K4040: weak bass

Hello together,

I´ve just finished the K4040 amp and I must say that this kit is great. The only problem I haved during build ( trimmer with wrong direction ) was already documented in this forum and so no problem.
The only thing I´m a little bit afraid is the weak bass.
The sound is bundled in the mid frequencies, high notes are ok but the bass is not present at all.
I´ve tried different speaker ( 4 ohm and 8 ohm ), always three way bass reflex ( I plan to build broadband horn speaker for this amp ).
May this be a problem with the speaker at the K4040 ?
May this be a problem of the used tubes ( EL34 or ECCC83 ) ? Are they probably not matched ?
Are they damaged because of the trimmer problem ( bias voltage ) ?
May I´ve made a mistake during build ?
Have anyone an idea for me ?

Best greetings from cologne

Joerg :?

Hello everyone,

I´ve just finished the back loaded horn speaker with one broadband speaker each. They could sound much better as everything I´ve heard before but at the K8020/K4040 the bass ist still missing.
Has anyone an idea how to figure out the problem. Could it be C1 on the small PCB of the K4040 ? May it be a problem of the preamp K8020 ?
I didn´t have any seperate pre-amp for testing.
May this be a problem of the output transformers eg. different colours of the cables ?

Who can help me ?



As this time, we cannot determine if there is a problem or not. ‘Weak bass’ is subjective, we need figures to be able to determine if there is something wrong or not. If you have acces to a scope, a generator and a 4 or 8 ohm resistor load, you can measure the output level of the amp over the complete audio range and check if it is within the specifications. Maybe a friend or college can help you with that.


the cause was quite simple. The bias adjustment went completely wrong. So After adjusting the bias setting over half an hour the setting become stable. Then I´ve changed the resistors with the speakers and the bass appears.
I only must have a look at the bias settings more often.