18 watt fizz: please help me diagnose this?

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katopan
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Post by katopan »

The other way to think of a Zobel in this application is it's just a filter putting the resistor in parallel with the speaker after a certain cutoff freq, and capping the max impedance. There is a theoretical value that will cancel the inductance of the speaker, but to me the extra brightness (and extra bass around resonance) is the big plus of a high output impedance amp (ie. valves with no feedback). You don't want to kill all of that effect! But the speaker range is typically up to 4 or 5kHz.

From Josh's original CRO shot of the overshoot, the width of the spike is around 0.15 div on the scale, and the wave period about 5.3 div. He said the test waveform was 333Hz. If the overshoot spike is indicative of half the period of an oscillatory frequency, it's in the ballpark of 5.3/0.3 x 333Hz = 5.9kHz. Setting the RC to have a cutoff freq around or a bit below this will reduce the spike but not have too much effect on the upper end of the speaker response. With a 10ohm resistor that would suggest a 2.7uF or 3.3uF cap.
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jhaas
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Post by jhaas »

Is a polarized cap OK in a Zobel filter?

Web searches show it with both polarized and non-polarized caps, and when polarized, orientation is inconsistent.
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ontariomaximus
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Post by ontariomaximus »

Yes I used polarized caps, and have the negative toward ground.
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zaphod_phil
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Post by zaphod_phil »

Surely, with the AC coming out of the OT secondary, you should be using a non-polarised cap in a zobel filter.
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ontariomaximus
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Post by ontariomaximus »

Far less AC voltage there than the ripple at the first cap in a PS. No problem using typical electrolyics.
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zaphod_phil
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Post by zaphod_phil »

Yes, but ripple is still direct current (ie unidirectional) even though fluctuating in voltage level. What comes out of an OT reverses polarity every half cycle, and that's what will destroy a polarised cap sooner or later.
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ontariomaximus
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Post by ontariomaximus »

I'll defer to you and your EE then :D
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JdJ
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Post by JdJ »

So much for the email notification feature...

The Zobel seems to do the trick with my amp, although the 100k blockers are rolling a little too much high end off so I'm going to reduce them to 55 to see what happens. Using my neck pickup with 2uF + 10ohm resistor sounds like the tone is rolled down a bit which I attributed to going from 1uf to 2, but I'm sure it is as much or more to do with the big blockers.

Anyhoo- implementation seems to be an issue here with the Zobel and an impedance selector. If you haven't looked at it, the GDS Vintage OT has taps that have to be tied together or separated depending on the speaker load. This makes for a more complicated selector switch (not a big deal really), and an impossibility for different resistors to be connected at that same selector for the Zobel. So I now have to decide whether I add a separate selector just for the Zobel resistor switching, stick with the 10ohm resistor for all selector positions (fixed across output jack), or if I go to the Wattkins conjunctive filter setup. The Wattkins seems like the easiest to implement. Does anyone here have experience with that filter setup? I know Katopan was suggesting a standard conjunctive would only be working part of the time, but since the Wattkins is balanced I would think it should do the trick...

Is that a light at the end of the tunnel I see?!

Cheers,

Josh
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snoof
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Post by snoof »

I was buttoning up a 18watt build last night, and for grins I took out the 10k/.001uf conjunctive filter that I had in there and upped the el84 grid stoppers to 100k and added a 220k stopper to the PI input. I had bypassed the conjunctive filter in the recent past, and put it right back in, too much fizz. But the changes to the grid stoppers and the added PI stopper cut the fizz down enough to get rid of the conjunctive for now. This amp can still get bright though. I need to try the Zobel on another amp that i have that gets fizzy as soon as I get round tuit.
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ontariomaximus
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Post by ontariomaximus »

If you want to convert a zobel to a conjunctive filter or vice versa just multiply or divide by 1000.
eg. a Zobel with a 10 ohm R and 2.2uf C on the 8 ohm secondary is equivalent to a 10k R and .0022 cap on the primary (which is 8k)
Shouldn't make any difference.
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JdJ
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Post by JdJ »

ontariomaximus wrote:If you want to convert a zobel to a conjunctive filter or vice versa just multiply or divide by 1000.
eg. a Zobel with a 10 ohm R and 2.2uf C on the 8 ohm secondary is equivalent to a 10k R and .0022 cap on the primary (which is 8k)
Shouldn't make any difference.
I'm just wondering if there's a different interaction with the PA having the filtering on the primary instead of the secondary, but it sounds like you say there shouldn't be. In either case having a different impedance speaker will result in the resistor needing to be changed, right?
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snoof
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Post by snoof »

ontariomaximus wrote:If you want to convert a zobel to a conjunctive filter or vice versa just multiply or divide by 1000.
nice bit of info, thanks.
Last edited by snoof on Mon 02/08/10 4:39 pm, edited 1 time in total.
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kleuck
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Post by kleuck »

JdJ wrote: I know Katopan was suggesting a standard conjunctive would only be working part of the time, but since the Wattkins is balanced I would think it should do the trick...
I think a regular cunjunctive filter is working all the time, just as a post-Pi tone or master volume is.
Can we imagine that the Watkins twin filter is more a way to have an even impedance through the two halves of the OT, even with a poor built OT ?
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ontariomaximus
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Post by ontariomaximus »

JdJ wrote:
ontariomaximus wrote:If you want to convert a zobel to a conjunctive filter or vice versa just multiply or divide by 1000.
eg. a Zobel with a 10 ohm R and 2.2uf C on the 8 ohm secondary is equivalent to a 10k R and .0022 cap on the primary (which is 8k)
Shouldn't make any difference.
I'm just wondering if there's a different interaction with the PA having the filtering on the primary instead of the secondary, but it sounds like you say there shouldn't be. In either case having a different impedance speaker will result in the resistor needing to be changed, right?
There shouldn't be a difference because what happens on the secondary is reflected back to the primary, and therefore the R+C load on the secondary is as if it's on the primary. This increasing-load-with-frequency is in parallel with the speaker and it doesn't matter if you use 4, 8, or 16 ohm speakers. But you may want to tweak the cap to taste, as different speakers obviously have different voice coil inductances. The 2.2uf I suggested is a good all around value, but the setup - notice I am saying setup, not just amp - may sound better with values 1 - 4.7uf ( OR .001 to .0047uf if on the primary). I have mainly used zobels, but there is no drawback going to the primary instead.
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kleuck
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Post by kleuck »

I think i understood the reason for a twin cunjuctive filter in the Watkins :
When both tubes are working together ("A class") they "see" the whole impedance of the primary of the OT, BUT, when only one is conducting, it "sees" only 1/4 of the primary impedance.
So with a cunjunctive filter for each half of the OT there should be a more even impedance on the primary all along the conduction cycle of the tubes.
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