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FDP Forum / Amp Mods, Repairs, and Projects / Vox AC15 C1 speaker ohm question

Ryder
Contributing Member
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Northwest Missouri

Butterscotch Blues
Feb 25th, 2017 07:21 AM   Edit   Profile   Print Topic   Search Topic

I'm confused from reading online.

Is the 8/16 ohm switch only for an extension cab and or for the internal speaker which is 16 ohm stock?

pdf64

UK

Feb 25th, 2017 10:52 AM   Edit   Profile   Print Topic   Search Topic

It may be like that so as to facilitate the user connecting the amp to 8 ohm or 16 ohm loads, however they may be arranged / comprised.

(This message was last edited by pdf64 at 01:02 PM, Feb 25th, 2017)

ejm

usa

Feb 25th, 2017 11:47 AM   Edit   Profile   Print Topic   Search Topic

I looked at the manual. I'm not surprised that you're confused.

If what I see is correct, there is a difference between the External jack and the Extension jack. They are NOT the same and are wired differently. I haven't seen everything on the planet, but I was surprised at this.

The Extension jack is for using the internal speaker AND and extension cabinet.

The External jack is for using an external cabinet ONLY.

From the manual:

EXTENSION SP:
This speaker jack runs parallel with the internal speakers which are wired for 16 Ohms. The extension cabinet must be 16 Ohms.
EXTERNAL SP:
This speaker jack will mute (disconnect) the internal speakers and you can hook up either a 16 Ohm or 8 Ohm cabinet. Be sure to set the Output
Select switch accordingly.

Once you know this, then from the manual for the Impedance switch:

16 OHMS: Combos allow for the following configurations:
1) Set it to this if you use the internal speakers only.
2) Set it to this if you connect an external 16 Ohm speaker cabinet through the External jack.

8 OHMS: Combos allow for the following configurations:
1) Set it to this if you connect an extension cabinet through the Extension jack.
You’ll be running the internal and external speakers in parallel. The impedance of
the extension cabinet must be 16 Ohms.
2) Set it to this if you connect an external 8 ohm speaker cabinet through the External jack

Edited to add: Make sure that the amp is always seeing a load. Without thinking about it too much, this switching system offers a lot of flexibility, but it also could potentially be a recipe for disaster.

The end.

(This message was last edited by ejm at 03:45 PM, Feb 25th, 2017)

Ryder
Contributing Member
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Northwest Missouri

Butterscotch Blues
Feb 25th, 2017 04:26 PM   Edit   Profile   Print Topic   Search Topic

So what you are saying is, I can't put in an internal 8 ohm speaker?

Ryder
Contributing Member
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Northwest Missouri

Butterscotch Blues
Feb 25th, 2017 06:10 PM   Edit   Profile   Print Topic   Search Topic

Here is where I got confused. From Tonesmith site.
Question:

"I’m getting ready to install the Bad Cat Celestion and I noticed that the Green back I’m removing is a 16ohm speaker. Can I really just replace a 16ohm speaker with an 8ohm?"

Reply:

tonesmiths says:
February 19, 2016 at 12:52 pm
"You just need to slide the ohm selector to 8 ohm. It’s global, so it controls the ohm load to any and all speakers connected internally or externally."

Back to me. So, does this make sense?

(This message was last edited by Ryder at 08:11 PM, Feb 25th, 2017)

pdf64

UK

Feb 26th, 2017 02:34 AM   Edit   Profile   Print Topic   Search Topic

Yes, that makes sense.
To use an 8 ohm load, however it's arrived at, switch the amp to 8 ohms.

Ryder
Contributing Member
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Northwest Missouri

Butterscotch Blues
Feb 26th, 2017 12:18 PM   Edit   Profile   Print Topic   Search Topic

Ok thanks. It makes sense but I seem to recall the manual being confusing on that.

Tonesmith was touting the Celestion speaker, 12 in., for Bad Cat amps. "It takes away the shrill high of the alnico blue and greenback."

I've never noticed that so much as my ac15 sounds like it has a "cover" on it.

Not to mention, the bad cat speaker is $159, whereas the blue is $279.

(This message was last edited by Ryder at 05:24 PM, Feb 26th, 2017)

Ryder
Contributing Member
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Northwest Missouri

Butterscotch Blues
Mar 1st, 2017 04:19 PM   Edit   Profile   Print Topic   Search Topic

Check this out from the gearpage.com.

"if you have a new AC15C1 - per Vox you cannot do what I have done above. The Ohm switch only applies to the output jacks for either an extension or external cab. The internal speaker will continue to run on 16 ohm resistance, thus if you change the internal speaker with a lower resistance speaker (8ohm or less) you will run hot and possibly damage the output transformer over time and add additional wear to the tubes. Two things you can do; attach the internal speaker to the external cab jack and set the switch to 8 ohms (you must use the external and not the extension jack to do this for obvious reasons. Also, you could add an in line 8-ohm resistor to the speaker line to increase the impedance."

That was the one I was looking for.

Te 52

Laws of Physics

strictly enforced
Mar 2nd, 2017 10:49 AM   Edit   Profile   Print Topic   Search Topic

"...Also, you could add an in line 8-ohm resistor to the speaker line to increase the impedance..."

Bad idea. That would protect the OT, but change the speaker frequency response -- and hence the tone -- a lot.

Also doesn't mention that if you *did* decide to do this, the resistor must be a high wattage power resistor, not one of the the usual small 1/4 watt resistors used in guitar circuits.


Ryder
Contributing Member
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Northwest Missouri

Butterscotch Blues
Mar 2nd, 2017 02:01 PM   Edit   Profile   Print Topic   Search Topic

I'm not going to mod mine, and I'm also only using a 16 ohm internal speaker.

FDP Forum / Amp Mods, Repairs, and Projects / Vox AC15 C1 speaker ohm question




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