I have just assembled my K4040. All dc voltages on both channels check out close to the specified values, and they are virtually identical between the left and right channels. All bias adjustments are settable and the voltage on the output tube cathodes can be set very close to 0.4V.
The left channel is working properly. I checked it out with a function generator and oscilloscope (my trusty PCS500 combo) and it delivers not far from the rated 90W into 8R resistive load. Clipping is progressive and symmetrical. Frequency response measures as expected and a quick look on the PCS500 spectrum analyzer shows odd and even harmonics well down. I also have access to a Prism dScopeIII analyzer for more detailed performance measurements once I’ve sorted out the problem I have.
The problem is this: The right channel is not working properly. The output waveform degenerates from fairly low power, say a couple of volts output, resulting in distortion. There is also a very slight buzzing sound that can be heard on the output.
Here’s what I’ve checked:
- Components are all installed as per the instructions and the component placement diagram.
- Soldered joints are all good (I went over them just in case),
- I have checked all resitor values in situ with an ohmmeter. All are what one would expect from an in-situ check, and resistor values between left and right channels are almost identical.
- The tubes are OK. I established this by swapping the tubes from the left and right channels - the fault stays on the right channel.
- I scoped the waveform at output of V10 (input amplier stage) of both channels by probing at the junction of R20/R21 (left channel) and R26/R27 (right channel). The waveform on both channels is the same at this point.
- I scoped the waveform at the junction of R17/R84 (left channel) and R23/R87 (right channel). The faulty channel shows a much higher signal amplitude than the good channel at this point, and the waveform deteriorates markedly as input to the amplifier is increased.
This would suggest that there may be something amiss within the feedback loop, causing some kind of instability? Under ‘Troubleshooting’ on p23, the assembly instructions describe a mod of increasing R65 and R68 in value e.g. 1k or 1K5 which would reduce the negative feedback - I have not made this change yet, in case the problem is more simply solved.
Do you have any ideas on what may be causing this problem, and can you give me some pointers on where to look?