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Post by humanbn on Feb 13, 2008 19:11:26 GMT -5
Hello, I am new to working with amplifiers and I have a couple of questions. I play through a 2001 Hotrod Deluxe. It has recently begun to have a crackle in the speaker, even in the clean channel. This crackle is not constant but it is noticeable when it is there. I was looking on this website studentweb.eku.edu/justin_holton/plateloads.htmland I'm not sure how to measure one side of the resistor. Do I accomplish this by placing the positive lead of my volt meater on the resistor and then ground the negative to the chassis? What can I do to keep from getting a jolt? I take it that the amp should be plugged in while doing this. Does it need to be off standby or can I leave it in standby mode? Any help will appreciated. Thanks. |
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Post by andy on Feb 13, 2008 19:40:21 GMT -5
Hi there, welcome to Guitarnuts!  I don't know about how to go about making the check you are asking about, but there are guys here who will fill you in on it. It may seem like a silly question, but have you tried changing the valves first? Every crackling problem I have had with an amp so far has been solved with new valves, except a solid state one (of course) which, incedentally, gave me an electric shock as I tried to work out what was up with it! |
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Post by ChrisK on Feb 13, 2008 20:02:42 GMT -5
NO! Always connect the ground side first. Otherwise, the entire meter is at plate voltage until the ground is connected. Also, there are two ends of this resistor. The B+ end won't tell you anything about the resistor, only the plate end will.
A plate resistor goes from the plate of a tube to a few to several hundred volt DC supply rail.
Take it to someone else (after all, someone's always the fuse).
One should use a clip lead to the chassis, and a clip lead to the plate or plate side of the resistor, and THEN turn the amp on and measure the plate voltage. If the resistor has failed, it will be lower than it should be. (You DO have the Fender amp schematic from their web site?)
I expect that others with more pertinent empirical experience (Ouch, dang, that was shocking) will be along to comment.
From your link, this is a dire warning in the form of a clue;
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Ignore my tag line below. THIS IS HIGH VOLTAGE, IT WILL KILL YOU.
I would be tempted, if it was my body involved, to find another way to determine the state of that there resistor. One might turn the amp off, wait a while and then ensure (as in make sure) that all high voltage caps have discharged, and measure the aforementioned resistor with a digital voltmeter to determine its resistance.
If it were me, since I would be going into the amp to measure this anyway, I'd just replace the resistor.
From your link;
Most techs go ahead and replace every plate load (R4, R11, R16, R22, R57, R58), which may be a good idea.
I agree, since
When asked about the problem a Fender representative said, "we had a vendor problem with these resistors. The problem would cause the resistors to eventually hair-line fracture and open under normal operating conditions. The vendor has since analyzed and corrected the problem with their process."
may well mean (as in marketing speak) that SOMEONE didn't take into account the actual power dissipation in the resistors in the design and they're failing in the field.
This happens when "plumbers" (not electrical design engineers) think that a 2 watt resistor can dissipate 2 watts forever, in a hot environment, with generally obstructed airflow.
Even the highest reliability resistor is only rated at its rated power (well, internally) for 10,000 hours.
The standard rule that I use (with excellent results over three decades) is to only use a resistor at half of its rated power dissipation AFTER taking into account, and derating for, the environment that it is operating in.
BTW, in spite of all the "vintage whining", carbon comp resistors are fairly crap, and generate a fair amount of noise.
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Post by humanbn on Feb 15, 2008 16:27:42 GMT -5
Thanks much.
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