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Question about a resistor in my amp.
Question about a resistor in my amp.
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Old 8th February 2012, 01:17 AM   #1
whatisacup is offline whatisacup
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Default Question about a resistor in my amp.

So I've been playing bass for years and recently bought my first non Ampeg amp. I bought a Mesa Carbine M6 new , I love it so much I bought a used Mesa 750 as a backup. One of the things I love about mesa is how clean their PCB's are, and the amount of room.

I was looking at the board of the Mesa 750 today and noticed a resistor that looks out of place. I posted pictures below. The resistors are tied in together and each of them say:
5W 3.9 Ohm J

I'm pretty OCD, so I could be obsessing over nothing, but since the amp is used, I want to take action before the 30 day return policy is up. These two resistors that are wired in series wobble pretty easy with a slight touch of the finger.

Should I be concerned by this?

Is there a way for me to remove this and resolder in something that looks more durable?

Attached Images
File Type: jpg Mesa1.jpg (70.9 KB, 71 views)
File Type: jpg mesa2.jpg (40.8 KB, 69 views)
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Old 8th February 2012, 01:44 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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Looks like poor design to me.
The resistors are also hemmed in by electrolytic capacitors which dont like heat !
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
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Old 8th February 2012, 02:56 AM   #3
polychrome is offline polychrome  United States
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I doubt you'll ever have a problem from those ceramic cased wire wound resistors, because of the mounting, but if your OCD compels you, there are similar resistors with both leads at one end for PWB mounting, like the caps nearby. As first glance those looks like 2W ceramic resistors. Without knowing the purpose of this 7.8 Ohm load, I don't know if heat is a concern, they may be sized for some transient event and there's no reason to worry about them baking the adjacent electrolytic caps. If they went in during a repair, the rework may have beefed up the replacement of a 1 watt carbon comp resistor that didn't survive the transient. Since you have confidence in the design house otherwise, you can probably assume the thermals are OK, or you can use a non-contact thermometer to get an operating temperature.
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Old 8th February 2012, 05:51 AM   #4
Enzo is online now Enzo  United States
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You could even call Mesa and ask them about it.

I can;t recall which one at the moment, but I have some factory schematics for something that describes doing exactly that in some circuit.

A pair of 5w resistors like that instead of one 10 watt part takes up less board space. And it probably offers up more radiating surface in the bargain.

7.8 ohms with whatever it is voltage across it is going to radiate X watts of heat no matter what, so you could place some other shape part there, but it will still be next to that cap.
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Old 8th February 2012, 08:21 AM   #5
godfrey is offline godfrey  South Africa
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It could well be part of a Zobel network, it which case it shouldn't get hot in normal use.
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Old 8th February 2012, 12:09 PM   #6
Enzo is online now Enzo  United States
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Mesa Big Block 750

Turns out I have one of those in the shop right now. Power amp output transistors bit the dust. I looked inside just now.

That resistor is on the power supply board, not the power amp board, so it is part of the power supply.

The amp I have is done EXACTLY the same way, so I have to think it was intentional from the factory.
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Old 8th February 2012, 12:50 PM   #7
Jay is offline Jay  Indonesia
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Location: Jakarta
The 5x caps are Panasonic and the 1x resistor is Dale. The ceramic just do not belong there in term of quality and installation. I guess the other 1x blue capacitor is also not original (It must not be a Panasonic?).

I see small diodes in sets of 4 so they must be a low current power supply for the front end. Probably single rail supply for tube amp, so it is probably be CRCRC circuit. The 1x Dale resistor is 10 Ohm, so may be the ceramic is put in series to achieve similar resistance with common 4.7 or 3.9 Ohm resistors.

If the original resistor was a Dale, and if it was broken and been replaced, may be you can see a "trace" of burnt on the PCB?

EDIT: LOL it must be dangerous if it is high voltage

Last edited by Jay; 8th February 2012 at 12:53 PM.
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