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Old 18th April 2010, 10:08 AM   #1
Cammo is offline Cammo  Australia
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Default Melody Roll

Hi tubemen of the valve,
Ive recently replaced the Power tube on my Melody SP3 II with Winged C 6l6GC I'm burning them in at 1.3v bias on idle, any suggestions for bias voltage to get the most out of these tubes in this amp?
Cheers.
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Old 18th April 2010, 05:05 PM   #2
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I hope you are not running at -1.3 volts bias. Is that 1.3 volt number perhaps the voltage across a cathode resistor?
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Old 18th April 2010, 05:35 PM   #3
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If you have a schematic, you could post it here and we could answer your question more easily. If not, there are three key numbers you will need to know. First is plate voltage and second is cathode voltage and the third is the resistance of the cathode resistor that (I think) you are measuring that "bias voltage" across.

I don't know your level of experience, but directly measuring the plate voltage could get you killed. You might want to inquire of the manufacturer that spec. The bias resistor value you can measure with a volt/ohm meter. Simply turn off the amp and wait a couple of minutes. Then place the probes of the VOM into the bias ports like you did when you measured the voltage, but this time measure the resistance.

Now, you can use ohms law to calculate cathode current from that voltage number. Divide the voltage figure you measured by the resistance. The result is amps. Multiply by 1000 and you have milliamps.

Then you multiply the cathode current by the difference between plate voltage and cathode volatge. That's power in watts. Let's say the plate voltage is 350volts higher than the cathode (and I have no idea the real number) and the idle current is 60 ma. Then the power is 350 * .060 or 21 watts. You want to operate your tubes at about 70% maximum power at idle because they will produce more power at volume. They reach peak power at normal listening levels and then actually fall of some at higher levels.

This calculator is helpful. Weber Bias Calculator

Last edited by Captn Dave; 18th April 2010 at 05:40 PM.
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Old 18th April 2010, 09:33 PM   #4
Cammo is offline Cammo  Australia
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Default Bias

Thanks dave, I measured the volts using probe holes at both sides of amp, it says 1.15V above these and you can adjust the bias via a screw. My mate, who's as silly as me, told me to bump up the bias voltage at idle here. When I first measured it numbers were jumping from 1.2 to 1.4 on each tube in the quad, I adjusted until all four sat at 1.3, thought this was ok. Your extra info is great, thanks for taking the time to explain. Cheers.
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Old 18th April 2010, 11:31 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by Cammo View Post
Thanks dave, I measured the volts using probe holes at both sides of amp, it says 1.15V above these and you can adjust the bias via a screw. My mate, who's as silly as me, told me to bump up the bias voltage at idle here. When I first measured it numbers were jumping from 1.2 to 1.4 on each tube in the quad, I adjusted until all four sat at 1.3, thought this was ok. Your extra info is great, thanks for taking the time to explain. Cheers.
Never good to do something without some understanding of what you are doing and why. Without knowing the resistance of that cathode resistor and the plate voltage applied to the output tube you cannot determine whether or not you are running the tube beyond its plate dissipation rating.

Running a tube beyond its plate dissipation rating can have serious consequences ranging from shortened output tube life to catastrophic failure with the potential for significant to serious damage to the amplifier itself. Doing so also significantly increases the load on the power supply and you had better be sure that the supply can handle the increased current.

Unless your mate designed the amplifier in question and knows it is safe to increase the idle current I would set it back to the recommended level until you have done your homework.

In one or two instances where a misguided client damaged his amplifier doing this I had to void the warranty (I clearly stated that deliberate operation outside of maximum limits would void the warranty - and this limiting value was well beyond my recommended operating conditions btw) - you might want to think about that in the event your amp is still under warranty. Consult with the designer/manufacturer!

Doing this might make the amplifier sound and measure better, but before you do you need to know it is safe to do so, otherwise it is just foolish...
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Old 19th April 2010, 12:18 AM   #6
Cammo is offline Cammo  Australia
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Thank you for pointing out that I am an idiot. I have returned volts to factory level. Now, I have another question, is it ok to stick a fork into a toaster while toasting a crumpet?
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Old 19th April 2010, 12:30 AM   #7
kevinkr is offline kevinkr  United States
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Originally Posted by Cammo View Post
Thank you for pointing out that I am an idiot. I have returned volts to factory level. Now, I have another question, is it ok to stick a fork into a toaster while toasting a crumpet?
It could prove to be "quite" interesting, but haven't we all done that or something similar at one time or other?

(I have done just that, well into the crumpet anyway, and got away with it somehow or other.. )
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Old 20th April 2010, 02:03 PM   #8
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I second what kevinkr said in his first post. There is an excellent chance that the manufacturer has specified the correct bias. It is, after all, in his best economic interest to see the tubes operate through the warranty period while producing hi-fi sound. Your interests are aligned.

Until you know all of the parameters, increasing bias will only ruin those new tubes. That said, post the voltages and resistances when you learn them.
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Old 26th April 2010, 02:05 AM   #9
Cammo is offline Cammo  Australia
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Default Melody Schematics

I tracked down schematics for the SP3, does this help in answering my question about correct bias for 6l6GC power tubes? I'm still running them at factory 1.15V. I've also included an interior image.
Attached Images
File Type: jpg Power schematic.jpg (101.3 KB, 80 views)
File Type: jpg Tube schematic.jpg (183.5 KB, 81 views)
File Type: jpg Mel interior.jpg (69.1 KB, 80 views)
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Old 26th April 2010, 02:38 AM   #10
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Nice amp, looks like Russian made. But I did not see where you can measure bias and get 1.25V voltage. Are you sure it is this amp's schematic?
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