18 watt fizz: please help me diagnose this?

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Re: 18 watt fizz: please help me diagnose this?

Post by JMPGuitars »

ampmadscientist wrote:
Tue 05/26/20 5:19 pm
There are 2 types bias in 18 watt amplifiers:

1. is the cathode bias method, this is class A.
The current is 90-100% of the rated plate dissipation.
The bias is adjusted by changing the cathode resistor.

2. Grid bias: this is class AB
The current is typically 70% of the rated plate dissipation.
the bias is adjusted by B minus voltage on the grids of the power tubes.

I know a little bit about it. I have worked on tube amps for 45 years.

But no matter which method, Class A or Class AB, the bias is adjustable.
Cathode bias does not make it class A. Nor does grid bias make an amp AB. It sounds like you've been holding on to misconceptions for a long time.

The 18 watters discussed on this site are almost all cathode biased. They're pushed towards class A, but that doesn't make them class A.

You sincerely need to do some reading about cathode biasing so you can understand how it works. Yes, bias can be adjusted, but once set correctly for a cathode biased amp circuit, it isn't that likely that it will need any adjusting unless you're changing to drastically different tubes.

Again, nearly none of these amps are biased cold. If you look at the circuits, and the voltages on this site, you will see most people are pushing 85% to 100% dissipation (and yes, those at or near 100% should bias them better).
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Re: 18 watt fizz: please help me diagnose this?

Post by zaphod_phil »

Bieworm wrote:
Tue 05/26/20 1:10 pm
Or maybe a .1uf.. you'd get fuzz😃😃😃
Maybe more like flubb :lol:
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Re: 18 watt fizz: please help me diagnose this?

Post by zaphod_phil »

JMPGuitars wrote:
Tue 05/26/20 5:44 pm
ampmadscientist wrote:
Tue 05/26/20 5:19 pm
There are 2 types bias in 18 watt amplifiers:

1. is the cathode bias method, this is class A.
The current is 90-100% of the rated plate dissipation.
The bias is adjusted by changing the cathode resistor.

2. Grid bias: this is class AB
The current is typically 70% of the rated plate dissipation.
the bias is adjusted by B minus voltage on the grids of the power tubes.

I know a little bit about it. I have worked on tube amps for 45 years.

But no matter which method, Class A or Class AB, the bias is adjustable.
Cathode bias does not make it class A. Nor does grid bias make an amp AB. It sounds like you've been holding on to misconceptions for a long time.
JMPGuitars wrote:
Tue 05/26/20 5:44 pm
The 18 watters discussed on this site are almost all cathode biased.
Actually about 99% of them are, apart from some experimental circuits. Likewise for Vox AC30s, AC15s, Matchless amps and Fender Tweed Deluxes. They're also all Class AB, as a pair of EL84s in Class A can't provide any more than 9W total. Only Class AB operation enables them to output 15W to 18W per pair.

JMPGuitars wrote:
Tue 05/26/20 5:44 pm
You sincerely need to do some reading about cathode biasing so you can understand how it works. Yes, bias can be adjusted, but once set correctly for a cathode biased amp circuit, it isn't that likely that it will need any adjusting unless you're changing to drastically different tubes.

Again, nearly none of these amps are biased cold. If you look at the circuits, and the voltages on this site, you will see most people are pushing 85% to 100% dissipation (and yes, those at or near 100% should bias them better).
Cathode bias is sometimes referred to as "auto bias", as it self adjusts. Basically, as the tube's current flow increases, the voltage across the cathode resistor also rises, biasing the tube colder, which then reduces current through the tube. So then the only "adjustment" needed is to chose a cathode resistor value to put the tube approximately in the right bias range, and then the auto-bias will take care of the fine adjustment.

I also recommend this article to clear up questions on Class A vs Class AB amplifiers. http://aikenamps.com/index.php/the-last-word-on-class-a
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Re: 18 watt fizz: please help me diagnose this?

Post by PaulRuby »

The fizz has nothing to do with design of the bias. It was very clear when studied with a scope. When the EL84's were over-driven, grid voltage swings up above cathode voltage and the grid current very quickly charges the grid coupling cap which forces the bias to go cold and cross-over distortion results. The resulting kinks in the output current hit the OT and the OT reacts with kick-back. This kick back shows up as large voltage spikes on the EL84 plates and in the output signal at those kinks. Adding zeners to clamp the grid voltage eliminates the cross over distortion. I documented this long ago.

http://paulamps.com/18watterbuzz.html

What I was never able to figure out is why some amps have the fizz sound and some don't. They all have the blocking distortion happening.
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Re: 18 watt fizz: please help me diagnose this?

Post by JMPGuitars »

PaulRuby wrote:
Fri 08/21/20 8:25 pm
What I was never able to figure out is why some amps have the fizz sound and some don't. They all have the blocking distortion happening.
Hey Paul!

I assume it has to do with inconsistencies in the manufacturing process from one transformer to the next (even among the same lines). I could be wrong, obviously it's just a guess, but I figure it has to do with human error in the manufacturing process.

Thanks,
Josh
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Re: 18 watt fizz: please help me diagnose this?

Post by PaulRuby »

I tried swapping OTs between a good and bad amp. The problem did not follow the OT. Also left the OT unbolted and tried moving it around a bit as I wondered about the magnetic spike coupling elsewhere. That didn't work either. Very frustrating. I really wish I could solve that because I still consider the zener trick a band-aid for something I can't figure out. The last hypothesis I had was ground paths for the current spikes, but old repurposed amps mostly tied right to the chassis willy nilly, so is not an easy experiment if that's the bad one.
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Re: 18 watt fizz: please help me diagnose this?

Post by JMPGuitars »

PaulRuby wrote:
Sat 08/22/20 10:27 am
I tried swapping OTs between a good and bad amp. The problem did not follow the OT. Also left the OT unbolted and tried moving it around a bit as I wondered about the magnetic spike coupling elsewhere. That didn't work either. Very frustrating. I really wish I could solve that because I still consider the zener trick a band-aid for something I can't figure out. The last hypothesis I had was ground paths for the current spikes, but old repurposed amps mostly tied right to the chassis willy nilly, so is not an easy experiment if that's the bad one.
Weird! That's much stranger than my (wrong) guess. If the problem doesn't follow the set of tubes, or the OT, then it has to be ghosts. 👻👻👻
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Re: 18 watt fizz: please help me diagnose this?

Post by crgfrench »

It could be "ghosts in the OT". Let me identify potential ghosts in the OT, with a list of factors that come into play -- The ghosts are parasitic capacitance and leakage inductance. Both of these ghosts can be influenced by the following factors:

1) is the core a strip wound C-type with grain oriented steel or is it a stamped/laminated silicon-iron alloy?

2) is it a single coil shell or a 2 coil core construction? If single is it E&I or T&U layout, and if double is it U&I or L layout?

3) is it sectionalized to reduce leakage inductance, and if so is the sectionalizing vertical, horizontal, or both?

4) are the windings connected in series or parallel?

5) is it interleaved, and if so what is the interleaving pattern (see chart below)?

6) what are the wire gages?

7) are the windings layered or random, and if layered what is the insulation material?

8 ) is the secondary winding(s) paralleled using smaller gage?

9) what is the distribution of the primary winding around the center tap (is it 50% of the windings, 50% of the resistance, or something else)?

10) what specific insulation material is used?

11) what is the bobbin constructed of?

12) does it use a U (conventional traverse) or a Z (flyback) winding style?

13) what is the wire material and wire insulation material (polyurethane etc)?

14) is there an air gap (I know its double ended for PP operation, but there could be one) - and if so what is the gap length?

15) does it have shielding and if so what type?

With respect to #5, leakage inductance decreases with higher levels of interleaving, see the attached chart below. One manufacturer likely uses different interleaving patterns than the next, and perhaps even changes patterns over time within a given firm (e.g., later production runs are improved with higher N2, vs. earlier production runs).

With respect to #7 and #10, even otherwise identical OTs could have different "fizz" characteristics if A) The bobbin was wound using random winding rather than layer winding -- since, by definition, random winding will have more variance sample to sample; and B) the type and thickness of insulation used between the layers of the primary when the layer winding method is used (not random winding). Over time, acid in the paper can corrode the wire insulation. Paper also may not maintain its thickness over time, and is hygroscopic. Over time, "fizz" may develop in trannies that use "vintage" paper insulation between layers, as opposed to those using mylar, polypropylene or kapton. This is because of the increased parasitic capacitance which develops as the dielectric constant of the paper insulation degrades over time.
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Re: 18 watt fizz: please help me diagnose this?

Post by PaulRuby »

That is all very cool stuff. But, swapping the OTs between a good and bad amp, the problem stays with the bad amp. The davis 104 amp has the problem, no matter what OT I put in it. Magnetic coupling?? Ground paths??? I don't know.
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Re: 18 watt fizz: please help me diagnose this?

Post by crgfrench »

Is the fizz different than the hiss from carbon comp resistors?
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Re: 18 watt fizz: please help me diagnose this?

Post by crgfrench »

ampmadscientist wrote:
Tue 05/26/20 5:19 pm
There are 2 types bias in 18 watt amplifiers:

1. is the cathode bias method, this is class A.
The current is 90-100% of the rated plate dissipation.
The bias is adjusted by changing the cathode resistor.

2. Grid bias: this is class AB
The current is typically 70% of the rated plate dissipation.
the bias is adjusted by B minus voltage on the grids of the power tubes.
This is inaccurate.
Class of operation is independent of bias method.
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Re: 18 watt fizz: please help me diagnose this?

Post by Daviedawg »

"Is the fizz different than the hiss from carbon comp resistors?"

Yes, very. Hiss is always a low background sshhh. Fizz takes over the note as it decays in a distortion of the tone almost like sawing. It is a sound you will recognise if you hear it.

And Paul - thanks for your lovely simple band-aid.

Dd
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Re: 18 watt fizz: please help me diagnose this?

Post by Bieworm »

Daviedawg wrote:
Sun 08/23/20 2:25 am
"Is the fizz different than the hiss from carbon comp resistors?"

Yes, very. Hiss is always a low background sshhh. Fizz takes over the note as it decays in a distortion of the tone almost like sawing. It is a sound you will recognise if you hear it.

And Paul - thanks for your lovely simple band-aid.

Dd
I refuse to call it a band aid. It's a good solution!
We don't call the filter caps a band aid either?
:)
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Re: 18 watt fizz: please help me diagnose this?

Post by Daviedawg »

It was a joke on Paul's own description of it as a band-aid. Sorry maybe I should have added a smiley face.

I have put the mod in all my 18 watts so am very dependant on it.

Dd
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Re: 18 watt fizz: please help me diagnose this?

Post by Bieworm »

Daviedawg wrote:
Sun 08/23/20 9:00 am
It was a joke on Paul's own description of it as a band-aid. Sorry maybe I should have added a smiley face.

I have put the mod in all my 18 watts so am very dependant on it.

Dd
Me too.. on both ;)
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Re: 18 watt fizz: please help me diagnose this?

Post by PaulRuby »

The zeners will absolutely stop blocking distortion. The reason I call it a band-aid is that blocking distortion in a guitar amp is not a bad thing in the vast majority of them. I really want to know why. Maybe it's time to dig the davis out of the shed and try again...
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Re: 18 watt fizz: please help me diagnose this?

Post by Bieworm »

PaulRuby wrote:
Sun 08/23/20 11:38 am
The zeners will absolutely stop blocking distortion. The reason I call it a band-aid is that blocking distortion in a guitar amp is not a bad thing in the vast majority of them. I really want to know why. Maybe it's time to dig the davis out of the shed and try again...
Paul, you should do that on livestream and charge $$$ for watching. I bet you'd be surprised the amount of cash that will be. By now about the whole world is on the tip of its chair to witness thát climax 🤟🤟🤟
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Re: Clips

Post by cuffers65240 »

TomOlsen1 wrote:
Fri 01/15/10 2:41 am
Thanks guys for the quick fix! Right... here we go:

Apologies for the use of the Seymour Duncan demo riffs, needed something that would cover all the bases in less than 30 seconds.

The sound comes in over the open E chord at the end of the clip.
How do I get access to the sound clip? I do not see a link
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Re: 18 watt fizz: please help me diagnose this?

Post by cuffers65240 »

Sorry, one last question. Does the fizz sound like a metal part vibrating? I have a sound that I have chalked up to a resonant frequency issue regarding either the chassis sheet metal or some other part. Now that I have read through this, I decided to create an experiment and separate the chassis from the cabinet (1x12 combo). Now the chassis is sitting on the workbench and the cabinet is on the floor and I still get a sound like a resonant frequency vibration on certain notes. I did lightly tap on each tube with the eraser end of a #2 pencil to check for microphonic tubes and heard no noises.

I ordered an assortment of zener diodes that will arrive Monday so that I can tackle my horrific blocking distortion on the TMB channel (it sounds awful when volume pot is anywhere past 25%) that would also clear up the fizz (if that is what I am actually dealing with). I have a PC based scope that also can generate sine waves at any frequency so I will follow the signal and post pics of the waves like in Paul Ruby's wonderful article.

I used the voltage chart posted by Josh in the downloads section (18 watt TMB) and have played with the power transformer taps and some resistors to make my Ted Weber kit match those values (voltages were all over the place before). I am getting 97.5% dissipation per EL-84 using values plugged into Ron Robinette's Tube Bias Calc (cathode resistance is 150 ohms) and great sounds on the non TMB channel. The extra stages in the TMB channel are right now just too much, fart noises unless I really dial back the volume.

I just want to say that I am thankful for the people that really know what they are doing that contribute their time to this forum. Since I decided to take the plunge I have modified two guitars, built this amp and now am addicted and owe the confidence to those of you out there that are patient with us all. I am an EE but I do not swim in these waters professionally, I deal with power distribution and automation mostly and am trying to catch back up.
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Re: 18 watt fizz: please help me diagnose this?

Post by Bieworm »

The Ruby mod does the fix in most cases.
You say you have a 150R cathode resistor? Our advice is to use a 180R 5W and try to land around 75 to 85% dissipation.. that is much sweeter
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