm speakers and had the same results.
Checked with extension cab plugged in and had the same results.
No changes when switching between the 4 and 8 ohm selector on the amp.

First thought was a short in the speaker cable, but results are the same with other cables.

Leads me to believe there is some sort of short inside the chassis?
Will be pulling the chassis this weekend, but am not sure what I should be looking for.

Lil amp history in case it could be relative...
I've had this amp for 15 years. Never checked the ohms on a speaker while in it before, so I have no idea how long this has been going on.
Have had the chassis out a couple times to clean the pots, but never done anything to any of the wires.

One more thing... in 2002, this amp spent a day under water when my basement flooded to the ceiling.
I let it dry out for a couple months, turned her on, and it sounded as good as it ever did. Wouldn't be surprised if this started a slow failure process of some sort though.

Cap change is on short order, but I'm not sure if that would be effecting the speaker load?

Anyone have any ideas as to what might be going on here?

Thanks.
 

Blue Strat

Member
Messages
30,595
You can't measure the speaker resistance/impendance with it connected to the amp. What you measure when you do that is the secondary of the output transformer (in the amp) which is close to zero ohms.

The readings you get with the speakers disconnected are right and the only readings that matter.
 

mark norwine

Member
Messages
17,229
Back up.

Does the amp not work?

What malfunction causes you to be making these measurements (meaningless though they are) in the first place?
 

fishinjim

Member
Messages
101
What malfunction causes you to be making these measurements (meaningless though they are) in the first place?
Why are you even bothering to make that measurement? As Blue Strat said, it is meaningless.
No malfunction. Was just changing some speaker and cabinet configurations around and checking some unmarked speakers for impedance. Curiosity led me to checking the speakers hooked up.

You can't measure the speaker resistance/impendance with it connected to the amp. What you measure when you do that is the secondary of the output transformer (in the amp) which is close to zero ohms.

The readings you get with the speakers disconnected are right and the only readings that matter.
Thank you, Blue Strat.
I had tried searching cyberspace for whether or not this was the case, but came up empty on answers.
Checked the connected speaker in a small Fender solid state amp and that speaker read the same either connected or disconnected. This led me to assumption about the Music Man. I'm guessing that solid state circuitry isn't set up quite the same.
Long way from getting my electricians license here. lol.
Thanks again.
 

bluesky636

Member
Messages
3,025
Checked the connected speaker in a small Fender solid state amp and that speaker read the same either connected or disconnected. This led me to assumption about the Music Man. I'm guessing that solid state circuitry isn't set up quite the same.
Long way from getting my electricians license here. lol.
Thanks again.
Tube have a high output impedance and require a transformer to match the low impedance load of the speaker.

SS amps, on the other hand, have a low output impedance closer to that of the speaker and don't require a matching transformer though some early SS designs did use an OT to match the speakers.
 

drbob1

Gold Supporting Member
Messages
29,513
So, the consequence of what Bill said is that when you measure the impedance "in circuit" of a speaker in a solid state amp you measure the speaker in parallel with the output transistor (which should be a very high impedance) and get 6 ohms or so for an 8 ohm speaker.

With the tube amp the speaker will be in parallel with the speaker. The impedance of the output coil of the OT is near zero, so the combined impedance will be near zero.
 




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