18 watt fizz: please help me diagnose this?

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gtrplayr1976
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Re: 18 watt fizz: please help me diagnose this?

Post by gtrplayr1976 »

Where can I find clips of this "fizz" ? , so I can make sure I don't have it.

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

Post by Bry »

Has anyone tried the 2 x 3k3R's + 2 x .005 caps like in the Watkins amp?
http://www.18watt.com/forums/viewtopic.php?f=25&t=24434

Isn't this a cross between a conjuctive filter and a snubber? (looks like)

I built my amp (LiteIIb) in an aluminum chassis from mouser(hammond) and the PA tubes make a rattle sound at loud volumes so I just set it on a stand next to the cab instead of setting it on the cab,so I'm guessing its not the fizz the Paul Ruby mod fixes? But the fizz is supposed to sound like that?

BTW for the Zep sound I used a Jstation(w/bypass looper pedal) set to "British" so that when turned up to gig level (amp distortion + Jsation distortion) sounded dead on to the first 3 Cd's,which I know Page used a modified Supro 8O to record those,but at every gig at least one person would ask how I got the tone so "spot on" :lol:
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Re: 18 watt fizz: please help me diagnose this?

Post by Bry »

I tried the zobel ,I used a 12R and non polar 2u2 it seemed just a hair darker at higher volumes.
The interesting thing was with the postPIMV turned down there was very little pre amp distortion.
Before adding the zobel the pre amp was distorting nice with the PPIMV turned down,the volume maxed.
The other thing I noticed is before the Zobel the last bit of the volume knob from 10.5 -11(max) gave a distinctive harder crunch.(with MV maxed)
Now it doesnt have that extra grind at the max position of the Volume (with MV maxed)
I have been comparing using my LP and the same EVJ 1x12 cab with the stock Lady Luck spkr.
Tommorow I am going to put the Zobel on a switch.FWIW I am using the 16R tap (the Lady Luck measures 13.6R) with the zobel from the 8R tap to ground.
Any reason this would give me less pre amp distortion and missing that extra grind with the amp maxed?
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Re: 18 watt fizz: please help me diagnose this?

Post by oh7hhi »

I'm still using this: http://www.guitarplayer.com/article/smo ... -ride/5505

My amp didn't have a fizz-problem but more like a high-pitch ringing when played loud.
Now it's gone :wink: ...
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Re: 18 watt fizz: please help me diagnose this?

Post by gtrplayr1976 »

It was suggested to me to change the cathode bypass caps on V1 ,and V3 to 1uf or lower. I did this ,and it did take the fizz out. I'm just not sure what else it did yet. I may have to go back to the 25/25 caps just to see what the difference is. Will dropping the caps this low cause issues ?
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Re: 18 watt fizz: please help me diagnose this?

Post by CurtissRobin »

No "issues," just a bit of voicing change.

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

Post by gtrplayr1976 »

I think I'm going to change it to the Ceriatone schematic ,or use the Marshall schematic.
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Re: 18 watt fizz: please help me diagnose this?

Post by 500 Four »

Old thread here but I figured I'd chime in with my experience.

I built an 18 watt according to the Lite iiB schematic/layout last summer, I used a 14 gauge aluminum cake pan for a chassis (which worked great), Classictone PT and an OT from an old Baldwin organ that was run w/a push pull pair of 6BQ5s. It was my second build and I was very pleased with it, dead quiet at idle, nice overdriven tones from about 1/4 of the way up and onward. Then I tried to use it on some recordings...ugh.

I don't know if you could characterize it as 'fizz' or 'buzz' or crossover distortion or blocking distortion or what, but listening back I could hear this kind of shrill, unpleasant high end frequency accompanying the sound. I successfully mixed it out by rolling down a lot of the high frequencies in 'post', but I was prompted to scrutinize the sound in person to see if it was some fault of my recording technique. Sure enough, playing in a room with the amp a couple weeks later it was absolutely coming from the amp. A lot more tolerable in person but there nonetheless and getting in the way of the amp achieving what I would call 'good tone'. I went through a few tries at modding it out but none seemed to work and I wrote it off as a bad amp and carried on.

Then, last week I was working on a sort of cross between a 36 watt and single channel AC30 inspired amp that I intended to run 6GK6s in. Only have the one quad of 6GK6 I decided to re-wire the 18 watt to accommodate 6GK6s in order to find out definitively if the tubes were faulty. I re-wired the output tube sockets, but now with a couple more builds under my belt I was able to see other parts of the circuit that I could change. If I screwed it up or it didn't work, what the hell, right?

So I changed two things on the amp, 1) I swapped out the 8.2K grid resistors on the output tubes for 33K carbon films, and 2) I changed the cap that connects the plates of V1 to the volume control from .0047 to .01
Now I don't know if it was a different characteristic that the 6GK6s possess which EL84s don't, or if something I did made a difference but not only did I find out the 6GK6 tubes were healthy but holy crap, the 18 watt now sounded strong, able, glorious, confident and just good! I couldn't believe how much better it sounded. The high end harshness was gone, almost all the way up the dial. Topped out the volume did get a bit mushy but that was to be expected and is of no real concern since max volume is achieved much earlier on the dial.

The 30 (36?) watter got switched to EL84s since I had more to go around and I didn't want to change whatever magic happened in the 18 watt. I can't wait to record with it again :)

Anyhow, just wanted to share what worked for me. Death to all fizz, buzz and unwanted high end harshness!
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Re: 18 watt fizz: please help me diagnose this?

Post by ilcaccillo »

Bry wrote:Has anyone tried the 2 x 3k3R's + 2 x .005 caps like in the Watkins amp?
http://www.18watt.com/forums/viewtopic.php?f=25&t=24434" onclick="window.open(this.href);return false;
Isn't this a cross between a conjuctive filter and a snubber? (looks like)
It seems like 2 conjunctive filters from each plat to CT of the output transformer, I guess the peak voltage on each filter will be halved.

Anyone can chime in on this?
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Re: 18 watt fizz: please help me diagnose this?

Post by ampmadscientist »

I'm going to add something to this:
The fizz is caused by cold bias of the output tubes.
Since all output tubes are not the same..
The bias should be adjustable. And to get the cleanest decay of notes, the bias should be adjusted to 70%.
I see from the schematics that many of these amps have no adjustable bias.
That was a design error. The bias must be made adjustable, an easy mod to the amp.

What's happening technically:
The output tube is trying to shut off, during the decay of the note.
Because the bias is set too cold.
This is what creates the fizzy sound in the decay of the note.

On a scope:
You will see crossover distortion. This is the fizz sound.
And if you had a scope, you could recognize these problems quickly.

So you can go on doing all kinds of tweaks and mods...BUT:
you will never solve this until the bias current of the output tubes is set to match whatever type tubes you are using.
And as I said:
Not all tubes are the same. There is very wide variations among EL84 output tubes.

If you need more help, go ahead and ask it's OK. Have fun w/ it.
AMS
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Re: 18 watt fizz: please help me diagnose this?

Post by JMPGuitars »

ampmadscientist wrote:
Tue 05/26/20 11:54 am
I'm going to add something to this:
The fizz is caused by cold bias of the output tubes.
Since all output tubes are not the same..
The bias should be adjustable. And to get the cleanest decay of notes, the bias should be adjusted to 70%.
I see from the schematics that many of these amps have no adjustable bias.
That was a design error. The bias must be made adjustable, an easy mod to the amp.
It's not a design error. It's cathode biased. You should read more about cathode biasing.

You would also benefit from studying this circuit more. It's significantly more common for these amps to be biased too hot rather than too cold.

Crossover distortion can cause fizz, but that's not the only reason it happens. A lot of the time these tweaks mentioned above can be compensating for OT issues, among other things.
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Re: 18 watt fizz: please help me diagnose this?

Post by Bieworm »

ampmadscientist wrote:
Tue 05/26/20 11:54 am
I'm going to add something to this:
The fizz is caused by cold bias of the output tubes.
Since all output tubes are not the same..
The bias should be adjustable. And to get the cleanest decay of notes, the bias should be adjusted to 70%.
I see from the schematics that many of these amps have no adjustable bias.
That was a design error. The bias must be made adjustable, an easy mod to the amp.

What's happening technically:
The output tube is trying to shut off, during the decay of the note.
Because the bias is set too cold.
This is what creates the fizzy sound in the decay of the note.

On a scope:
You will see crossover distortion. This is the fizz sound.
And if you had a scope, you could recognize these problems quickly.

So you can go on doing all kinds of tweaks and mods...BUT:
you will never solve this until the bias current of the output tubes is set to match whatever type tubes you are using.
And as I said:
Not all tubes are the same. There is very wide variations among EL84 output tubes.

If you need more help, go ahead and ask it's OK. Have fun w/ it.
AMS
When experimenting with bias settings on fender amps I have concluded cold bias to result in thin and sterile, not fizz. Thanks to my friends here at 18watt I learned that doing the PR mod with the zeners and diodes the fizz is eliminated to the level that a little bit of fizz on top adds a lot of charm and complexity. The PR mod is only like 1$ on components and some fun on top. 😄
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Re: 18 watt fizz: please help me diagnose this?

Post by zaphod_phil »

500 Four wrote:
Thu 01/15/15 11:45 pm
Old thread here but I figured I'd chime in with my experience.
I don't know if you could characterize it as 'fizz' or 'buzz' or crossover distortion or blocking distortion or what, but listening back I could hear this kind of shrill, unpleasant high end frequency accompanying the sound.
So I changed two things on the amp, 1) I swapped out the 8.2K grid resistors on the output tubes for 33K carbon films, and 2) I changed the cap that connects the plates of V1 to the volume control from .0047 to .01
Now I don't know if it was a different characteristic that the 6GK6s possess which EL84s don't, or if something I did made a difference but not only did I find out the 6GK6 tubes were healthy but holy crap, the 18 watt now sounded strong, able, glorious, confident and just good! I couldn't believe how much better it sounded. The high end harshness was gone, almost all the way up the dial. Topped out the volume did get a bit mushy, but that was to be expected and is of no real concern since max volume is achieved much earlier on the dial.
Anyhow, just wanted to share what worked for me. Death to all fizz, buzz and unwanted high end harshness!
I'll put my money that it was swapping the 8.2K grid resistors on the output tubes for 33K which fixed the fizz issue. Changing the cap that connects the plates of V1 to the volume control, from .0047 to .01uF will have improved the amp's low end response. You could also have used a 0.022uF there, for yet more bass response.
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Re: 18 watt fizz: please help me diagnose this?

Post by Bieworm »

zaphod_phil wrote:
Tue 05/26/20 12:55 pm
500 Four wrote:
Thu 01/15/15 11:45 pm
Old thread here but I figured I'd chime in with my experience.
I don't know if you could characterize it as 'fizz' or 'buzz' or crossover distortion or blocking distortion or what, but listening back I could hear this kind of shrill, unpleasant high end frequency accompanying the sound.
So I changed two things on the amp, 1) I swapped out the 8.2K grid resistors on the output tubes for 33K carbon films, and 2) I changed the cap that connects the plates of V1 to the volume control from .0047 to .01
Now I don't know if it was a different characteristic that the 6GK6s possess which EL84s don't, or if something I did made a difference but not only did I find out the 6GK6 tubes were healthy but holy crap, the 18 watt now sounded strong, able, glorious, confident and just good! I couldn't believe how much better it sounded. The high end harshness was gone, almost all the way up the dial. Topped out the volume did get a bit mushy, but that was to be expected and is of no real concern since max volume is achieved much earlier on the dial.
Anyhow, just wanted to share what worked for me. Death to all fizz, buzz and unwanted high end harshness!
I'll put my money that it was swapping the 8.2K grid resistors on the output tubes for 33K which fixed the fizz issue. Changing the cap that connects the plates of V1 to the volume control, from .0047 to .01uF will have improved the amp's low end response. You could also have used a 0.022uF there, for yet more bass response.
Or maybe a .1uf.. you'd get fuzz😃😃😃
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Re: 18 watt fizz: please help me diagnose this?

Post by ampmadscientist »

JMPGuitars wrote:
Tue 05/26/20 12:19 pm
ampmadscientist wrote:
Tue 05/26/20 11:54 am
I'm going to add something to this:
The fizz is caused by cold bias of the output tubes.
Since all output tubes are not the same..
The bias should be adjustable. And to get the cleanest decay of notes, the bias should be adjusted to 70%.
I see from the schematics that many of these amps have no adjustable bias.
That was a design error. The bias must be made adjustable, an easy mod to the amp.
It's not a design error. It's cathode biased. You should read more about cathode biasing.

You would also benefit from studying this circuit more. It's significantly more common for these amps to be biased too hot rather than too cold.

Crossover distortion can cause fizz, but that's not the only reason it happens. A lot of the time these tweaks mentioned above can be compensating for OT issues, among other things.
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.
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