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Old 6th January 2009, 09:59 AM   #1
Skorpio is offline Skorpio  Denmark
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Default 1. time biasing...

I have a few questions about biasing a tube amplifier, more specific a PP fixed bias amplifier with KT88 output tubes.

There are individual potentiometer for each tube and also 10Ohm kathode resistors for controlling current.

I am second owner and I have been informed that tubes are not new. So I will have to learn before a tube fauld occour.

The previous owner has adjusted bias to 88mA pr. tube and I would like to reset that to standard value = 40mA. Is the best way to adjust on tube at a time to 40mA or to dismount tubes and adjust bias voltage to higher voltage and then remount tubes and fine adjust?

My concern is if large differences in bias can damage the amp?

Also, when changing to completely new tubes, what is recomended start value for bias? Same as for the old tubes or a preset start value?
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Old 6th January 2009, 10:35 AM   #2
45 is offline 45  Italy
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Default Re: 1. time biasing...

Quote:
Originally posted by Skorpio
I have a few questions about biasing a tube amplifier, more specific a PP fixed bias amplifier with KT88 output tubes.

There are individual potentiometer for each tube and also 10Ohm kathode resistors for controlling current.
In this way you will always measure the sum of the anode current and the screen grid current.
Possibly, you could estimate the screen grid current from data sheets or in some other way depending on the operative connection and/or power supplies of the valve.


Quote:
Originally posted by Skorpio
I am second owner and I have been informed that tubes are not new. So I will have to learn before a tube fauld occour.

The previous owner has adjusted bias to 88mA pr. tube and I would like to reset that to standard value = 40mA. Is the best way to adjust on tube at a time to 40mA or to dismount tubes and adjust bias voltage to higher voltage and then remount tubes and fine adjust?

My concern is if large differences in bias can damage the amp?

Also, when changing to completely new tubes, what is recomended start value for bias? Same as for the old tubes or a preset start value? [/B]
88 mA for a KT88 can be a safe current...it depends on the other operative parameters.
For example, if you have an ultra-linear configuration with 425V anode voltage and -50V bias then (for the average tube) the total (anode + screen grid) current will be around 88 ma.
The total power dissipation would be 37.4 W which is safe in comparison to the absolute maximums of 42W plate dissipation and 8W screen grid dissipation.

You need to dismount the tubes only if you don't know anything about the voltages. However I think that if the amp has been working until now there would be no problems.
You can just switch off the amp and turn the bias potentiometer for its full negative value (you can know this by measuring the resistance between the central pin and one of the end terminals).
This is also a safe mode to plug in new tubes (if you have enough room for bias).


Cheers,
45
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Old 6th January 2009, 02:40 PM   #3
Skorpio is offline Skorpio  Denmark
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Thank you for your answer!

Yes, the amp is working nicely...and it is UL with 480V +B voltage.

Do you mean that I should turn the bias to maximum negative voltage before readjusting for 40mA?
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Old 6th January 2009, 05:07 PM   #4
45 is offline 45  Italy
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Quote:
Originally posted by Skorpio
Thank you for your answer!

Yes, the amp is working nicely...and it is UL with 480V +B voltage.

Do you mean that I should turn the bias to maximum negative voltage before readjusting for 40mA?
To go for lower current the bias has to be more negative. With 480V anode voltage in UL configuaration you will need approx. -65V to set the current (anode + screen grid) at 40mA.
With -60V it should be approx. 60 mA which is still very safe (less than 30 W dissipation) and get aroud 70W output power with 2-3% THD.

I don't know how your bias network has been designed.
First, you should know the voltage at the first filter capacitor. Then kwnowing the value of the series resistors in the network (including the potentiometer) you could figure out the current and thus the drop across the potentiometer and any other resistor. Then depending where the potentiometer is placed you can know the useful range for your bias.

Typically, after the rectifier, you should have a CRC filter, the potentiometer and another series resistor to ground, at least.


Cheers,
45
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