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Old 25th March 2009, 03:01 PM   #1
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Default MeiXing MC-805AA biasing


I have to replace the two 805 O/P valves in a Meixing MC-805AA integrated amp. Did anyone do this before --- having any information about the (bias) alignment procedure? Unfortunately, I do not even have a user`s manual...

thanks in advance

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Old 25th March 2009, 07:36 PM   #2
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I'm going to take an educated guess here. Assuming that your 805 amps are single ended, they are most likely self-biased. Meaning they use cathode bias via a power resistor in the cathode (filament) circuit to ground. This resistor may be bypassed with a capacitor. Pull off the bottom plate and see if you see a big resistor connected to the filament terminal to ground. (Or the filament center tap if it uses one) In this case, there is no manual adjustment to make. Just replace the tubes.

805 triodes are a pretty high gain tube that does not require a high negative grid voltage to keep it under control. So cathode bias is easy, and less expensive, to implement. You might also try contacting this Hong Kong dealer who may be able to help in obtaining a manual or conformation that it uses self bias.

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Old 25th March 2009, 08:42 PM   #3
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Hello Victor,

thanks for your advice --- Yes, its a single ended triode class A amp. It is definitely not self-biasing.
I e-mailed meixing (ming-da) twice in the last days, but they do not reply ... after all, I have set the anode current for 100mA, which seems to be a reasonable value.... warning to everyone else who should be working on such a monster: plate voltage is between 800 and 1000 volts -- this ain`t fun!

regards, audiomatic
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