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 Post subject: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 12:07 am 
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I'm using 6v6 tubes in my amp with ccs cathode bias.
I've set the ccs for 25ma
The LM317 w/a 50 ohm resistor between the "out and adj" pins will dynamically adjust its internal resistance ( approx 540 ohms) for 25ma.
With good or new tubes this results with a -13.5v bias voltage.
I have two old tubes, one tests "replace" the other as good but on the low end.

When I put these two tubes in the amp they seem to sound and work quite well ... as each is drawing exactly 25ma.

But one creates -11.75v bias the other almost -15v.

I can understand that the weak "replace" tube would need -11.75 volts to turn it on harder to get 25ma.

But what is happening with the other tube at almost -15v bias for 25ma. Is it more sensitive?
I'm not sure I know why it tests on the low-end of the "good" scale but the bias needed at -15v is so much leass than 13.5v to create 25ma.

And even though they both operate at 25ma nicely... they must not be responding to signal changes properly I wouldn't think.
Thoughts?

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 1:03 am 
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Off the top of my head: bias is dependent on tube geometry which can vary.

Can I assume that CCS means constant-current supply?


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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 1:05 am 
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Is each tube loaded exactly the same? Is each circuit precisely the same?


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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 1:06 am 
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Peter

Tubes with higher emission need more negative bias. Difference you found could by type of tube tester and how it supplies power?

Where was your weak tube tested? On a Hickok all bets are off. This tester doesn't look for emission. A tube slightly gassy would also need more negative grid bias for a specific current.

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 1:25 am 
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Norm Leal wrote:
Peter

Tubes with higher emission need more negative bias. Difference you found could by type of tube tester and how it supplies power?

Where was your weak tube tested? On a Hickok all bets are off. This tester doesn't look for emission. A tube slightly gassy would also need more negative grid bias for a specific current.

Hi Norm:
So the one at -15v must simply have higher emission?

The way I tested these two tubes was on two separate testers.
1.) Eico 666 - They both test good but low .. with one reading "82" ( -11.5 in amp) the other "88" (-15v bias in the amp)
2.) Precision 1040 - The weaker one tests as "replace" and the other just into the "green"

As for the amp:
Both are seeing the exact same system load in my amp... because any other new or "good" tubes all create -13.5v bias at 25ma as they should.
Image

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 1:28 am 
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Alan Douglas wrote:
Off the top of my head: bias is dependent on tube geometry which can vary.

Can I assume that CCS means constant-current supply?

Yes CCS means Constant Current Source (LM317 w/ 50 ohms)
1.25v ref voltage / 50 ohms = 25ma

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 6:33 am 
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Logically it seems that forcing a tube to operate at a specific current locks in the operating point on the characteristic curve. If the two tubes are biased differently when current is the same, then the gain of each has to be different based on operating curves. This is why you run into some crazy or complex AC and DC bias and balancing adjustments on some of the high end vintage tube amplifier designs.

You could take those two tubes in question, bias them at the voltage which your CCS sets them to, and then run curves to see if the two are indeed operating the same or not. You will likely find that one has more gain than the other, which means your output stage is unbalanced for AC operation (signal) even though you have the DC current set to exactly the same point. It would be interesting to know what the distortion is as well, I presume you have equipment to measure all those parameters?

If you want the best sound and least distortion, which I presume is always the goal of high fidelity audio amplifier design, then you need a system that will provide AC balance to get the signal equal at the plates of both tubes regardless of where the DC cathode current is. The question is, how far off can AC balance be before it is noticeable? No doubt that you can see it on a scope long before you can hear the effects. Having the DC current equal only affects the steady state magnetism of the output transformer core and doesn't necessarily assure you that the gain of each tube is the same unless of course the tubes are perfectly matched.

Personally, I don't believe CCS is a valid design in vacuum tube audio amplifiers, but you and some others are using it. I don't want to see any solid state components in the signal path of a tube amplifier.

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 7:12 am 
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Mr. Detrola wrote:
Logically it seems that forcing a tube to operate at a specific current locks in the operating point on the characteristic curve. If the two tubes are biased differently when current is the same, then the gain of each has to be different based on operating curves. This is why you run into some crazy or complex AC and DC bias and balancing adjustments on some of the high end vintage tube amplifier designs.

You could take those two tubes in question, bias them at the voltage which your CCS sets them to, and then run curves to see if the two are indeed operating the same or not. You will likely find that one has more gain than the other, which means your output stage is unbalanced for AC operation (signal) even though you have the DC current set to exactly the same point. It would be interesting to know what the distortion is as well, I presume you have equipment to measure all those parameters?

If you want the best sound and least distortion, which I presume is always the goal of high fidelity audio amplifier design, then you need a system that will provide AC balance to get the signal equal at the plates of both tubes regardless of where the DC cathode current is. The question is, how far off can AC balance be before it is noticeable? No doubt that you can see it on a scope long before you can hear the effects. Having the DC current equal only affects the steady state magnetism of the output transformer core and doesn't necessarily assure you that the gain of each tube is the same unless of course the tubes are perfectly matched.

Personally, I don't believe CCS is a valid design in vacuum tube audio amplifiers, but you and some others are using it. I don't want to see any solid state components in the signal path of a tube amplifier.
I don't like SS in the signal path either but it's bypassed, signal goes through the capacitor, so it isn't in the signal path. Not that it matters here as the preamp is opamps.

DC balance is all it's for. AC balance is another matter and this simple amp doesn't have it (and neither do a lot of 'Hi-Fi' amps either).


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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 9:38 am 
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This is like complaining that your car doesn't run right with jelly in the gas tank.

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 9:50 am 
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Leigh wrote:
This is like complaining that your car doesn't run right with jelly in the gas tank.

- Leigh
What is? The OP isn't complaining about how the car runs at all. He's just curious why a supposedly low emissions 6V6 takes more bias than a 'good' tube.


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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 3:28 pm 
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Also a good example of why even good tube testers are not so good in testing power tubes.

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 4:49 pm 
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Cathode bias itself is a form of current source, just not as much gain in the feedback loop.

If you wanted to make a research project out of this, you could try changing the heater voltage (emission), and see how it affects a particular tube.


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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Thu 09, 2015 9:47 pm 
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Mr. Detrola wrote:

You could take those two tubes in question, bias them at the voltage which your CCS sets them to, and then run curves to see if the two are indeed operating the same or not. You will likely find that one has more gain than the other, which means your output stage is unbalanced for AC operation (signal) even though you have the DC current set to exactly the same point. It would be interesting to know what the distortion is as well, I presume you have equipment to measure all those parameters?


Thanks...
yes I see.
I guess that is exactly the point or question. How do they perform?

I really like the concept and action of a CCS because it automatically adjusts the DC bias point perfectly; so that as tubes age I don't have to try to readjust the bias or worry about "matched" pairs.
But as they age or exhibit differences ... even though DC bias (idle-current-wise) remains the same, the bias-voltage-point on the curve changes. So then the question is: How might that influence or affect the performance?

Yes, it would be interesting to examine distortion. I don't know what equipment I'd need to determine that or to "run curves"

The concept of variably-adjusting the DC-bias current is popular in high end amps:
My Antique Sound Labs (AQ1003DT) 30watt el-34 PP amp ... has a similar approach but they do it using fixed-bias.
They provide a meter on the amp which samples the voltage drop across a small cathode resistor.
The meter has a small green spot in the center indicating correct bias. Each of the 4 tubes has its own external variable-fixed-bias trim-pot to center the meter.
So each tube is adjusted for the exact-correct DC idle current regardless of the actual bias-voltage required to achieve that current-point.
From time to time as the tubes age, it is necessary to re-adjust.

However with a CCS system, such as I use here, the corrections are automatic.

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Mon 13, 2015 6:02 pm 
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One way you can do a rough measurement is to use an oscilloscope.

Dual channel will be best, but single channel will work as well.

Connect one channel to one output tube plate and the other channel to the other output tube plate.

Feed a sinewave to the amp of an audio frequency (I like to use 400Hz) and see how the waveforms look. You can invert channel two and switch the scope to add mode.

Ideally if all is balanced you should see a straight line. If there is any waveform then there is imbalance in the output stage.

Another way to check for balance and proper phase is to invert the second channel and adjust the traces so both are on the center vertical line of the graticule. If all is balanced properly the display will look like one waveform. Any deviation from perfectly balanced will show one waveform larger than the other.

The scope is also a good way to view distortion as the waveform on both plates should look like a proper sinewave.

Any part of the waveform that doesn't look like a sinewave (flattening, sharp peaks ETC...) can be considered to be some form of distortion. Whether or not it is audible depends on how bad the distortion is.


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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Mon 13, 2015 10:26 pm 
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Tube Radio wrote:
One way you can do a rough measurement is to use an oscilloscope.

Dual channel will be best, but single channel will work as well.

Connect one channel to one output tube plate and the other channel to the other output tube plate.

Feed a sinewave to the amp of an audio frequency (I like to use 400Hz) and see how the waveforms look. You can invert channel two and switch the scope to add mode.

Ideally if all is balanced you should see a straight line. If there is any waveform then there is imbalance in the output stage.

Another way to check for balance and proper phase is to invert the second channel and adjust the traces so both are on the center vertical line of the graticule. If all is balanced properly the display will look like one waveform. Any deviation from perfectly balanced will show one waveform larger than the other.

The scope is also a good way to view distortion as the waveform on both plates should look like a proper sinewave.

Any part of the waveform that doesn't look like a sinewave (flattening, sharp peaks ETC...) can be considered to be some form of distortion. Whether or not it is audible depends on how bad the distortion is.

Thanks.

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 Post subject: Re: Help explain "weak" tube's performance using CCS bias?
PostPosted: Apr Tue 14, 2015 1:13 am 
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