Best way to cure output osscilation

[catalin gramada]

It started with 16db of nfb before tested by ear with the speaker I received for cab. I didn't received any issue till I started to scope found those instability at 300kcps around. Seems reducing the nfb from 100/470 ohm to 100/1500 for ca 6db amount it compromise the response in lows region.
Thank You for support gents!

[Helmholtz]

It started with 16db of nfb before tested by ear with the speaker I received for cab. I didn't received any issue till I started to scope found those instability at 300kcps around. Seems reducing the nfb from 100/470 ohm to 100/1500 for ca 6db amount it compromise the response in lows region.
Thank You for support gents!

The amount of feedback also depends on the OT secondary tap used.

Can you post an updated schematic?

Which HF compensation methods did you try to fight the 300kHz instability?

[catalin gramada]

But also the huge amount of nfb don.t help. It seems the OT is not able full power at those frequencies

[catalin gramada]

The amount of feedback also depends on the OT secondary tap used.

Can you post an updated schematic?

Which HF compensation methods did you try to fight the 300kHz instability?

The amount of feedback was measured to the output in respect with signal applied but was done at 400cps. I choose 16 ohm tap just for convenience, the tranny have also a 100V tap

[Helmholtz]

The amount of feedback was measured to the output in respect with signal applied but was done at 400cps. I choose 16 ohm tap just for convenience, the tranny have also a 100V tap.

If the open loop gain at 40Hz is much lower than at 400Hz, increasing feedback won't help and only cause trouble at HF.

[Chuck H]

Something to consider, regarding it's use as a bass amp, would be the Ampeg 8x10" cabinet. The way I read it was that the 8x10" isn't actually that good down to 40Hz, but because the second harmonic generated at that low frequency is strong enough the whole spectrum is effectively "implied". So, you could always trim some coupling caps to slightly reduce amplitude at 40Hz. Hopefully reducing the distortion and hopefully getting a similar effect.

[catalin gramada]

And a sketch




Edit. Is a misprint regards coupling caps from PI , was 100nf before, reduced at 47nf from yours suggestion

[catalin gramada]

If the open loop gain at 40Hz is much lower than at 400Hz, increasing feedback won't help and only cause trouble at HF.

That's all ready happen

[Justin Thomas]

The Ampeg book said that original cab was desined to not reproduce 40Hz anyway due to the onstage noise problems. The natural rolloff was reportedly a bit below 80 as designed. And I think it was absolutely brilliant that way. That's part of why those originals sound so damn good and punchy & clear. It takes advantage of natural human hearing instead of trying to force-evolve us into liking something unnatural. There was also no such thing as a "bass" speaker.

I miss those SVT amps...

Justin

[catalin gramada]

I have my own opinions regards that ,considering always a closed back is better suited for a classic bass tone and imagine how much bass gain you achieve acoustical coupling 8 speakers on the same baffle.

[Enzo]

Those 8x10s are right there right now in your face. If you need a bowel loosener, get a 1x18 and add to the rig.

[Justin Thomas]

There's aggressive bass, and internal-organ-jostling. I prefer the former. I also forgot to mention, those old 8x10"s are pretty shallow, by modern-day standards of "bass tone." Huh. Well, I still think they got it right way back then...

Justin

[Chuck H]

Ok... Just by mentioning the 8x10 cab I managed to derail this thread. catalin gramada has a sort of intense subject going and now I've turned it into a discussion on bass cabinet properties.

My point was simply that you might not need a huge amount of 40Hz to get good bass tone and that it might be ok to minimize the distortion by reducing that frequency. Test by ear of course.

[g1]

Ok... Just by mentioning the 8x10 cab I managed to derail this thread.

Maybe not.
I was just coming in here to say I wished I had a scope shot of a vintage SVT at 40Hz. Part of the bias setup involves a 40Hz signal, and it always struck me when I did those adjustments how funny the waveform looked at 40Hz.
Compared to that, I don't find these scope shots that terrible looking.

[Chuck H]

BING BING BING BING BING ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

And there you go. The real world through the eyes of someone that's seen it.