Treble lower on r/h channel K5881 mk2

[shadders44david]

Hi there, Wonder if anyone could offer some advice on a problem i`ve recently noticed on one channel on a K5881 mk2. The treble and output are lower on the right channel than the left. I have swapped the outputs over to confirm it`s the amp at fault. I have tried swapping the valves over, cleaning the valve pins and sockets, checked all resistor values, checked live voltages, resoldered the right channel tag board all to no avail and all the capacitors are fairly recent. I have now exhausted my knowledge and would welcome any suggestions. Regards, David.

[Richard]

Hi David, you've covered the usual checks so check the signal path. I think you have input coupling caps fitted, and pio too, which are not the most reliable,
http://www.world-designs.co.uk/forum...ead.php?t=6116
so check/change/remove them and see if one's faulty. Not many amps have them fitted, they are a throwback to the original MkI version and are only needed if
1 any of your sources have dc on their output
2 they are part of a deliberate input filter for some reason
Check also for any resistor dividers fitted on the channel inputs to lower sensitivity as this amp was very sensitive (75mV?) and it may have been attenuated.

[shadders44david]

Hi Richard, thanks for the quick reply. I have tried your suggestion of checking the input capacitors, swapped them over but unfortunately with no audible difference to the lowered treble level in the right channel. I understand they are not necessary but the amp seems to sound slightly fuller with them fitted ! I did try the Ampohm PIOs (tried both orientation) you mentioned but even after extended listening they really didn`t do anything i liked and reverted to the Jantzen Superiors which to my ears sound vastly better. Also, the amp doesn`t have input dividers.
Thankyou for your suggestions, do you have any other thoughts ?

[Richard]

Hi David, well if you're sure all the dc voltages are ok and the problem doesn't move with the valves I'd continue looking at the ac signal path. Next likely failures that would cause loss of gain and perceived treble would be cathode bypass caps and I'd go for op stage first and use 50uF 100V rated ones.

[shadders44david]

Hi Richard, Yes i`m sure the voltage readings are there or there abouts but you may have a point with the cathode bypass caps. Due to a lack of bass and gain several months ago i changed the ageing cbc to Nichicon 50uf @63volts which improved the sound considerably. Maybe one of the two in the right channel has now developed a fault but i don`t have a capacitor tester/analyser to confirm this, therefore after googling Nichicon cap`s and finding some info` on premature failure i`ve ordered 4x Elna silmic 47uf @ 100volts. The only major problem i`ve experienced in 14 years of owning the K5881 was an exploded cbc (originals) about 10 years ago so the amp may be "heavy" on cbc ? Also, the low treble right channel has now developed a faint hum/buzz.
Thankyou for your suggestions and i will report back after fitting the new cap`s. Regards, David.

[Richard]

Hi David, still do the cathode caps but in light of your last comment you might also want to change C3/4/9/10 decoupling and smoothing caps. C9 and 10 are common to both channels so probably not the cause, though they get a hammering and would be better changed after several years, but C3 and 4 are specific to each channel and one of them going down might cause this.