Hi all
I need help in solving a problem I cannot understand
I just finish my building a KT88 SE .This was meant to experiment different solution and not aiming to the highest quality. So far so good I am happy with what a learn and also from the sound of the ampli but there is a problem.
When I switch off the ampli the woofer of the speaker oscillate badly for few seconds.this happen only on one channel. I think may be a bad capacitor in the power supply?
Any idea/suggestion how isolate the fault?
Thanks for any help.
I need help in solving a problem I cannot understand
I just finish my building a KT88 SE .This was meant to experiment different solution and not aiming to the highest quality. So far so good I am happy with what a learn and also from the sound of the ampli but there is a problem.
When I switch off the ampli the woofer of the speaker oscillate badly for few seconds.this happen only on one channel. I think may be a bad capacitor in the power supply?
Any idea/suggestion how isolate the fault?
Thanks for any help.
Beginners generally get more help if they show us the circuit they built, not one of the many millions of circuits they didn't build but which they think are vaguely similar to what they did build.
Oscillation on switching off may be a sign of too much feedback, putting the amplifier normally in a state called 'conditional stability' - which means that if the gain is reduced then oscillation can occur.
Oscillation on switching off may be a sign of too much feedback, putting the amplifier normally in a state called 'conditional stability' - which means that if the gain is reduced then oscillation can occur.
point take on your observation DF96 it just happen I did not have an updated schematic ready as said this was a kind of experience workshop for me changing everyday.
Noted your advice regarding the NFB shall try to reduce it is just strange happening on one channel only?
thank you again
Noted your advice regarding the NFB shall try to reduce it is just strange happening on one channel only?
thank you again
One channel only probably means that the effect is marginal, and component tolerances play a role. For example, electrolytics may be 20% or more in tolerance.
Try changing capacitor values up or down by a factor of around 2. It may be that you have been unlucky and just happen to have a lot of phase shift at some subsonic frequency.
Try changing capacitor values up or down by a factor of around 2. It may be that you have been unlucky and just happen to have a lot of phase shift at some subsonic frequency.
Beginners generally get more help if they show us the circuit they built, not one of the many millions of circuits they didn't build but which they think are vaguely similar to what they did build.
Oscillation on switching off may be a sign of too much feedback, putting the amplifier normally in a state called 'conditional stability' - which means that if the gain is reduced then oscillation can occur.
when turn off the tube amp ..... should power tubes in output circuit go to cut off as fast it means no sound in speaker .
in tubes we should know abut tube bias .
if problem in tube quality ... can not be quit . so hearing sound or vibration .
Coupling caps, decoupling caps. Anything which defines low frequency phase shift in gain or feedback, including unwanted feedback via the PSU.ciccio64 said:Whats caps you are refering too?
Anyway, a simple change is to halve the feedback by doubling the feedback resistor. If this cures the problem then you may have had conditional stability.
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