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Biasing tube amp-under bias and over bias?


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I would like to ask these technical questions when we manually bias tubes in power amplifier. I am a bit confused as what is under bias and over bias? Say if the recommended bias is 0.5VDC then 0.4VDC is under bias and 0.6VDC is over bias? Also, under bias will over heat the tube while over bias will cool the tube? Please correct if that is not correct. Thanks.

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Posted (edited)

It's the other way around. If you under bias, the value will be less than the desired value. This is referred to as cold biasing. Conversely, if you over bias, the value will be more than the desired value. This is referred to as hot biasing.

 

I would recommend downloading  the FTBC lite app  for your phone and finding out what bias value is in the green zone. Otherwise, if you share the amp details and what sort of tubes you are trying to bias we can help out further. 🙂

 

Edit: Example screenshot included:

 

20210505_225306.jpg

Edited by xlr8or
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Thank you so much for your respond. My amp is the Lamm ML2.1. which the recommended bias is 0.31DVC. I found at times, the valves can drift quite high to 0.33DVC so I am biasing it at 0.27DVC. Am I doing it right?  I somehow read from another forum that higher valves means cold biasing and lower valves means hot biasing? Hot biasing can be danger to the amp? I am a bit confused right now and please help me!

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Yes - that's fine as AC mains voltage fluctuates. Close up the lid, sit back and enjoy the music. 👍

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2 hours ago, LunA said:

Thank you so much for your respond. My amp is the Lamm ML2.1. which the recommended bias is 0.31DVC. I found at times, the valves can drift quite high to 0.33DVC so I am biasing it at 0.27DVC. Am I doing it right?  I somehow read from another forum that higher valves means cold biasing and lower valves means hot biasing? Hot biasing can be danger to the amp? I am a bit confused right now and please help me!

 

 

Tube bias is measured in ma.  It is the current the tube conducts.  You are measuring voltage.  Tubes use negative bias (usually)  so a higher voltage means you get less current.  The inverse relationship causes the confusion :) 

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So in my case should I bias my amp a little higher then 0.27DVC, say to 29-30DVC to be on the safe side? cheers.

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20 minutes ago, LunA said:

So in my case should I bias my amp a little higher then 0.27DVC, say to 29-30DVC to be on the safe side? cheers.

 

Depends where the voltage is being measured and what the circuit is. This the output valves of some amplifier right?   0.27 is very low compare to normal bias voltages, so I suspect they have put a small resistor in series with the current for measurement purposes and are not measuring the cathode bias directly.

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Yes it is the out put valves 6c33c.The amp operates single end and at 18W. It use same tube for regulator running at 0.175V only! So in my case. 0.27 is cold and 0.34 is hot?

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Posted (edited)
10 minutes ago, LunA said:

Yes it is the out put valves 6c33c.The amp operates single end and at 18W. It use same tube for regulator running at 0.175V only! So in my case. 0.27 is cold and 0.34 is hot?

 

Again, can't be sure without seeing the circuit  (or reading the instructions).  If the point voltage is representing the plate current, then higher is hotter.  If it is measuring the bias voltage, then higher is colder.  I am suspecting the former now, but don't wish to bet on it.

Edited by aussievintage
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I would set it closer to 31.5 to be on the safe side then. At the past, when it gone over to 35-36 then the out put tube starts to make popping sound and failed. That is why I keep the bias down a bit.

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Posted (edited)

It might help sharing some photos of the measurements you're taking. Also, please share a photo of what the manual says on adjusting the bias. Something is not right here with the adjustment. The sensitivity on the pot appears to be the issue. How can the bias jump so high?

 

You mentioned before the range was 0.27 to 0.33mV but now indicate it's 0.35 to 0.36mV. Running the bias at these latter values also causes the tubes to make a popping noise. That doesn't sound right.

 

What AC mains voltage is the unit specified at?

Edited by xlr8or
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If you want to have long tube life you might want to be around 0.25mV witch equals around 250mA of current. The 6C33 tube runs on very high current compared to other tubes.

AC fluctuation should not be a problem as your second 6C33 tube acts as a voltage regulator for the first 6C33. The higher you go the shorter the life of your tube.

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Posted (edited)

@Ihearmusic is 100% correct regarding 6C33 operating points. I just checked the tube datasheet for the 6C33 and it states for 2 triode operation, 600mA is the absolute maximum plate current. For single triode operation, the absolute maximum is 300mA. Average operating conditions are 550mA and 275mA, respectively. The lowest plate current values are specified as 450mA and 225mA, respectively. In conclusion, safe bias would be anywhere in the range of 225-275mA (0.225-0.275v).

 

I missed the point about the second 6C33 acting as a B+ voltage regulator and not being influenced by AC mains fluctuation. Thanks for confirming. 👍

Edited by xlr8or
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  • 2 weeks later...

Sorry guys I haven't been following up on this threads. I have also found the answer too. It's should be between 30.5-31.5vdc and not higher for V2. I know I have been doing that in the pass. In regards to that 35+vdc measurement, it was an one off incident that the V1 tube was actually the problem. I remember I had to replaced that tube and the fuse and the amp works fine again. On the whole, the Lamm is pretty stable to work with but needs periodically checking on the biasing. I am managing but wish I have more technical knowledge. Thanks very much for all the help and concerns. cheers.

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