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Old 1st July 2004, 06:15 PM   #1
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Default help for DHT bias in PP

I need some advise regarding best method to bias the two triode tubes in PP.
Tubes are 300B and have slight different characteristics i.e. measured under Vp=300V with grid bias = -61V they present Ip = 54 - 57 mA and 5.0 - 5.2 mA/V.
Which is the best between to use only one cathode resistor for both triodes (in so doing there will be some unbalanced current flowing into the O/P transformer primary) or use a different cathode resistor for each triode trimmed in order to have no unbalanced current into the transformer (in so doing the different amplification factor of the 2 tubes will cause however an unbalanced current under use of a real music signal) ?
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Old 2nd July 2004, 04:33 AM   #2
SHiFTY is offline SHiFTY  New Zealand
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Try both and see which is better. I use separate cathode resistors for each tube in my pp amp, which means that if a tube dies that the other one is not affected. Also it is nicer theoretically.

However some people say that shared cathode resistors are more musical.
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Old 2nd July 2004, 08:57 AM   #3
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Default to PP or not to PP...

If the stage is to operate in push pull then the signal current flow must be from cathode to cathode. So if seperate resistors are used to bias the valves a large coupling capacitor must connect the two cathodes to enable the push-pull condition to be met.

Alternatively a pot can be placed between the two cathodes and the single shared cathode resistor connected to the wiper. The pot can be adjusted to create an offset between the two valves that will move both the dc and ac balance points of the PP pairs operation. Some people, including myself, think this sounds better - however there is a caveat to unbalancing the push pull pair and that is the danger of core saturation in the OPT.

If your OPT will not tolerate a small amount of dc current then set for dc balance. If your OPT will tolerate some dc current then set for best sound.

You could move to fixed bias and individually set each valve... There are several different methods of setting up the bias for valves and they can be mixed up as well - for instance 80% cathode bias and 20% fixed bias. Most can be adapted to allow the individual op points of the valves to be tweaked. Just keep an eye on signal current paths to ensure that push-pull operation is maintained - assuming that is what you want.

There are a lot of differences of opinion on what is best - so try different methods and see which works best for you.

good luck!

ciao

James
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