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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 7:46 am
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I will repeat this one last time and then I am done.

You CANNOT assume that the plate voltage will remain constant for any given plate current. Saying that you measured your plate voltage yesterday at, for example 440 VDC and your plate current was 38 mA (which gives you 56% max plate dissipation for a 30 watt 6L6GC), and now you want to set your bias at 66% of max plate dissipation for a 30 watt 6L6GC, you cannot use the same plate voltage and say that your plate current should now be set to 45 mA. Increasing the plate current to 45 mA will cause the plate voltage to DROP from 440 VDC. What it will drop to will depend on the tube characteristics and the robustness of the power supply.

I provided this example the other day:

bluesky636 wrote:
According to the schematic, the factory setting of 30 mA cathode current (about 29 mA plate current) on my Blues Deluxe Reissue results in a plate voltage of about 430 VDC. That equals about 50% of the maximum plate dissipation of the stock Groove Tube Sovtek 5881WXT (assuming the tube is a 25 watt tube). Way cold in my opinion. My preferred setting for a JJ 6L6GC of 53 mA cathode current (about 50 mA plate current) results in a plate voltage of 403 VDC. Quite a difference. That is about 67% of the maximum plate dissipation for a 30 watt JJ 6L6GC. That is why you need to measure the plate voltage each time you adjust the bias setting.


Will your plate voltage drop that much with a different tube or as you change the bias setting. You will never know unless you measure the plate voltage EACH TIME you change the bias setting.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 8:58 am
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+1 :!:

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 9:46 am
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DLW, I think you may have been mistaken when I posted: "Truly, the only way to really know the output wattage is to measure the secondary AC voltage with a DVM or scope."

Biasing the tube to a set manufacturer's value, for the tube and the amp, is how to setup the IDLE BIAS. And more importantly, the IDLE WATTAGE, of each tube prior to the output tranny. Idle wattage is how you properly setup output tubes, for the Class of amp and the specific chanracteristics of each tube and the amp, itself. NO TWO AMPS ARE ALIKE. Maybe close, but never alike.

If you think about it. assuming the plate voltage as constant betwwen amps is EXACTLY like assuming the bias current of every JJ 6L6GC is the same. And we know this isn't true. And the current draw and plate voltages are interactive. As Bluesky stated, changing one will always affect the other.

I only added the blurb about amplifier output --- as it differs from your calculated idle wattage, due to specific output tranny & speaker characteristics.

And to confused you more. Sorry, about that. :D

Check out this video. BTW... the final calculation (which gets cut off in the YT clip) =

5 X 5 divided by 4-ohm speaker load.

Equals 6.25 watts. Or roughly the output of most Champ and their clones.

http://www.youtube.com/watch?v=1b2jQWK8xlQ


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 10:09 am
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BMW2002Ti wrote:
DLW, I think you may have been mistaken when I posted: "Truly, the only way to really know the output wattage is to measure the secondary AC voltage with a DVM or scope."

Biasing the tube to a set manufacturer's value, for the tube and the amp, is how to setup the IDLE BIAS. And more importantly, the IDLE WATTAGE, of each tube prior to the output tranny. Idle wattage is how you properly setup output tubes, for the Class of amp and the specific chanracteristics of each tube and the amp, itself. NO TWO AMPS ARE ALIKE. Maybe close, but never alike.

If you think about it. assuming the plate voltage as constant betwwen amps is EXACTLY like assuming the bias current of every JJ 6L6GC is the same. And we know this isn't true. And the current draw and plate voltages are interactive. As Bluesky stated, changing one will always affect the other.

I only added the blurb about amplifier output --- as it differs from your calculated idle wattage, due to specific output tranny & speaker characteristics.

And to confused you more. Sorry, about that. :D

Check out this video. BTW... the final calculation (which gets cut off in the YT clip) =

5 X 5 divided by 4-ohm speaker load.

Equals 6.25 watts. Or roughly the output of most Champ and their clones.

http://www.youtube.com/watch?v=1b2jQWK8xlQ


I disagree with this approach.

Is he measuring peak output? Some "nominal" output? Why is he considering the speaker to be a 4 ohm resistive load instead of a complex impedance that varies with frequency? Amplifier manufacturers (at least in the hi Hi-Fi business) measure power output of an amp at a specific frequency and distortion level into a dummy load, with the input driven by a signal generator. Exactly the way Weber says not to do it.

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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 5:13 pm
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Bill,

Actually, whatever method you use to find the actual wattage output is kinda moot, as it doesn't take into account the speaker efficiency and room dimensions. To my ears, it's hard to relate output wattage and loudness. My Matchless two EL84, cathode biased amp is LOUD. And prolly only makes 12 watts.

A friend's BF Deluxe Reverb (in stock form) not as loud as the Matchless. Yet, we measured the output at 22 watts.

And there's the exponential factor of loudness versus wattage.

Just showed the Gerald Weber method, as a quick-&-dirty way to check if there is something amiss with the output tranny --- in an amp that measures nominal plate voltages and idle bias wattage.


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Post subject: Re: Help please with a 65 Twin Re-issue making some odd nois
Posted: Fri Apr 12, 2013 5:43 pm
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BMW2002Ti wrote:
Bill,

Actually, whatever method you use to find the actual wattage output is kinda moot, as it doesn't take into account the speaker efficiency and room dimensions. To my ears, it's hard to relate output wattage and loudness. My Matchless two EL84, cathode biased amp is LOUD. And prolly only makes 12 watts.

A friend's BF Deluxe Reverb (in stock form) not as loud as the Matchless. Yet, we measured the output at 22 watts.

And there's the exponential factor of loudness versus wattage.

Just showed the Gerald Weber method, as a quick-&-dirty way to check if there is something amiss with the output tranny --- in an amp that measures nominal plate voltages and idle bias wattage.


I don't disagree with what you say, but I disagree with Weber's statements, spoken or implied, about the "textbook" method of determining wattage.

Amp power in watts is something that can be measured without regard to speaker efficiency or room size. All those things only come into play when you want to talk about loudness.

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