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Old 16th December 2009, 09:32 AM   #1
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Default 7868 tube biasing and output trans protection

Here are the question

1) What is the recommended bias for 7868 tubes and i am not sure the brand of tubes. Most probably it is Toshiba, original tubes for my Pioneer SA810.

2) In order to measure the bias current, Bias FAQ
upon completion of adjustment can i leave the 1 ohm resistor indefinitely in the circuit?

3) If it is safe to put a 1 ohm resistor as mention in item 2. Can i select a low power metal film resistor to act as a fuse in case of valve failure to protect my precious output transformer? Or is it better to put a fuse instead of the fuse? What will be the fuse and resistor value?

4) In the SA810 there is only one pot to control the bias for the 4 tubes. Will it be better to modify 4 pots to adjust individual tubes.

Thanks in advance for your precious advice..
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Old 16th December 2009, 11:20 AM   #2
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1. Depends on the plate voltage and plate to plate load. I'd start with the recommendations on the datasheet.

2. Yes.

3. Perhaps, though be careful that normal use doesn't cause the resistor to go nonlinear. Generally, the fusing is done on the power supply end and that's what I'd recommend. Alternately, you could rig up an overcurrent sensing circuit with a comparator and a relay to cut off the B+ if current exceeds a preset limit.

4. Yes, individual adjustment is more of a pain in the rear, but will get you lower distortion and better balance as tubes age. Try to start with well-matched tubes, but they won't stay that way forever.
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Old 16th December 2009, 12:52 PM   #3
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Thanks Sy for your comments, however i do not know how to interpret from the datasheet further do not know the brand of the 7868 tubes. I assume is Toshiba coz it is the preferd tubes in my country long time ago..
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Old 16th December 2009, 01:02 PM   #4
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Don't worry about the brand, just grab a 7868 datasheet (RCA, GE, whatever) and see which of their conditions (plate to plate impedance and plate voltage) is most similar to the unit you're working on. Then use their recommended idle current as your starting point.
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