Bias Instructions for COPLAND 401 Integrated Amp?
TK Lee
the bias procedure for it. The distributor, Divergent Technologies, has
been pretty un-helpful.
If anyone has the instructions, or some common sense advice, I'd really
appreciate some help.
Thanks - TK
doctorjohn
biased.
In autobias amps you will not find trim pots and even weaker tubes will still
likely function. In fixed bias amps you should have at least 1 trim pot for
each channel pair of tubes. You have to adjust them using a multimeter but a
manual would be helpful.
TK Lee
The amp is from Sweden. 2 12ax7 for the phono stage, the amp stage is 1
12ax7 driving 2 12au7s (1per channel). The outputs are 4 EL34s, in
ultralinear mode. I guess the 12ax7 provides 2 gain stages, and the 12au7
in each channel amplifies the two inverted outputs which drives the 2 EL34s
in AB mode?
The bias pots on the circuit board are:
1) BIAS ADJ
2) LEFT BIAS BALANCE
3) RIGHT BIAS BALANCE
There are 4 small resistors (bias resistors?) sticking out. They are
mounted on extenders, and the leads are in turn bent into a "U", so they
stick up from the circuit board by over an inch. I can see that the trace
on the bottom of the board goes from one end of each resistor to one of the
EL34s.
What I'm not sure of is where to measure from. From one side of the bias
resistor to ground?
The procedure would be to measure each EL34, and use the BALANCE to make
sure both EL34s measure the same. The the BIAS ADJ would be used to set the
overall bias. That's the other thing I'm not sure of. What the overall
voltage should be.
Maybe you have a pretty good guess, knowing the topology of the circuit, and
the type of tubes involved? ie. I would guess there must be a typical bias
range? The output power is 32 watts per channel.
Hope it looks simple to you.
Regards - TK
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