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mbeards 5th December 2012 11:09 PM

PP KT88 Amp Low frequency Ringing
2 Attachment(s)
Hi everyone,

I noticed today that when on the scope, my mono KT88 amp has some "ringing". It only occurs on the lower half of the waveform when the amp is working at medium levels and beyond. It actually seems to get better when the amp is clipping slightly. It is more prominent at low frequencies say 100Hz down but I think may still be there at higher frequencies.

I need to double checked but I don't believe the other amp has the same issue.

Swapped power tubes and preamp tubes with no change.

Photo 1: Medium Drive
Attachment 316014

Photo 2: Diven into Slight Clipping
Attachment 316015

Any Ideas?

SY 5th December 2012 11:13 PM

That looks like HF oscillation. If you can post a schematic, we can suggest ways to stabilize it.

DF96 6th December 2012 09:32 AM

Grid or screen stoppers?

richwalters 6th December 2012 03:57 PM

yup....Zobels missing in the o/p stage UL..or something way out with the global feedback components....... Worth squarewaving the amp at 1kHz at medium o/p level as this will show the ringing more clearly. You don't mention if the instability (which it is) occurs with a dummy output load or with LS. Big difference as LS's are reactive.


mbeards 6th December 2012 05:28 PM

Hi everyone thanks for the posts.

I will be looking at the second amp tonight and see if it suffers from the same issue. altough I dont think it does.

I do not have any resistors on the screens of the KT88. Maybe that will help.

NFB is around 15 dB with a small value cap across the feedback resistor. I think 68pf off the top of my head.

Unfortunatly I dont have my suare wave oscialator right now. Using an old Tek SG505.

More to come tonight..

Any more thoughts are apprciated

DF96 6th December 2012 06:03 PM

Screen stoppers are your best bet, maybe 470-1k. Unlikely to be the whole loop oscillating, although this is possible.

SandyG 7th December 2012 01:20 AM

Looks familiar!

Check out this thread. Most all of what mentioned here was tried with no effect on cleaning up the waveform. It ended up being something fundamentally wrong with the PCB layout or the initial design of the input section of the amp.

Had to try all the common fixes before accidentally finding something that corrected the problem (and possibly not the correct fix)


richwalters 7th December 2012 06:06 AM

68pF in the global feedback return is very low... you may have misread it at 680p which is more likely. Need to know what the value of the feedback resistor too....or As SY mentions.....schematic PLEASE to make real progress.


LinuksGuru 7th December 2012 06:09 AM

1) You need to disconnect NFB and look if oscillation continues. If yes, this is problem of output stage (is it UL???), very likely related to transformer characteristics/design. Also, as you should add grid stoppers as wisely suggested by other forum members.

2) If oscillation stops with GNFB off, you may try to decrease it let's say by 3 dB and see what happens next.

PS. Please post a schematic, blind disgnostic is not the best solution.

20to20 7th December 2012 01:33 PM

Try to find the first point it appears.

Output plates? Grid? K? Driver? Try stretching it to see what freq. it is.

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