• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Trouble biasing Copland CTA-501

Baard

Member
2002-05-08 2:31 pm
Oslo
2 years ago I bought a used Copland CTA-501. And I wonder if I could ask a question about biasing a Copland 501.

FROM COPLAND BIAS PROCEDURE
1. Set P1-P2 in middle position. Turn P3 fully in clockwise direction.

2. Measure with + on the left side of R31 and - on the left side of R32. Adjust P1 to 0 mV. Repeat with R131-R132 and P2.

3. Measure mV over R32 and R132, choose the one with highest rating and adjust with P3 to 430mV

MY PROBLEM IS AS FOLLOW:
I have no problem adjusting P3 to 430mV for both pars of EL34. And I reach 0mV with P2 for R131-R132

BUT for P1 I get not mV but about 28-32 Volts! with + on the left side of R31 and - on the left side of R32.

When I got in contact with my dealer he just say: " If its sound okay, it's okay" The sound is okay though, but I¡¦m not particularly happy with that answer.

Why can I not adjust P1 to 0mV for R31-R32?

http://www.audiokit.it/ENG/Frames/Introduction1.htm
Here is a schematic for Copland-CTA-401, its out put stage is similar to my CTA-501

I have replaced diodes in the power supply with soft recovery diodes. And I noticed a more solid bass. I¡¦m very pleased with my amplifier, especially in the midrange and heights. But low bass is to slow, compared too a good solid state design. Anybody with an advice on this:

„h With it help changing the original power supply caps?
„h Use a choke in the PS?
„h Regulate the filament PS?

I hope this was not too much.

Best regards

Baard
 

Baard

Member
2002-05-08 2:31 pm
Oslo
Baard

I think I would make sure that all of the resistors R25 - R32 are the correct value. This should be done with the power off of course.

If the resistors measure OK then power the unit and measure the voltage on the cap side of R27 & R28. I would compair to the voltage readings of R127 & R128, they should be close in value. If the measured voltage at one of these points is very different from the others check for a bad tube or a leaky coupling cap C10 or C11.

I hope this helps. Good luck
 
Baard

The easy way to find a leaky cap, if that's the problem, is to swap the tubes around and rebias. If the problem moves with the tube, then the tube is the problem. If the problem is unchanged then the cap may be the trouble. To check this remove the output tubes, power the unit and measure the voltage at R27 & R28 this should be a - voltage of several volts. If one of these is less - or + in voltage then the cap is leaky.

To measure the cap with a RCL bridge would require removing the cap from the circuit. If you go to that much trouble just replace the part. I would think about replacing all of the coupling caps in that case.