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Old 17th February 2009, 02:03 AM   #1
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Default Cabinet Help!!!!

Here's what I have:

Fender Blues Junior - 8 ohm speaker

Here's what I want to do:

Build a cabinet, and plug in both the internal speaker AND the cabinet. BUT that changes the load. What will happen to my amp and sound if I:

1 - straight up, put a 16 ohm load on this amp?
2 - put the cab & internal in series, and parallel that with a 16 ohm resistive circuit, for 8 ohms?
3 - put the cab & internal in parallel, and series that with a 4 ohm resistive circuit for 8 ohms?
4 - do either #2 or #3, but with an inductive circuit (in which case, how do I build one...??)?

PLEASE, GUIDE ME TO THE TONE ZONE!!!!!!!
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Old 17th February 2009, 05:19 AM   #2
Jeb-D. is offline Jeb-D.  United States
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What is your motive?

If you want to be able to drive the amplifier harder while playing at lower volumes, get an L-Pad or one of those "hot plates" (it's pretty much a heavy duty fancy looking L-Pad). It will allow you to attenuate the signal going to the speaker while maintaining an 8ohm impedance.

Whatever you decide to do, don't improperly load the amplifier. Check the output transformer on the amplifier. It may have a 4ohm tap.
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Old 17th February 2009, 02:44 PM   #3
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I just want to be able to run the internal speaker, and run the cabinet speaker, and run both speakers together. The amp is open backed, the cab will be closed, different speaker, different tone possibilities. It's not an attenuation thing, it's completely tonal. If it's not possible, I certainly don't plan on improperly loading. I know the tone I'm looking for, and I'm in the research phase right now, but I'm having trouble finding the amp/load info I need in order to start designing.

I don't know anything about transformers - how would I tell if it has a 4 ohm tap? That would certainly be easiest...
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Old 17th February 2009, 05:03 PM   #4
Jeb-D. is offline Jeb-D.  United States
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Quote:
I don't know anything about transformers - how would I tell if it has a 4 ohm tap? That would certainly be easiest...
Track the wire that is connected to the speaker, it will lead you to the transformer. On the secondary side of the transformer (the side that goes to the speaker, not the tubes) See if there are any wires or solder lugs that aren't being used. Or, if there are any wires that are just connected to each other. If it does, let me know what you find and I can help you with what to do with it.

You may consider just switching out the existing speaker with a different type to get the tone you want. Or make it so that you can use the internal speaker OR the cabinet. To do this you can just wire up a 1/4" shorting type jack so that when nothing is inserted the signal goes to the internal speaker. But when the external cabinet is plugged in, the signal goes to that instead.

Your thoughts 2 and 3 will work, but half of your amplifiers output power will be wasted as heat in the resistors. The resistors would have to be fairly large. It's will also alter the impedance plot and damping characteristics. May give better tone, but may give worse.

Your thought 1 will work, but the amp was optimized for 8ohm (on the tap being used at least). You will loose half the output power and some low frequency extension by loading it with 16ohms. I wouldn't recommend this option, but nothing should blow up. Whatever you do, don't load the 8ohm tap with 4ohms. That would probably cause something to blow.
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Old 17th February 2009, 05:46 PM   #5
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The Blues Jr. has only an 8ohm secondary wiring. So unless this is a "clone" or had the OPT replaced with a more "universal" replacement you are stuck with that.

What you could do is wire in a "Universal" Type OPT. A Hammond 1620A would allow you 4 - 8 - 16 Ohm configurations along with better response and power handling over the original.

Pretty simple to wire in and costs like $100

http://cgi.ebay.com/Hammond-1620A-20...742.m153.l1262
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Old 17th February 2009, 05:54 PM   #6
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What you would do is wire the stock jack to the 8 ohm secondary and Plug in when using just that speaker, unplug and insert the external cabinet plug for just the cabinet.

Wire 2 additional jacks in Parallel to the 4 Ohm and plug cab and internal speakers to those when you want them together.

I can't really see a use for the 16Ohm tap, but then again I am an Audio Guy not a Guitar guy so I am sure you might think of something.

BTW...Be sure to wire the speakers in PHASE !
Unless you want to try the cab out of phase it might make some interesting sound???
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Old 17th February 2009, 06:20 PM   #7
Jeb-D. is offline Jeb-D.  United States
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Quote:
What you could do is wire in a "Universal" Type OPT. A Hammond 1620A would allow you 4 - 8 - 16 Ohm configurations along with better response and power handling over the original. Pretty simple to wire in and costs like $100 http://cgi.ebay.com/Hammond-1620A-2...1742.m153.l1262
A universal OPT is a good suggestion. But typically HiFi transformers aren't desirable for guitar amps.
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Old 18th February 2009, 02:23 AM   #8
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This is fantastic - thanks for all the help.

Now, I'm liking the "do nothing - go with 16 ohms" option, because it sounds by far like the easiest. I just didn't know if it would destroy the transformer or tubes. If it's not going to do that, and judging by what has been said here, and since the Blues Junior has a flabby low end, and since I've never had it up past 4 (even in gigs or practice), let me run this by you, and correct me if I'm wrong please:

Halfing the output would not significantly decrease the audible volume, which I can compensate for by turning up the Master. However, since that's a linear pot, not an audio taper, that would not help as much. Therefore, I could just increase the preamp volume, increasing the overdrive. This would solve the volume issue, saving the output tubes and transformer, and then I could just clean up the tone as desired by rolling back on the guitar volume.

Am I on glue? Since this is a "no mod" approach, even if I suffer tone loss because of the increased load, I can always just go with the internal or the cab.
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Old 18th February 2009, 03:02 AM   #9
Jeb-D. is offline Jeb-D.  United States
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Give it a try. You can always undo it.
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