Little help for beginner?

[ciccio64]

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.

 

[waltube]

Switch with the other channel the power tube first, then the input tube and loook.


Walter

 

[ciccio64]

Hi Walter thanks for replay
I did it already no change
It is not tube dependent...

 

[waltube]

Send the schematic, please


W

 

[ciccio64]

This is the schematic

 

[waltube]

Try to take off the input tube and switch on the amp only with power tube to check if the issue is still there

W

 

[ciccio64]

ok shall try
grazie Walter

 

[DF96]

The 100uF cap across the feedback resistor is presumably a mistake?

 

[ciccio64]

hi DF96 yes that is an early schematic
The final version use an 820 hom resistor and no cap in the nfb loop
also the tube in the pre are now ecc88

 

[DF96]

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.

 

[ciccio64]

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

 

[DF96]

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.

 

[ciccio64]

Hemmm... Whats caps you are refering too?

 

[waltube]

Please send also a photo of your amp.

Walter

 

[ciccio64]

Here is the picture. The look is meant to match my listening area..
On other end I did as you said and removing the input tube the problem do not occur.
Not sure what this mean tough..

 

[hooman]

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 .

 

[hooman]

remove coplage cap in g1 of kt88 in channel vibration.happen again or not ?
if happen again ...chang your new kt88.

 

[DF96]

Whats caps you are refering too?

Coupling caps, decoupling caps. Anything which defines low frequency phase shift in gain or feedback, including unwanted feedback via the PSU.

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.

 

[waltube]

Check carefully the voltage of input stage (on resistors) on good channel and then on the other and compare; a little difference is normal
Also check the solder points on socket.
After that, swap the coupling capacitor between channels


Walter

 

[ciccio64]

Ok Walter the voltage on the imputati stage is at least 10v l'ess then on the Good channel the poter suply seems ok the voltage drop as i Connect the wire to the socket what colui be?