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Old 16th January 2012, 09:31 PM   #21
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Originally Posted by Soonerorlater View Post
Cogline, just noticed your join date, welcome to the forum........
Thanks!

The pictures in that link are not the amp I have, but a different version on which you cannot change the bias! I shall post some pictures of the amp i have and the insides of it!

Just need to find them agaain...

ahh yes here you go a little gallery http://imageshack.us/g/266/pc200048i.jpg/

No i do not have any other speakers that i can use unfortunately!

Not that any of you care but i just got a place at uni, masters degree in Physics for Energy Science and Technology.

Last edited by Cogline; 16th January 2012 at 09:39 PM.
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Old 16th January 2012, 09:58 PM   #22
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Cogline: Do you know how to check/set the bias current? Do you know what the target bias current (or voltage) is?

If you know either one of these, and you have a multi-meter, we can probably talk you through this....

Can you borrow some other speakers?

Does your camera have a macro setting? can you snap and post a clear pic or two of the area under the output tubes? You can post pics directly here as long as the file size is under 200K.

Last edited by boywonder; 16th January 2012 at 10:02 PM.
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Old 16th January 2012, 10:22 PM   #23
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No i don't know how to bias it or what the bias current and or voltage should be. If you think I'm overloading it surely i should just turn it down?

I could see about borrowing some speakers and i do have a Multimeter at hand!

The only information i have to go on is some manuals of a version of my amp that also has a pre-amp stage integrated.

If you could talk me through this it would be amazing, i have quite alot of knowledge from studying electronics basically at a university level.

Thanks for you interest and quick replies through tonight guys very much appreciated!
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Old 16th January 2012, 11:29 PM   #24
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Default After you phone-up one of your buddies and ask to borrow speakers....

Look at the bottom of the PCB where the 4 output tubes are for each channel.

Notice that pins 3 and 9 are connected together for each tube, and that connection goes to ground through a resistor (those are your cathode resistors, and are used to set the bias current). There are 4 resistors, from the blurry pic, it appears that each tube has it's own resistor to ground. Please verify that this is correct. Sometimes pairs of tubes share resistors, and your pics are somewhat blurry.........

What are the color bands of these resistors? Again, hard to tell from the pics......

The two blue trim pots are used to set the bias current in the tubes; leave these alone for now.

The way you check the bias current is to simply connect your meter across each cathode resistor and measure the voltage drop. Set the meter to DC volts. This part of the circuit is at low volts but use caution when measuring.

So just measure the voltage drop across each cathode R with the amp warmed up and idling, ie no input signal. Once we know the value of the cathode R's then use ohm's law to calculate the bias current I=E/R.

As a first check, just verify that the drop across each R is roughly the same. You have 8 to check if you are checking both channels.

so, verify that each tube's pin 3 and 9 are connected to ground through a separate resistor for each tube

determine the resistance of the cathode R's by interpreting the color bands

Post your results for voltage drop for each cathode R.......and the calculated idle current based on the cathode R values.

Last edited by boywonder; 16th January 2012 at 11:32 PM.
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Old 17th January 2012, 02:06 AM   #25
chrish is offline chrish  Australia
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Speakers moving in and out slowly. Motorboating?
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Old 17th January 2012, 02:15 AM   #26
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Quote:
Originally Posted by chrish View Post
Speakers moving in and out slowly. Motorboating?
Probably.
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Old 17th January 2012, 03:23 PM   #27
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Default Your amp may not play well with your speakers.....

Cogline: Sounds like Chrish and Wavebourn may have diagnosed your problem.....

"motorboating" is a low freq oscillation of the amp output with a given speaker load due to marginal low frequency stability of the amp. It involves gain and the amount of negative feedback the amp utilizes.

For clarity, when the problem occurs, do the speaker cones slowly oscillate in and out?

Chrish and Wavebourn: Would adding series resistance at the output of the amp help diagnose a motorboating problem?...or is disconnecting the feedback req'd?
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Old 17th January 2012, 03:26 PM   #28
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yes the movement of the cone you described matches exactly what happens!
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Old 17th January 2012, 04:23 PM   #29
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Cogline: Do you happen to have any 8 ohm power resistors, like 5-10W power rating?
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Old 17th January 2012, 05:38 PM   #30
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Well... there could be some places for me to pick em up, could pinch some from school if they have any or maybe my stepdads work!

What would i do once i obtain the resistors? just wire them to the output terminals then to my speaker wire?
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