When should bias reading be taken?

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When should bias reading be taken?

Postby kevco » Sat Sep 26, 2009 11:30 am

I notice that my ST70's bias rises and falls based on how long it's been operating. I always short the inputs and bias with a speaker load. My question is: How warmed up should it be before bias is adjusted? Do I want bias set after the unit has been on for 15 minutes, or after it's been playing for a couple hours? There is a fair amount of difference between the two readings. If I set bias at my usual 400mV per tube after a 15 minute warm up, it will generally rise about 425-430 mV after extended use of several hours. Both channels rise at the same rate, and the balance on the driver board remains equal. Do I want it biased warm, or hot? All responses are always greatly appreciated.
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Postby Geek » Sat Sep 26, 2009 2:17 pm

Hi,

For new tubes, I bias them initially, then after ten minutes and finally after an hour.

If there's more than a 10% variation with older tubes, they may need replacing (heats up things expand, grid distances and cathode emission change, too much may be real bad for the power or OPT)

Some brands (Philips are terrible for this) need a 24 hour burn-in if they've sat for a long time.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby kevco » Sun Sep 27, 2009 3:56 pm

Perhaps I should rephrase: Generally, bias voltages creeps upward during several hours of play. If the amp has a per tube bias of 400mV after a short warm up, and a per tube bias of 425-430 after some good use, should I adjust it, or leave it alone? After it's cooled down, the bias will drop again. Is this normal for output tubes with about 200 hours of use?
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Postby Geek » Sun Sep 27, 2009 4:15 pm

Hi,

I would redajust it back to 400mV after the "good use" (if that's what it's supposed to be), then forget about it [:)

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby Bob01605 » Wed Oct 28, 2009 1:07 pm

What will normally happen with an ST-70 with a tube rectifier is that the bias will creep up slightly during the first 15 minutes to 1/2 hour of playing. Setting the bias to the correct setting for your individual amp after 5 minutes will usually result in a bias voltage that is too high after 1/2 hour. What I would do with new output tubes is set them a little low after the first minute or so. If your target bias is .400 volts DC then set each output tube at .360 or so. If you monitor the bias you should notice a slow upward creep during the first 15 minutes or so. After 1/2 hour the bias should be stabilized and you can make your final setting.

Understand also that your line voltage can change UP or DOWN slightly and if it does then the bias will follow suit. Two other things > 1. If you change your rectifier tube you will have to rebias the amp and 2. Don't ever have a signal going through the amp when you set the bias.

Bob Latino
Bob Latino Custom Dynaco amplifiers

http://www.tubes4hifi.com/bob.htm
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