Good day one and all.
I have a Cayin A88-T Mk 2, that's been doing it's job quite well for a while. After transporting it to a friend, I noticed that the higher frequency had left in the right channel, it all sounded very muted. A quick frequency sweep confirmed this. I noticed that the power tubes in one channel stayed quite cold, although the heaters are glowing normally.
I first suspected the tubes, and swapped them with the other channel. The problem persisted with the tubes plugged into the right channel. I then suspected a bias problem. The tubes in fact refuse to be biased, the meter just goes completely to the right and doesn't react to adjustments. However, when I pulled the tubes and measured the bias voltage at the socket, it was identical to the left channel and also regulated identically when I turned the potentiometers. All the other voltages, especially anode voltage, was equally identical to the left channel. I haven't opened the amp up, because I had to leave my friend's place that day. I'm a little stumped though: It's not the tubes, not the bias, not the B+ - do you have any good propositions where I should propably look first when I go there next to open the amp up?
Thanks a lot and best regards from Germany
I have a Cayin A88-T Mk 2, that's been doing it's job quite well for a while. After transporting it to a friend, I noticed that the higher frequency had left in the right channel, it all sounded very muted. A quick frequency sweep confirmed this. I noticed that the power tubes in one channel stayed quite cold, although the heaters are glowing normally.
I first suspected the tubes, and swapped them with the other channel. The problem persisted with the tubes plugged into the right channel. I then suspected a bias problem. The tubes in fact refuse to be biased, the meter just goes completely to the right and doesn't react to adjustments. However, when I pulled the tubes and measured the bias voltage at the socket, it was identical to the left channel and also regulated identically when I turned the potentiometers. All the other voltages, especially anode voltage, was equally identical to the left channel. I haven't opened the amp up, because I had to leave my friend's place that day. I'm a little stumped though: It's not the tubes, not the bias, not the B+ - do you have any good propositions where I should propably look first when I go there next to open the amp up?
Thanks a lot and best regards from Germany
There would be small-value bias-set resistors from the cathodes to ground, like 1 to 10 ohms. You could check those and see if they've blown.
It might be cheaper to have this amp repaired when tools spares and knowledge is available then a length try and test method that also could damage other parts of the amp.
The choice is the owners
The choice is the owners
Check all the pins on the tube. Probably the cathode resistors are o/c. They will be designed to fail if the tube goes short, the bias is missing or the screen voltage is missing (UL switch faulty)
You are right. In a forum where members build amplifiers, they give the address of a repair shop to fix the problems.
Hey.
Thanks for the input!
There's a total of one valve technician in the greater area, and he's booked for more than a year. And though I lack the knowledge how to design tube amps in detail, I've at least built a few from schematics and understand a few bits and pieces.
I don't think the amp has power tube biasing via cathode resistors, but fixed bias. There is a bias potentiometer, and there's a negative grid voltage.
Thanks for the input!
There's a total of one valve technician in the greater area, and he's booked for more than a year. And though I lack the knowledge how to design tube amps in detail, I've at least built a few from schematics and understand a few bits and pieces.
I don't think the amp has power tube biasing via cathode resistors, but fixed bias. There is a bias potentiometer, and there's a negative grid voltage.
Are all the cathode resistors ok? Measure their resistances with all power off. These often open due to tube current surges.
All four resistors should be the same.
All four resistors should be the same.
Yep first thing to check. Also check the connection from the screen to HT or plate on the tube socket again with power off.
The output tubes also have screen resistors, one per tube. The same applies to those.
Measure them with all power removed and the AC line unplugged. All four resistors should be the same.
Replace any bad parts, and clean the related screen switches.
Measure them with all power removed and the AC line unplugged. All four resistors should be the same.
Replace any bad parts, and clean the related screen switches.