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Post subject: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 5:13 pm
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Hey guys, I recently started using an extension cab with my bass amp (SWR RedHead) and the first thing I noticed is that the speaker kill switch does not kill the extension cabinet. It only kills the internal speaker. I would prefer it kill them both so I decided to take a look inside and see if there was something really simple that even a layman like me could do to rectify the situation. I have not yet touched anything except to poke around with my ohm-meter to see what goes where.

Here is the amp as it sits on my kitchen table.

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The green and white wires are the only wires that go to the speaker jacks from the power amp board. There are three other wires connecting to that board. One goes to the XLR line-out pad knob and the other two head over to somewhere underneath the O/T which is quite a bit larger than I was expecting and I am surprised to see has Merc Mag written on it.

I think what I have discovered is that the extension jack (the one on the right with the green wire) has power to it from the power amp all the time. I have deduced the green wire is a chassis ground which would explain why the internal speaker jack (the one on the left with the black wire) only has one wire soldered to it. The white wire from the power amp appears to be the hot side, goes to a fuse, on the other side of the fuse there's a solid jumper to the hot side of the extension jack and a red wire that heads to the speaker kill switch. On the other side of the kill switch it comes back as a black wire to the hot side of the internal speaker jack.

It makes sense to me that if I desolder the solid jumper from the fuse to the external jack and resolder it across the hot sides of both jacks then the switch would then kill both jacks. It makes sense to me but I am no amp tech which is why I am asking you guys. I'm not going to do anything unitil I feel safe it's the right procedure.

Am I seeing this correctly?

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 5:54 pm
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BMW-KTM (I like KTMs!) :D This is a direct capacitor-coupled solid-state amp. You should NEVER have the output see no load.

I've seen ppl that have placed a large 25-50watt, 10-ohm resistor with a DPDT style switch to take the speaker(s) out of the circuit, but keep an appropriate load on the output transistors.

Warning... should be good quality wire wound types. And 25-50 watt ones are BIG:

http://angela.com/images/products/detai ... istors.jpg

PS: that's the power tranny. There is no output tranny. Study the circuit a bit more. If the switch takes out the internal speaker (or speaker cab 1) and not the exteral (cab 2) --- there must be a compensating load built into the internal speaker circuitry.

BTW--- I own an SWR Strawberry Blonde. One of the best acoutic guitar amps I've ever used.


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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 6:55 pm
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BMW2002Ti wrote:
This is a direct capacitor-coupled solid-state amp. You should NEVER have the output see no load.


The speaker-defeat switch is operating just as it was intended -- to provide a fail-safe for the output stage's protection. I see no reason to tamper with it.

JMOOC

Arjay

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 7:11 pm
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OK, like I said, I'm no amp tech. I defer to your knowledge of these things ... but .... there is a speaker kill switch which does effectively kill the internal speaker. I have used this switch for over ten years without a problem. I just never used it with an extension cab before. If there is a compensation built in I still wonder if that issue confronts me for what I want to do. I can see the entire wire from the fuse to the switch and back to the jack. There are no stops along the way. If there is a compensation built into this circuit it must be somewhere else which makes me wonder if I'm not still Ok with this. Maybe it's built into the switch? Is that possible. Can you tell from this pic?

Image

That's the headphone out TRS jack to the right of the switch.

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 7:19 pm
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BMW-KTM, solid-state amps often have logic circuits that can sense current load. The "compensation" circuit maybe on the boards around the output transistors. I can see a hand wound choke in one photo.

The one thing about ss amps. They can be several levels of magnitude more complex than your 1950-60 era tube amp. And they can also be very unforgiving. I don't know how many blown-up ss amps I've seen (both guitar and high-end hi-fi), that were redone by so-called knowledgeable techs. Either with mods or even simple adjustments (like biasing). They are completely different animals than old Fender amps.

Just a warning. I'd hate to see you send that Redhead into an unrecoverable spin --- if you catch my drift.

:)


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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 7:25 pm
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(Heavy sigh .....)

Well, that just plain sucks.

I go to use a cab and now my "stand-by" (sorta) switch is no longer functional.

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 7:34 pm
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By the way, I've looked in the manual and also on the SWR web site FAQ and they do say you can run the amp with no speakers connected. I'm not saying I disbelieve you on the need for a compensatory load but it still leaves me with the gut feeling that wherever that load is located it might not affect what I want to do. Not to worry, I am heeding your cautions. I won't do anything yet. I might contact SWR and see what they have to say.

Scroll all the way down. It's about 6" from the bottom.

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 9:57 pm
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BMW-KTM wrote:
By the way, I've looked in the manual and also on the SWR web site FAQ and they do say you can run the amp with no speakers connected....


Yes, this is true. The SS amps are quite the opposite of tube amps, the SS does not require a speaker or load to be connected to the amp while operating. Basically, they have an RC network across the output, so there is already a "load" before a speaker is connected.

It appears that you are correct about the amp output going to the fuse (white wire), providing output to the extension jack, and the "kill" switch just opens to "kill" power to the main jack, or closes to provide power to the main jack. It looks like you could cut the bare wire at the extension jack terminal, then solder a jumper from the black wire on the main jack, to the terminal on the extension jack where the bare wire was connected. This will allow the switch to "kill" both jacks. :idea:

Oh, be sure that the bare wire doesn't contact the chassis or another place after cutting it from the extension jack. In fact, cut the bare wire all the way back to the fuse terminal to get it out of the way completely. :!:

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Last edited by shimmilou on Mon Nov 05, 2012 10:31 pm, edited 1 time in total.

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 10:25 pm
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Shimmy, disconnecting the load off this amp maybe true. It seems to be the classic push-pull PNP-NPN output transistor amp. So I guess he could build a switch to disable the connection to the external speakers. But, this is not a universal rule-of-thumb.

A lot of solid-state amps are not the classic push-pull PNP-NPN topology. And they can be ruined by seeing no load. Esp the new one runing in Class "D: mode.


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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 10:57 pm
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Check the circuit. The "kill" switch is a simple on-off switch. The switch is simply open in the off position, and closed in the on position (this can be checked with a simple ohm meter). Note that the white wire coming from the amp to the fuse is the output from the amp. The red wire connected to the other side of the fuse is connected to one side of the kill switch, and the black wire on the other side of the kill switch connects back to the main speaker jack. The bare wire from the fuse (same connection as the red wire going to the switch) goes directly to the extension speaker jack, so power is always present at the extension speaker jack, while the main speaker jack is disabled by the kill switch. So, with the kill switch off, and no speaker connected to the extension jack, the output of the amp is wide open, no external load, which SWR confirms is OK to use with no speaker connected. If you clip the bare wire from the extension jack terminal, and jumper the black wire from the main jack to that same terminal on the extension jack where the bare wire was connected, both jacks will be disabled by the switch. Easy, breezy, beautiful.

There are a very special few SS devices such as high-end VFDs that have special transistors that shouldn't be operated without a load connected to the output. These transistors are IGBTs (Internally gated bipolar transistors) and although it's been awhile since I've encountered them, they are not used for instrument amps.

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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 11:13 pm
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Shimmy, thanks, for clarification. I think many of today's cheap Class D amps can be ruined by seeing no load.


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Post subject: Re: Seeking assistance from the pros
Posted: Mon Nov 05, 2012 11:32 pm
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Maybe. I would have to look at the circuit. But, SS amps require the RC network across the output for frequency response compensation, I think that it's about the bass frequencies. So, there is a load on SS amps whether or not speakers are connected. The last SS amp that I had that blew was my old Checkmate, but it had a speaker connected. :shock: I am so tired right now, I can't even think about what kind of circuit a class D amp has. Going to bed.........zzzzzzz.

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Post subject: Re: Seeking assistance from the pros
Posted: Tue Nov 06, 2012 12:12 am
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Thanks, shimmi.

I thought that made sense but I wasn't sure.
It appears you guys have access to a circuit drawing or schematic for my amp.
I do not. Is it possible you could direct me to one? I've been looking with Google but I find lots of user manuals and no schematics.

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Post subject: Re: Seeking assistance from the pros
Posted: Tue Nov 06, 2012 7:38 am
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BMW-KTM, Sorry about the mis-information. Here's a blurb I found from SWR site:

Q: Where can I find an owner's manual for my SWR?
A: You can find both plain text and/or PDF versions of most SWR owner's manuals available for free download on this website, in the "products" section. Hard copies of SWR owner's manuals are available for $5 each, plus shipping and handling. To order, contact the FMIC Consumer Relations Department, 480-596-7195.

Q: Where can I order replacement parts for my SWR?
A: Replacement parts can be ordered directly from your local SWR retailer or any authorized FMIC Service Center.

Q: How can I order a schematic?
A: To request a schematic, please provide your full name and mailing address (or fax number), along with the model and serial number of your amplifier. Send your request to swrcustserve@fender.com, or contact the FMIC Consumer Relations Department, 480-596-7195.


The only solid-state amps that need a load, in the guitar amp world, are the newer Class "D" amps. Good luck with your amp!


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Post subject: Re: Seeking assistance from the pros
Posted: Tue Nov 06, 2012 11:27 am
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I'd already seen that and fired off an email last night but thanks anyway, Bimmer.

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