18 watt fizz: please help me diagnose this?

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: CurtissRobin, colossal, zaphod_phil, Daviedawg, Graydon

Post Reply
katopan
Frequent poster
Frequent poster
Posts: 914
Joined: Mon 12/10/07 2:00 am
Location: Melb, Aust
Contact:

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.
0 x

jhaas
Occasional poster
Occasional poster
Posts: 109
Joined: Wed 07/25/07 2:00 am

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.
0 x

ontariomaximus
Occasional poster
Occasional poster
Posts: 155
Joined: Wed 12/27/06 2:00 am
Location: Unknown

Post by ontariomaximus »

Yes I used polarized caps, and have the negative toward ground.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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.
0 x
Nature abhors a clean tube amp

ontariomaximus
Occasional poster
Occasional poster
Posts: 155
Joined: Wed 12/27/06 2:00 am
Location: Unknown

Post by ontariomaximus »

Far less AC voltage there than the ripple at the first cap in a PS. No problem using typical electrolyics.
0 x

User avatar
zaphod_phil
Builder, Admin
Builder, Admin
Posts: 15208
Joined: Wed 03/19/03 2:00 am
Location: YYZ

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.
0 x
Nature abhors a clean tube amp

ontariomaximus
Occasional poster
Occasional poster
Posts: 155
Joined: Wed 12/27/06 2:00 am
Location: Unknown

Post by ontariomaximus »

I'll defer to you and your EE then :D
0 x

User avatar
JdJ
Newbie
Newbie
Posts: 54
Joined: Tue 05/05/09 2:00 am
Location: Seacoast NH

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
0 x

User avatar
snoof
Occasional poster
Occasional poster
Posts: 211
Joined: Sat 09/23/06 2:00 am
Location: KC

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.
0 x

ontariomaximus
Occasional poster
Occasional poster
Posts: 155
Joined: Wed 12/27/06 2:00 am
Location: Unknown

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.
0 x

User avatar
JdJ
Newbie
Newbie
Posts: 54
Joined: Tue 05/05/09 2:00 am
Location: Seacoast NH

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?
0 x

User avatar
snoof
Occasional poster
Occasional poster
Posts: 211
Joined: Sat 09/23/06 2:00 am
Location: KC

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.
0 x

kleuck
Occasional poster
Occasional poster
Posts: 253
Joined: Sun 11/18/07 2:00 am
Location: France

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 ?
0 x

ontariomaximus
Occasional poster
Occasional poster
Posts: 155
Joined: Wed 12/27/06 2:00 am
Location: Unknown

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.
0 x

kleuck
Occasional poster
Occasional poster
Posts: 253
Joined: Sun 11/18/07 2:00 am
Location: France

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.
0 x

katopan
Frequent poster
Frequent poster
Posts: 914
Joined: Mon 12/10/07 2:00 am
Location: Melb, Aust
Contact:

Post by katopan »

kleuck wrote: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.
Kinda what I was talking about - on the primary a filter across the whole winding will act differently as one valve cuts off. On the secondary there is conduction through the whole cycle.
0 x

kleuck
Occasional poster
Occasional poster
Posts: 253
Joined: Sun 11/18/07 2:00 am
Location: France

Post by kleuck »

katopan wrote:
JdJ wrote:So what are the differences between a conjunctive on the primary and the Zobel on the secondary? ..... Right now I have a selector wired in, so I'd imagine putting a conjunctive filter on the primary would be the simplest to execute in my situation
A filter on the primary is subject to the valves cutting off through the class B part of the signal swing. On the secondary conduction is all the time. I have no idea on what difference this would make to the sound. You can still fix the filter to the 8 ohm secondary even if you are switching the speaker between taps. The idea is just to reflect a load in parallel to the speaker at higher frequencies to 'cap' the max impedance, and this can be on a separate winding.
Indeed :oops:
0 x

User avatar
JdJ
Newbie
Newbie
Posts: 54
Joined: Tue 05/05/09 2:00 am
Location: Seacoast NH

Post by JdJ »

So I finally got around to taking a quick screen shot with the Zobel in place. This is with the 10ohm and 2uF across the secondary and 15v Zeners a la Ruby:

After (Volume on 10 400mV 333Hz Sine wave)
Image

Before with no Zeners and no Zobel (note the Vol on 7 - @10 the leading edge spike goes through the roof!)
Image

So the rising impedance is the culprit indeed! Thanks so much guys - you all have been a great help. I can't believe how much I have learned over the past month. If I can assist anyone by putting together more screen shots or any other testing, just let me know.

Next I'm going to try it without the Zeners again to see how I like the crossover distortion...

Special thanks to Katopan who talked me through the testing.

-Josh

PS I'm finally going to gig it this Friday!!
0 x

katopan
Frequent poster
Frequent poster
Posts: 914
Joined: Mon 12/10/07 2:00 am
Location: Melb, Aust
Contact:

Post by katopan »

Awesome news! Nice to see the CRO traces too - thanks for updating.

I'm interested to see what you think with the zeners removed, which will indicate if the problem noise was a combination of the two issues or just the one. Also did you put the grid blocker resistors back to some normal sort of value and did that return all of your treble, or does it still sound like the tone control is turned down a bit? I'm hoping that the tone is full range because on paper the zobel shouldn't be having an effect except right at and above the top end.
0 x

TomOlsen1
Occasional poster
Occasional poster
Posts: 159
Joined: Thu 09/18/08 2:00 am
Location: Jo´burg, South Africa

Post by TomOlsen1 »

Congratulations Josh!
0 x

Post Reply