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Strange Stability Enhancement Trick

LinuxGuru said:
Hi, dcgillespie! Is there any other similar tricks? Actually I'm having problems not with Williamson, but with 2nd schematic which seem to be in need for total reworking. Small HF oscillation (about 8 - 20 mV) on output, at 30 - 60KHz, and LF oscillation, at around 1 - 2 Hz. I tried to suppress HF oscillation with RC networks shown in dark-red without any success.
I suspect this could be due high capacitance between output transformer primary and secondary windings - about 5nF, or long HV wires lying around table.
Leakage inductance is 9 mH for one model and 18 mH for another. so this should not be an issue.
I tried to replace LTP tubes (6N6P) with 12BH7 and 6SN7, adding screen stoppers with no success. With the oscilloscope check I found source of HF oscillation is LTP.
Click to expand...

For HF oscillation try 330pf in series with 10k resistor between anodes of phase splitter.
 
LinuxGuru said:
I think I will get rid of DC coupled LTP and will use AC coupled one based on 6SN76N8S/6CG7.
Click to expand...

That seems counterproductive to me. Why add another LF time constant to a circuit that already has LF stability problems, unless you plan to place it far above the other LF poles, which ruins LF bandwidth? Dave outlined a very sensible procedure that should eventually lead to a better solution.
 
This is just a guess, based on what I was commenting on regarding the way Heathkit "killed" oscillation in the W5- but Linuxguru, what coupling cap values have you tried, on C2 and C11? With the .3uf cap, it looks like the rolloff point of that stage (coupling from LTP to output) occurs around 7.5Hz. Usually, that's not a problem range, but it'd be interesting to move that point around, say an octave in each direction, to see if that changes the phase relationships enough to change your LF stability problem. Maybe try a .15uf, or something like a .68 or 1uf in those two positions?

I'd be inclined to try that, before adding ANOTHER cap coupling to the unit (AC coupled LTP)...

Regards,
Gordon.
 
One other question- what power supply cap values are you using, for your B3 and B4 supplies? THOSE can also interact with the plate resistors of the stages they're feeding, to produce ANOTHER LF phase shift pole! Crowhurst mentions this being used for stability control- in fact, he specifically shows a Williamson design, which used the value of the power supply cap to "boost" the LF bandwidth of the amp, changing the LF phase enough to avoid stability issues...

So, those PS cap values could be something to look at. Either smaller or larger- only experimentation or calculations based on all the cap and resistor values in the amp (including power supply), are really going to tell you which one is better...

Regards,
Gordon.
 
GordonW said:
One other question- what power supply cap values are you using, for your B3 and B4 supplies? THOSE can also interact with the plate resistors of the stages they're feeding, to produce ANOTHER LF phase shift pole!
Click to expand...

B0+ - 200 uF + 5H/400mA Choke + 100 uF
the rest - 50uF

I have not posted PS schematic because its typical.
 
GordonW said:
This is just a guess, based on what I was commenting on regarding the way Heathkit "killed" oscillation in the W5- but Linuxguru, what coupling cap values have you tried, on C2 and C11? With the .3uf cap, it looks like the rolloff point of that stage (coupling from LTP to output) occurs around 7.5Hz. Usually, that's not a problem range, but it'd be interesting to move that point around, say an octave in each direction, to see if that changes the phase relationships enough to change your LF stability problem. Maybe try a .15uf, or something like a .68 or 1uf in those two positions?
Click to expand...

I used 0.33uF film capacitors (polypropylene). Since I have not been able to kill HF oscillation, which is main source of instability, I decided to trash that assembly and start another one from scratch.

What I also suspect as source of instability is an output transformer, which have 5nF capacitance between primary and secondary (yet its leakage inductance is below 9mH).

With completely another schematic it will be possible to identify if output transformer could be a culprit.

I think that schematic was tuned to particular output transformer and may be even layout. One fellow from Japan reconstructed this amp with Hashimoto transformers, replaced triode pre-driver with EF86, CFB with UL, and lowered NFB from 20dB to 13dB. However, this mod suffered from very high THD (7% at 60W/1 KHz, yet original had high THD level only at frequencies starting from 10 KHz). Probably he had to lower NFB so significantly due to stability problem at high frequencies, or may be oscillation.
 
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I'm pretty much a babe in the woods here on stability issues but I'm tempted to join in despite that. As I understand it, oscillation will occur because a feedback loop that should be negative has phase shifted beyond a critical value and now has become positive feedback. (Or at least one possible cause of it). Looking at the schematic in post 9, I notice that the cathodes of the outputs are wired to output transformer windings on the output side of the transformer, creating a possible feedback loop. Could this be as simple as those are phased incorrectly? Just asking here.

Shelly_D
 
LinuxGuru said:
I used silver mica. Yes. its oscillation with NFB disconnected.
Click to expand...

Do you still have oscillation if you pull out VL1?

You can try 1k..10k resistor between plate ov VL1 and grid of VL2A

Also how well are B3 and B4 decoupled from each other?
 
shelly_d said:
Looking at the schematic in post 9, I notice that the cathodes of the outputs are wired to output transformer windings on the output side of the transformer, creating a possible feedback loop. Could this be as simple as those are phased incorrectly? Just asking here.
Click to expand...

Nope, this is just cathode feedback (CFB)
 
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