Can someone explain Auto bias

Kingrex

Well-Known Member
Feb 4, 2019
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174
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I am looking for new 845 tubes. I was considering Premium Linlai. My concern is as follows. I have some tubes in my stash that will redplate. Yet I am told the following about the autobios.

The auto cathode biasing, bias voltage varies up and down automatically to keep the correct bias based on the tube. For example too much bias current and it’s turned down automatically and too little bias current and it’s turned up automatically. So basically if your tube is looking for 65mA that what the auto biasing will give it ,, HT voltage does not change however at 960-980V. In some ways auto biasing is a bit like plug and play and takes away a lot of the setting up issues of fixed bias setups.

IF this is the case, why is it that some tubes redplate and others don't.
These are my shuguang tubes.

My B tube data says
Uf=10v
Ua=1250v
Ug1=200v
Plat currant 72
The B tube will faintly redplate. It is a mild discolor of black plate to faint opaque red.

My C tube data says the same, but the plate current is 84. The C does not glow red at all. The plate stays black. There is a perception its hot. It does run at 580* Fahrenheit

I also have a set of C that have a plate current of 70. I was told these will redplate in my amp. I have not tried them

The Linlai tube data says the tube has a plate current of 64 at 1000 volts. This is well below the 84 that runs fine and also much lower than the 72 that starts to glow red.

I don't understand Auto bios. How a resistor and cap will sense a tube demand. I can see a resistor and cap always supplying a steady set current, and being a circuit as the tube changes keeping it at a set point. So why are my tubes plates glowing red with the lower plate current tubes???

Thanks
Rex
 

Kingrex

Well-Known Member
Feb 4, 2019
394
174
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Bump. Any tech able to comment?
 

Direct Drive

Member
Sep 16, 2020
74
43
20
East Anglia, UK
In auto bias the amp has a built in circuit (feedback comparator) which will automatically set the mA current level of the tube circuit, compensating for tube differences and thus run them at the recommended/designed level.
It only alters the Ma not the voltage.
It seems yours is cathode biased, so do you know specs for the cathode and anode?
Calculating the bias resistance is relatively simple: divide the required bias voltage by the anode current (mA), multiply the answer by 1,000. This gives the value of the bias resistance in ohms, which is the resistor value.

.
 

Kingrex

Well-Known Member
Feb 4, 2019
394
174
50
In auto bias the amp has a built in circuit (feedback comparator) which will automatically set the mA current level of the tube circuit, compensating for tube differences and thus run them at the recommended/designed level.
It only alters the Ma not the voltage.
It seems yours is cathode biased, so do you know specs for the cathode and anode?
Calculating the bias resistance is relatively simple: divide the required bias voltage by the anode current (mA), multiply the answer by 1,000. This gives the value of the bias resistance in ohms, which is the resistor value.

.
I don't know the spec. I could ask. Per your response, low mA tubes should not red plate. The amp should sense the demand???? Why would some tubes red plate and other seem to burn just fine.
 

Direct Drive

Member
Sep 16, 2020
74
43
20
East Anglia, UK
don't know the spec. I could ask. Per your response, low mA tubes should not red plate. The amp should sense the demand???? Why would some tubes red plate and other seem to burn just fine
That will depend on the tube and manufacture, some plates being thiner and heating (glowing) under lower amperage. But it could be sign that the bias circuit is not working correctly and too much amperage is getting through.
 

microstrip

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May 30, 2010
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Some times cathode auto bias simply means having a significative cathode resistor (around several hundreds of ohm or even kohm) - if current increases the cathode voltage increases and the negative voltage between grid and cathode increases in absolute value, causing a decrease of current that compensates for it. It is self stabilization technique. The drawback is we are loosing power supply voltage in the cathode resistor, reducing the possible available power. Some Jadis amplifiers, such as the JA80, were biased in this way.

Auto bias in modern designs usually measures the current in a resistor of low value - typically 5 to 20 ohms and adjusts the negative grid voltage to keep a constant average current in the tube.
 
Last edited:

Kingrex

Well-Known Member
Feb 4, 2019
394
174
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I think I get it a little more. But It still leaves me scratching my head.

I would personally rather rip the auto bias out and put a multi meter in once a week and spin a pot to keep it set. When the pot bottoms out I know its time to change the tube.
 

ArnoFenn

Member
Oct 28, 2020
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Got a schematic? I am asking since recently I am fixing some old dynacord eminent2 amplifiers that used to burn a lot of EL34's (before)
 

microstrip

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The Marantz t1 has 845 auto bias. The middle points TP5 / TP6 are just used to check cathode current balance.
The good thing of cathode auto bias is simplicity - no extra circuits to introduce noise in your tubes, just a passive resistor and a capacitor connected to ground a1.jpg !
 

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