Best way to cure output osscilation

[Helmholtz]

One more question,please. Can I shift the point where core saturation start changing the plate voltage,please? Thinking to drop the supply from 450v around to some like 400v....?

The limit where the core non-linearity starts to show as signal distortion for a given OT is determined by primary peak voltage divided by signal frequency. If you reduce plate voltage, distortion at 40Hz and full power will reduce - but also will max. output power. Maximum output for a given OT primary impedance depends on the plate voltage squared.
So lowering plate voltage by 10% will reduce available output power by roughly 20%.

[jmaf]

[ATTACH=CONFIG]52560[/ATTACH]

[ATTACH=CONFIG]52561[/ATTACH]

Trying to post a pic, found is much better today, but still can see. The mains voltage get lower with 10v around from 232v yesterday to 223v now.....
I measured everything slightly before clipping in respect with the bias voltage was set
Sorry for images was not pretty accurate, it is still pretty noisy, I have a spider on the table

You should cancel the amplifier project and sell this amazingly low THD oscillator you've created!

[catalin gramada]

You should cancel the amplifier project and sell this amazingly low THD oscillator you've created!

Sorry, didn't get it. The amp is pretty stable, nothing oscillate around, at least in low frequency range. Did I missing some ?

[jmaf]

Sorry, didn't get it. The amp is pretty stable, nothing oscillate around, at least in low frequency range. Did I missing some ?

I understood that the trace you posted was your 300k oscillation?

[catalin gramada]

No way, this topic runs through a lot of issues regards my project. Hi freq. oscillations was suprimed. Those pics shows the output at 400hz before clipping and was related to supposed crossover distortions in my amp sort of bias shifting in fact.

[catalin gramada]

Despite the fact it comes from 100w PA amp, the Partridge was not able to perform full undistorted in whole audio range.It is really too small with no abilities in low frequency range. This one will fit better

[Chuck H]

Where are the end bells?

[catalin gramada]

The limit where the core non-linearity starts to show as signal distortion for a given OT is determined by primary peak voltage divided by signal frequency. If you reduce plate voltage, distortion at 40Hz and full power will reduce - but also will max. output power. Maximum output for a given OT primary impedance depends on the plate voltage squared.
So lowering plate voltage by 10% will reduce available output power by roughly 20%.

I just thinking... this test was done with a 4.2 ohm power resistor. With a speaker connected I can expect the load impedance to be more like that keeping the plate current into a region which avoid premature core magnetization of OT in those frequency region. I cannot do a test with speaker connected from social reason but I can connect a 8-16 ohm resistor over 4 ohm output to see how perform around 40 hz.thinking more voltage/current transfer Can be this test relevant,more close by what real happen with a speaker connected,please?

[catalin gramada]

Where are the end bells?

...this will be such of problem..

[Helmholtz]

I just thinking... this test was done with a 4.2 ohm power resistor. With a speaker connected I can expect the load impedance to be more like that keeping the plate current into a region which avoid premature core magnetization of OT in those frequency region. I cannot do a test with speaker connected from social reason but I can connect a 8-16 ohm resistor over 4 ohm output to see how perform around 40 hz. Can be this test relevant, please?

Not sure if I understand correctly.

Output power into 4 Ohm or 4.2 Ohm will be essentially the same. Greater deviation from nominal/optimal load tends to reduce output power, as power is maximal with optimal load (changing the load impedance means changing the load line).
Real speaker impedance rises considerably below 200Hz (up to a factor of 5 and more at bass resonance depending on speakers and enclosure).

Core magnetization is determined by signal frequency and PRIMARY voltage amplitude. The influence of the load on primary voltage is weak.

[catalin gramada]

Not sure if I understand correctly.

Output power into 4 Ohm or 4.2 Ohm will be essentially the same. Greater deviation from nominal/optimal load tends to reduce output power, as power is maximal with optimal load (changing the load impedance means changing the load line).
Real speaker impedance rises considerably below 200Hz (up to a factor of 5 and more at bass resonance depending on speakers and enclosure).

Core magnetization is determined by signal frequency and PRIMARY voltage amplitude. The influence of the load on primary voltage is weak.

It is the same, the problem is a 4 ohm speaker will not perform the same, showing more impedance in low freq region
Thinking to try to put a16 ohm resistor over 4 ohm output to do the test in more close condition like a speaker do at those low freq. I just wonder if is relevant or not, please ? Thanks

In other words why to try to scope at 40hz with a matched load, if the speaker shows x4-×5 nominal impedance at those low freq. Testing with a 16 ohm resistor over 4 ohm output should be close enough ?

[Helmholtz]

Thinking to try to put a16 ohm resistor over 4 ohm output to do the test in more close condition like a speaker do at those low freq.

No, you got it wrong. I understand you want to wire a second load resistor in parallel with the 4.2 Ohm resulting in a lower total resistance.
But speaker impedance at 40Hz will be higher than nominal (4 Ohm), e.g. 20 Ohm.

Please don't do any sine wave tests/measurement with a load higher than twice the nominal resistance at full power, as this is likely to melt power tube screens. Also speakers are not designed to withstand full power sine operation.

[catalin gramada]

No, you got it wrong. I understand you want to wire a second load resistor in parallel with the 4.2 Ohm resulting in a lower total resistance.
But speaker impedance at 40Hz will be higher than nominal (4 Ohm), e.g. 20 Ohm.

No, I want to replace 4.2 ohm resistor with a 16 ohm one connected at 4ohm OT tap, just to test for low freq range response. Just thinking is more close what is happen with a real speaker connected, at least for low freq range

[Helmholtz]

No, I want to replace 4.2 ohm resistor with a 16 ohm one connected at 4ohm OT tap, just to test for low freq range response. Just thinking is more close what is happen with a real speaker connected, at least for low freq range

Please see my edit above.

[pdf64]

My understanding is that below resonance, driver impedance drops quickly, and by 40Hz it may be below nominal.
For a driver in a cab, compared to its free air unloaded characteristics, the bass resonance freq, Q and magnitude may change; with a ported etc cab, strange things may happen to the freq / impedance chart.