nfb issue

[catalin gramada]

Hi
2V in PI (this is the signal of generator)400hz

output 7.5ohm/13V vith 2V in PI

output 7.5ohm/17V with 2.6V in PI (just in border of clipping)

output with 0.5V square in PI

preamp output was disconected for that test, the signal was injected in cap PI input
The blip is still here even with PI feed from generator but it is smaller compared when I injected the signal at input of amp
Presence pot was in 10

[pdf64]

How about with the presence at 0?
Sorry, I should have mentioned that previously.
Does the little cap across the LTP plates affect the blip, eg if it's lifted, doubled in value?
The blip phenomena seems to ring a faint bell with me, and I think that when I fiddled around with things, lifting the cap helped a lot.

[catalin gramada]

Hi
I was thinking about ...but not to remove. I was changed a SM type with a ceramic one at few kV. same value 47 pF.but did not any diference. I will remove and play with some values as You suggest. Thanks
Presence at "0" ussualy I saw attenuate the effect moving the blip from right side to the top of sine. It was very evident in previous tests.

[catalin gramada]

ok,here we go :
2v PI input , presence on 10

2V PI input,presence in 0 ( it was moved to the top, not to evident in pics but is there)

presence in 0 better view

fizz cap removed, presence on 10

fizz cap 220pF, presence on 10

[catalin gramada]

Thinking to change the transformer with a Hammond for a test. But this one is a good rated -139 replica made by Heyboer and provided by Marstran. Do You think it worth to change it for a test ?

[nickb]

Back in post #17 we demonstrated that the effect was happening prior to the power amp as it was present on the input to the PI. The presence, or adjusting caps, just changes the treble boost so emphasising the effect.

If you want to track it down, you have to look before the PI. That is why I suggested checking signal generator first; a binary search approach.

I had suggested that it was a 3rd harmonic effect but now I'm not so sure. I think for that to be true you would have to have odd symmetry in the waveform i.e. reflected through the origin - this is not the case. In other words, I have no idea what harmonic(s) to look for.

Whilst it would be interesting to know what is going on, I suspect it's beyond the scope of a forum format and considering the test equipment available. What I would to is go through it from input to PI input using a spectrum analyzer. Even then the going could get tough.

Honestly, if you're not going to lose sleep over it, I wouldn't worry.

[catalin gramada]

No way. I have not worries about because the amp is stable and more important it sound spectacular. I thought at begining it is a sign of instability and easy to fix, but both seems to not be true.I'm not a tech. I learn from my mistakes.Check in the test equipment is first on my list. Thank you. Cheers
Catalin

[Justin Thomas]

... it sound spectacular.

If this is true, then no, I wouldn't swap out a tranny just for a test! But do what you can to figure it out - that blip COULD be the reason the amp sounds so spectacular, and knowing the science might lead to repeatability. You MIGHT even stumble in the Trainwreck mojo! Just don't tell anyone when you do.

Justin

Justin