18 watt fizz: please help me diagnose this?

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

Post by zaphod_phil »

You could also be dealing with positive feedback in the higher-gain TMB channel, causing instability and signal overload, in which case you may need to check your layout, grounding and lead dress. Any longer signal connections should be made with shielded wire. Maybe also post some gut shots for the collective to examine.You never know, there might even be some faulty solder joints in the TMB channel. 8O

HTH
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HarryPotter69
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Re: 18 watt fizz: please help me diagnose this?

Post by HarryPotter69 »

You're discussing potential signal issues in the higher-gain TMB channel, suggesting checking layout, grounding, and using shielded wire. Sharing internal images could help identify faulty solder joints.
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