Fuzz/Phase shifting effect (muskito flying by) in the mids

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Who has Aluminium OR Steal chassis OR both with the SNAG???

Poll ended at Thu Jul 02, 2009 8:29 pm

Aluminium
1
20%
Steal
4
80%
had it on both type of chassis
0
No votes
 
Total votes: 5

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novosibir
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Post by novosibir » Tue Jul 29, 2008 8:31 pm

5150loveeddie wrote:Yes or No here :?: :?: :?: , merci beaucoup!!
Yes!

You also can measure on the board's turret, where the orange & green wires go off to the tubes. Because there's no risk, to slip off and shunt pin 5 to pin 4 or 6 by accident, what your output tube maybe won't forgive :wink:

Larry
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Post by 5150loveeddie » Tue Jul 29, 2008 8:45 pm

novosibir wrote:
5150loveeddie wrote:Yes or No here :?: :?: :?: , merci beaucoup!!
Yes!

You also can measure on the board's turret, where the orange & green wires go off to the tubes. Because there's no risk, to slip off and shunt pin 5 to pin 4 or 6 by accident, what your output tube maybe won't forgive :wink:

Larry
YEAH!!, Larry you rock as usual....... here we go...... Have my 43v Zener (only value I have for now..) assemblies ready, will pop that in and see what it does....
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Post by Billy Batz » Tue Jul 29, 2008 9:10 pm

raiken wrote:
Billy Batz wrote:What if your next set of tubes is far off or you use a different type of tubes? That would be a pain.
It shouldn't be that critical - that's why you want to clip a few volts over the bias level, but close enough to prevent pushing the signal down too far.

RA
OK. Certainly will do!

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Post by Froumy » Tue Jul 29, 2008 11:14 pm

Good Luck 5150!!!

You're 8V over neg bias voltage? Hoping that works. If it doesn't, I wonder if upping your Bias voltage slightly will yield positive results. If it doesn't? We'll find a solution. As far as I know, Dan is the only one that has done the exhaustive work to absolutely prove the problem to the PI section. That may benefit all of us. Crossing my fingers.... :shock:

Bob

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Post by Froumy » Tue Jul 29, 2008 11:36 pm

Ricky Lee wrote:With dual voltage switching in my 12xxx my bias on the low voltage side (400v) is set to 40ma, the high side (500v) is set to 33ma hopefully the 43v Zener will suffice both voltages. Most of my other amps have a 35ma bias setting so the 39v Zener is what I ordered for those and is probably the most popular since the -35 is probably the most used setting with the plexi's.
I have the same conundrum. I occasionally use the 100v tap out of the HO attenuator. I haven't measured mine, but It could possibly change my requirements. Worst case scenario, you could always switch a small diode in when you throw the high voltage switch, or something like that. I would initially err on the high side of your dual voltage amp, too. I didn't realize you had the issue in more than one amp. Is that correct?

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Post by 5150loveeddie » Tue Jul 29, 2008 11:53 pm

Froumy wrote:Good Luck 5150!!!

You're 8V over neg bias voltage? Hoping that works. If it doesn't, I wonder if upping your Bias voltage slightly will yield positive results. If it doesn't? We'll find a solution. As far as I know, Dan is the only one that has done the exhaustive work to absolutely prove the problem to the PI section. That may benefit all of us. Crossing my fingers.... :shock:

Bob
Ok first report:

I measured my bias voltage at my pin5's and have minus 31.5, tubes are biased at 28ma/& 33ma via a 1ohm cathode bias resistor method, I don't want to bias my tubes much lower then that, so I was way OFF with my 43v Zener clippers, seems to have more gain and more high ends, I was at minus16db on the hotplate, not sure if the Zeners ever kicked in there..... :? Not sure if the sound/reaction of the amp really changed of it's my imagination, swril was still there......

Good news I just found some 36v Zeners, so I will adjust my bias voltage to 32, tubes will be on the cold side but at least I'll near the Zener value and see what it does.....
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Post by 5150loveeddie » Wed Jul 30, 2008 1:32 am

Second report:

I think we are getting close now. Ok I've set my bias voltage the highest I could with the actual configuration, -32.5, my tubes are way cold (18-20mA) on there idle BIAS but who cares for know, I monitored the bias voltage with my multimeter hooked at ones of the 220k/.1uf junction, while playing fully dimed I could easely see the voltage going up to -40-45v sometimes a little more so the Zener was kicking in I suppose, I could fell the amp was "cleaner" noise wise, I still had some swril but kinda far away AND the fun part is I was attenuated like an animal at minus 16db room level in the hotplate, that is the best way to listen to the noise then, I think it is doing something in the good direction.

I'll leave it like that till I play the amp non attenuated and everything in between to really see tomorrow!!

Qts? What would be the tendancy of being just 1-2v above on the Zener diode and going higher or away 4-5v+, less/more swril if we have a smaller/bigger gap?? I was 3.5v on this one.
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Post by Froumy » Wed Jul 30, 2008 2:28 pm

The higher the Zener, the more crossover distortion you should get. If you clip it right at Bias voltage, I don't see any way that you could be experiencing crossover distortion. My understanding anyway. I've looked at a bunch of sine waves lately that had crossover, and in most cases it wasn't perceivable to the listener. That's why I've been thinking that getting the zener as high as possible shouldn't sacrifice any tone. We'll see.

Could be that your attenuator(like the Mass) is bringing it more to the forefront. Good luck with it unattenuated.

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Post by Ricky Lee » Wed Jul 30, 2008 3:15 pm

I ordered Zener values equal to or near my bias as well as other values as high as double my bias to hear any audible differences. The changes are so simple and cheap why not experiment.
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Post by 5150loveeddie » Wed Jul 30, 2008 3:39 pm

Froumy wrote:The higher the Zener, the more crossover distortion you should get. If you clip it right at Bias voltage, I don't see any way that you could be experiencing crossover distortion. My understanding anyway. I've looked at a bunch of sine waves lately that had crossover, and in most cases it wasn't perceivable to the listener. That's why I've been thinking that getting the zener as high as possible shouldn't sacrifice any tone. We'll see.

Could be that your attenuator(like the Mass) is bringing it more to the forefront. Good luck with it unattenuated.
Well with a hotplate or Mass, etc, it will make the "problem" worse much worse as the signal is all compress and every little noises will became concentrated orange juice.

So I continue today (didn't try it unattenuated yet, too mamy complaints here..), by just decreasing a haft a volts at a time my bias voltage while bringging my bias current up to par via my bias trim pot, so I went from 32.5 (Zener is 36v, 3.5v apart..) to about 30.5 (5.5v apart) bias voltage, amp seems more gainy and a tap more highs the higher I was going, no real change in the noise very far away, almost gone even. I think I like a hair of swril in my signal. Will need a lower Zener, it's ordered.....

My tube bias is still litle cold so I'll have to bring those up to 60-70% then adjust the Zener from there..

So in summary (so far) the "further away" you are with your Zener V versus the bias voltage the gainier the amp seems to get with a tab more high ends, less organic sounding also in some ways, not sure yet... :? Might just be my imagination and all the weed I smoked lateley!!
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Post by novosibir » Wed Jul 30, 2008 5:04 pm

Froumy wrote:The higher the Zener, the more crossover distortion you should get. If you clip it right at Bias voltage, I don't see any way that you could be experiencing crossover distortion.
If you clip it right at the bias voltage - today...

... tomorrow, or at another place you have 5 volts more from the wall, means 15...20V higher B+, and of course 2...3 volts more negative bias voltage - actually - but it's cut off by the zeners in this case.

Result: Your tubes are running too hot, because the higher B+ wouldt require more negative bias, but the bias now is *regulated* by the zeners, which don't allow more negative (as it wouldt be due to the higher wall voltage) :wink:

So better go at least a few volts higher with your zener's value in respect to your bias voltage!

Another reason for this - IMO still more important:

Keep in mind, that your output tubes still aren't in cut off at idle at the bias voltage the amp is adjusted to! They still are drawing current!

So allow the tubes - with the right choice of the zener - to go a step further to cut off while the negative signal cycle, riding on your pre-adjusted bias voltage. After my understanding, the zener shouldt only avoid an unneccessary low bias voltage below the tube's cut-off point - but the zener shouldt allow, that the tube still *reaches* its cutoff point.

The only reason of the zener is - as Randall already has pointed out in-depth - that when the signal swing on the bias voltage is so high, that the positive cycle reaches the zero volts point, where it can't 'grow' further through the roof, by still larger signal swing only the negative part of the cycle can go deeper and deeper into the still more negative direction.

The positive part is clamped on the roof at zero then (or just a few volts above the roof), but the negative part is steeping deeper and deeper into the negative area - with the result, that the DC/AC mixed bias voltage then is creating itself 'a new center line', another average bias voltage, which only can be more negative, then you'd adjusted before!

Hence the from itself created new bias voltage is more negative, the amp is running cooler with crossover distortion now.

And I still see another reason, why you shouldn't limit the bias voltage exactly at its adjusted value by the zener:

Keep in mind, that any class AB push-pull power stage at low input levels is operating in (true) class A - first when the input signal swing is increasing, it's sliding over into class AB mode. With a zener's value spot on the bias voltage level, the power stage will be forced, to operate in somewhat like a double SSB modulated SE mode and some signal cut out might appear.

Hope, that you all can understand, what I wanted to say with my weak German's English!

Still let me point out, that all what I wrote above I haven't verified by myself yet - it's just a theoretical based consideration!

Maybe Randall can chime in, to confirm or to deny it.

Larry
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Post by Ricky Lee » Wed Jul 30, 2008 6:19 pm

I never thought of the wall voltage swing. My wall voltage is never consistent, especially around meal times when my small little town is cooking - one minute 113vac the next 127vac, which naturally makes a dramatic swing in bias. And what about other places we play at, who knows what kind a voltage we get from some of these places. Of course one could get a voltage regulator I guess.
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Post by 5150loveeddie » Wed Jul 30, 2008 8:23 pm

Qts: Can we stick two Zener in parallel to make it's value smaller/lower like with resistors? Hard to find proper values.......
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Post by raiken » Wed Jul 30, 2008 8:41 pm

5150loveeddie wrote:Qts: Can we stick two Zener in parallel to make it's value smaller/lower like with resistors? Hard to find proper values.......
No.

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Post by Froumy » Thu Jul 31, 2008 12:32 am

A zener at bias voltage shouldn't have any crossover. I'd definitely stick with Randall's recommendations, too. Just trying to answer the question, as you lower the Zener will you get less swirl? I've thought all along that the higher you get it the better. But if you're still hearing it, you don't have a scope, your zeners are close? For testing, I'd slap 'em in there to at least be certain that it's crossover, not something else. I wouldn't think that it should cause other major issues to pop up. But....

FWIW, on Paul Ruby's 18watt link I posted, for test purposes only, a Zener(6.3V) at less than bias voltage(7.5V) was initially used. Crossover was completely removed, mosquitos were gone, and the scope view didn't indicate other problems. There was some extra clipping on the negative side of the waveform. If you encounter some crazy wall voltages, you might experience the same thing. Not sure of that other stuff, Larry. I'm still improving my knowledge of the long tail pair, but this post is definitely shedding some light on my misconceptions......Thrilled with that. Less relevant, that Marshall Major had 76v Bias voltage. used 151v Zeners referenced to ground(double the Bias voltage=152 for the Zeners.They do cutoff at .65V above their value, or basically = to bias voltage) He was all over the board with his plate dissipation, though.

In tracking Ruby's progress, for 18watters, seems like he's audibly noticed crossover in most new tranny's except Heybeor. Didn't hear it in most(but not all) old salvaged iron. So if you're building an 18 watter, take note... At least in his case, with that circuit, iron can make a difference. We've heard it with the old stuff though... And Dan has heard it with everything....(We should investigate iron, once we're sure we have a solution)
Last edited by Froumy on Thu Jul 31, 2008 1:16 am, edited 3 times in total.

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