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Old 28th Mar 2023, 9:39 am   #21
Valvepower
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Default Re: LF Instability TAD EL34B-STR

Hello,

Just seen this thread and felt it worth posting an article from a 1951 Wireless World about stabilizing amplifiers. Its a useful reference as regards instability.

At a tangent I’ve recently built a 50-watt guitar amplifier using original Partridge transformers salvaged from a very early 1970’s Sound City amplifier (not by me!), this would have been designed around original Mullard EL34’s in fixed bias – I’m using current production valves and it made me realize the subtle differences between the original Mullard and current production EL34’s – its hard to quantify, as I said they are very subtle, but they are there.

Terry
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Old 28th Mar 2023, 4:52 pm   #22
cathoderay57
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Default Re: LF Instability TAD EL34B-STR

Quote:
I would be contacting TAD and asking them why their valves are causing this problem.
. Hi Michael. Well, yes, I could do that. First, here are the links to the datasheets, first for the TAD EL34B-STR:
https://www.tubeampdoctor.com/media/...3-01-12-17.pdf
and here for the Mullard EL34:
http://www.r-type.org/pdfs/el34.pdf
As far as the inter-electrode capacitances are concerned, objective comparisons are difficult when the less-than operator is used. Others may wish to comment further...
Jerry
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Old 31st Mar 2023, 1:37 pm   #23
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Default Re: LF Instability TAD EL34B-STR

I'm happy to report that with lots of help from Gabe001, including his modelling of the complete amp circuit using LTSpice, shelf filters (2 per each channel), each comprising a 10nF and 1M resistor in parallel, were placed in series with the existing EL34 grid coupling capacitors (100nF, C9 and C10). Modelling predicted that this would significantly reduce the overall loop gain around 1Hz (where the LF oscillation was occurring). Indeed it did, and no ill effects at the top end either. Many thanks to Gabe001 and to all the others who contributed thoughts and ideas, particularly those who posted docs on shelf filters. Jerry
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Old 31st Mar 2023, 3:09 pm   #24
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Default Re: LF Instability TAD EL34B-STR

Thanks Jerry. It was a learning experience for me too. That shelf filter really kills off that low frequency boost, I may use it myself in the future. Without the shelf filter there is about an 8db low end boost and a very abrupt phase shift. See pictures
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Old 1st Apr 2023, 1:41 am   #25
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Default Re: LF Instability TAD EL34B-STR

Gabe001, I can see you stayed with the 10:1 ratio for caps (100nF, 10nF) in the step network. It appears you settled on a low value for n=1M/440k~2 for the step filter response characteristic, compared to say the design example shown by Roddam that used n=10, where Learned shows that n=10 has a significantly more pronounced mid-step shift of phase at circa 36Hz (10nF, 440k) as well as final LF gain drop by circa 3.6Hz, than say with n=2.

Did you explore the effect of higher n values with LTSpice, or were you playing it safe and seeing what minor level of n would stop the observed LF oscillation?
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Old 1st Apr 2023, 8:20 am   #26
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Default Re: LF Instability TAD EL34B-STR

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Originally Posted by trobbins View Post
Gabe001, I can see you stayed with the 10:1 ratio for caps (100nF, 10nF) in the step network. It appears you settled on a low value for n=1M/440k~2 for the step filter response characteristic, compared to say the design example shown by Roddam that used n=10, where Learned shows that n=10 has a significantly more pronounced mid-step shift of phase at circa 36Hz (10nF, 440k) as well as final LF gain drop by circa 3.6Hz, than say with n=2.

Did you explore the effect of higher n values with LTSpice, or were you playing it safe and seeing what minor level of n would stop the observed LF oscillation?
Yes I did play around a bit, and played it a bit safe as well. Plan b would have been to increase the resistor size if there still was oscillation with those values. How much is the margin of stability? If I'm interpreting the above correctly, the phase shift with an input sweep signal of 100mv amplitude (about 2w output) covering 0.1hz upwards, is 160degrees (360-200) at the 0db gain point, so the feedback is still negative at LF. Will it ring in the presence of a large 1hz signal at the input? probably, if the signal is large enough, but this shouldn't be a situation that will arise outside of a testing environment. Let me know if I'm interpreting this wrong. It's a complex topic

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Old 1st Apr 2023, 9:31 am   #27
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Default Re: LF Instability TAD EL34B-STR

To see if you've really nailed the issue & to test for LF stability you could try testing the amp without a load but at a very low power. To test if the amp is stable with difficult loads you could also try testing with a capacitor as the load, this is to test for HF stability, try a 10n followed by a 100n. In both cases you do a sweep from 10hz to 100khz. You can use paper or software like REW, Audmes or Arta.

On the subject of stability it's an idea to fit a Zobel network across the OPT secondary something like a 100n and 10r and to protect your OPT you can fit 2 or 3 reversed diodes from the anodes of the OP stage to ground, this prevents inductive HV spikes.
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Old 1st Apr 2023, 11:40 am   #28
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Default Re: LF Instability TAD EL34B-STR

I think it is difficult to predict what form of resonance or instability could occur. It may be that the original characteristic is as bad as it could get. A significant issue can be that the effective primary inductance of the OPT can vary with signal level, so for example an oscillation can build up but not get worse, or an oscillation can start with say a disturbance at one frequency and then the frequency can shift to the most 'unstable' condition the loop can find.

Of course trying to brick wall filter any disturbances/signals from getting to the closed loop is important, which may simply be just a CR filter on the amp input that rolls off below what the speakers really want to see, and which should then be ok for the circa 1-2Hz region.

It's also not easy to 'test' the amp as not everyone has a signal source down to 1Hz or below, nor do they have a scope with X-Y view and screen persistence control. And it takes time to set it all up and check the output signal for such an input signal and then change load impedance and check again.

Its even more onerous to try and determine gain and phase margins, as the output signal can be very distorted and can make measurement of magnitude and phase almost impossible from an X-Y plot.
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Old 1st Apr 2023, 12:47 pm   #29
cathoderay57
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Default Re: LF Instability TAD EL34B-STR

Thanks for the suggestions about deeper investigation. However, I'm not a perfectionist; I'm just grateful to Gabe001 that the swinging output valve current has been stopped without, to my ear, any adverse impact on sound quality. Furthermore, considering the cost of these transformers https://www.lundahltransformers.com/tube-output/ I'm not doing any tinkering that risks damaging them. Jerry
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