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Impedence of JBL D140F [message #27491] Mon, 09 September 2019 11:08 Go to next message
oldnavycdr is currently offline  oldnavycdr
Messages: 38
Registered: July 2006
Location: Sanford, NC
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I have a late 60's K100 with the single 15" speaker -- a JBL D140F. As I understand, these speakers were 8 ohms but I want to make sure before I try using a different amp head than the K100. I'm picking up a Vox AC15 Custom head which has outputs for either 8 ohm and 16 ohm speaker cabinets. One thing I want to make sure of is to avoid any damage to this vintage (and original) JBL speaker. It sounds wonderful with the K100 head but I want a variable wattage amp.

Old Navy
Re: Impedence of JBL D140F [message #27492 is a reply to message #27491] Mon, 09 September 2019 17:58 Go to previous messageGo to next message
thetragichero is currently offline  thetragichero
Messages: 29
Registered: April 2019
Junior Member
got a multimeter? even a cheap one from harbor freight?
i personally don't trust 1. the seeming lack of specs for 50 year old drivers 2. any particular 50 year old driver to not be held together with the equivalent of chewing gum

i could just be an incurable cynic
Re: Impedence of JBL D140F [message #27493 is a reply to message #27492] Mon, 09 September 2019 18:08 Go to previous messageGo to next message
oldnavycdr is currently offline  oldnavycdr
Messages: 38
Registered: July 2006
Location: Sanford, NC
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The speaker has held up well, rarely played at full volume and remains smooth with no break up. Cone is intact and no voice coil rattle. I have found specs for it but I'm wondering if Kustom ordered 16 ohm speakers rather than the stock 8 ohm. I do have a multimeter and can test but my question has more to do with the load on the amp. I want to match amp output impedance with the correct speaker impedance thus protecting both amp and speaker.

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Re: Impedence of JBL D140F [message #27494 is a reply to message #27491] Mon, 09 September 2019 19:33 Go to previous messageGo to next message
oldnavycdr is currently offline  oldnavycdr
Messages: 38
Registered: July 2006
Location: Sanford, NC
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Found my answer on the site. K100 are 0 ohm amps. All should work out.

Old Navy
Re: Impedence of JBL D140F [message #27495 is a reply to message #27491] Tue, 10 September 2019 06:06 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4212
Registered: June 2004
Location: NY
Senior Member
Yes that a 8 ohm JBL bass driver if it was original to the Kustom K100 amp.

Being a bass driver it only goes up to 2500 HZ in frequency responce so it's going to sound pretty dark if played thru by a 6 string guitar!

If your plan is to use the JBL on the Vox amps 16 ohm output tap then you will drop off 1/3rd of the Vox amps output wattage due to the impeadance mismatch ,and if the Vox amp was for example a 2-12" model then you also will loose 19% of the cone area which will further make for a sound pressure reduction!

The impeadance mismatch will also roll off some of the Vox amps bottom end.

JBL drivers where wired for reverse polarity , so keep that in mind and prove that out by testing it with a 9 volt Battery,
Re: Impedence of JBL D140F [message #27496 is a reply to message #27495] Tue, 10 September 2019 15:44 Go to previous messageGo to next message
oldnavycdr is currently offline  oldnavycdr
Messages: 38
Registered: July 2006
Location: Sanford, NC
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Thanks Steven. The Vox amp I'm considering has both 8 ohm and 16 ohm speaker jacks so I should be good.

Old Navy
Re: Impedence of JBL D140F [message #27497 is a reply to message #27496] Wed, 11 September 2019 09:40 Go to previous messageGo to next message
thetragichero is currently offline  thetragichero
Messages: 29
Registered: April 2019
Junior Member
because the vox is a tube amp do not mismatch the impedance between the load (speaker) and source (amp)
it sounds like you're already aware of that but there's a possibility someone may come to this thread 5 years from now via search engine so i would feel remiss if this thread ended without its mention
Re: Impedence of JBL D140F [message #27500 is a reply to message #27491] Wed, 11 September 2019 12:20 Go to previous message
stevem is currently offline  stevem
Messages: 4212
Registered: June 2004
Location: NY
Senior Member
Your posted comment on having a "variable wattage amp "is what made me post the info on what happens when you mismatch a speaker load on a tube amp , as that is the only way to change a tube amps output wattage without digging into its electronic's.
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