trinityamps.com

Unless your chassis is fully enclosed your amp will pick up air-born RF interference which typically sounds like a buzz riding on 60Hz (or 50Hz outside North America). You will hear the buzz whether the guitar is being played or not. In a combo you usually have metal foil lining on the inside to complete the Faraday cage.

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Great tones come in small glass jars!

there is foil on the wood back that goes on the bottom of the chassis but the Ceria-tone chassis does not have sides so the ends are exposed

That's not good. Especially if you increase preamp gain by cascading.

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Great tones come in small glass jars!

Do you think it would be sufficient to use some aluminum shielding foil on the sides?

I might as well try properly shielding it though this amp does not exhibit much more AC buzz or any pronounced RF vs the Trinity amp, at least at my house.

Have not really had it out in the world to say if there would be more interference elsewhere

OK so I think I have this thing licked - Unimind I have a good suggestion

My Trinity plexi buzz largely went away with the Ruby mod (still has a little but nothing like it was), but my Ceria-tone combo was much more stubborn. I've tried everything under the sun but today had a eureka moment and think this might work for your amp too.

Convinced that it is blocking and/or crossover distortion after reading Paul Ruby's description, Zaphod's and other comments and the success I had on the Trinity with the Ruby mod, I read and read and then came across this tidbit in a thread at the music electronics forum

"Zener on Rk to "fix" bias - I want to try using a zener across Rk on my cathode biased amp to help tame the crossover fizz at full dime. With 9.5 volts on top of Rk at idle and 18 volts at full output, I find a 13 volt zener gives good clean compression attack but snugs up nice and tight when cranked and helps a bit with crossover distortion."

intrigued and reading more I found Chuck posted this on a different thread a year later ..

"you can still have fixed bias without placing the el84 cathodes at 0 volts. You can just measure the voltage on the top of the cathode R. Lets say you measure 11 volts. Get a pair of 5 watt 6 volt zener diodes and solder them in series across the cathode resistor with the cathodes toward the voltage (instead of ground that is). Your amp will be just like it is now at idle. But now there can be no voltage rise at the cathode when current increases. This voltage rise in cathode bias amps cools the bias and creates that cathode bias sag. Your amp would now be fixed bias.

The '11 volts two 6 volt zeners' used above is an example. If you measured 10 volts you would use a pair of 5 volt zeners, for 13 volts use a pair of 7 volt zeners. You want the zener string voltage to be right at or just above the cathode voltage."

I tried it this afternoon

My amp now idles at 11 volts at the cathode resistor, and I installed a 12v zener as described in the 2nd post above across the cathode resistor and voila!!! - that killed 95% of the buzz that I had been unable to get rid of with all the other mods I tried - amp currently also has a 13v ruby zener mod, and big cathode bypass cap 1000uF - I also tried the Guitar Player mag snubber I posted earlier in this thread, and both conjunctive and Zobel style filters which are not on the amp now. None of the earlier trys killed the buzz - some helped marginally (the Ruby Mod)

did a couple before and after checks removing and reinstalling the zener to be sure my ears weren't fooling me and indeed it did kill the buzzies in my amp very nicely

Now I could use some input from the gurus as to the safety and reliability of such a mod - currently I have a single 5 watt zener installed which I assume may be a little light? I would guess that a couple higher rated zeners in series would give some redundancy and headroom but do not really know how to calculate what I would need

Interestingly I also have a Dana VVR scaling the whole amp and when I dial it down the buzzies come back as I turn it down, which I guess makes sense .. as I drop way below 12 volts the zener across the cathode and/or Ruby mod can no longer work as effectively the further I get from the zener values

Kinda confirms that they are clamping the buzzies though

Now to figure out how to scale the clamps... ha ha

edit.. maybe it would be easier to just switch to fixed bias??

A few points to clarify things:

If you use zeners to clamp or "fix" the cathode-bias voltage, you need to aim for a bias voltage that's somewhat higher than at idle, so that the power tubes run cooler, like in a regular fixed-biased amp - eg at 75% of max dissipation. If you don't do that you may injure your power tubes when the amp's cranking.

The power dissipated by the zener(s) is the zener voltage times the current through it. You would need to insert a meter in series with the zener itself to measure the current through it. Either that or add the standard 1 ohm series resistor and measure the voltage across it.

Any zener you use must be rated to at least twice the power it will actually dissipate while in use.

With multiple zeners in series, the overall zener voltage at the power tube cathodes is the sum of the rated voltage of each individual zener. So if say you want a maximum of 14V at the cathodes, you could use a 9V and a 5V in series. Never try to run zeners in parallel.

If you decide to convert the amp to "fixed" bias (better described as grid-biased), you will need a different kind of VVR that scales the boas voltage as well as the B+.

You will generally get clearer, crisper definition if you don't scale the preamp, just the PI & PA.

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Great tones come in small glass jars!

Thanks for the info as usual Zaphod -

If I follow you on the biasing, I should bias at something like 9 watts if amp is "quasi" fixed bias by using zeners across the cathode resistor?

This "band aid" really does clean up the buzzies my amp had but my Compu-Bias (yes I am lazy and thought this gizmo would make for less math - ha ha) indicates 345 volts plate, 41 milliamps cathode current, and almost 14 watts at idle, so I guess I need to bump up the Cathode resistor quite a bit to drop the dissipation - correct? Amp has a 135ohm 10 watt on it now

Assuming I drop it to roughly 9 watts, would you suggest I use a zener that is roughly 9 watts as well for the clamp? Or is a larger value safe?.... a bit larger might perhaps add a little cathode sag without buzzies provided I don't introduce dissipation issues with the clamp vs the regular cathode resistor

I also came across a post of yours from '06 at AX84 - "zener assisted cathode bias" that sounds like it would essentially do the same thing - Would this series zener/cathode resistor method you suggest offer any advantages vs the "clamp" zener across the resistor I am using now?

To refresh - "For even more stiffness you can use a zener diode in the cathode circuit. To do this, first measure the voltage across the cathode resistor at idle. Then chose a zener and resistor combination that will give the same voltage. Say for example, it's 25V across a 250ohm resistor (ie 10ohms/V bias). You could decide to use an 18V zener (or two 9V zeners), and make the remaining 7V up with the cathode resistor. The relationship is linear, and so you would use a 70 ohm resistor in series with the zener(s). This means that most of the bias voltage is now derived from the zener, but there will still be a small amount of squish due to the 70 ohm resistor.

You can basically then chose how stiff or how squishy you want to make the bias, depending on the proportion of resistive bias to zener bias you use. For example with a 21V zener and 40 ohm resistor, the amp would feel very much like a fixed bias amp. I don't know if it's the correct term or not, but I call this technique "Zener-assisted cathode bias".

Any insight would be greatly appreciated

I will post some clips soon of both my amps to demo before and after buzzies mods - I'm pretty stoked at how they sound now, particularly the Trinity head

Sorry to be late getting back to you on this.

No that's 75%, which is still a little too cold. Probably about 10W or 11W would be perfect.


Yes, 14W is really way to high for tubes rated at 12W max.


There's no correlation there. And in operation most of the cathode current will still flow through the cathode resistor with transients going through the zener. So I would gess that a 5W zener would be ample.


The series method is just easier to work out what ratio of fixed/variable bias you have. You will also get a constant amount of stiffness, whereas the parallel zener method allows the cathode voltage to swing freely up until the point it clamps. So the feel will be a little more dynamic. I really couldn't say which is better.

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Great tones come in small glass jars!

I finally fixed the fizz in my amp.

After re-reading the tech info on blocking distortion from Aiken Amps (http://www.aikenamps.com/BlockingDistortion.html) I came across this part:Now I had read that part before and in my amp I already had the Marshall values so I looked past it. However, the standard values for power tube coupling capacitors in an 18 watt are .01uF not .022uF. Slightly lower value. So I figured I would give it a try, saying as nothing else I have done has worked, and low and behold the fizz is gone.

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Les

http://www.soundclick.com/unimind

So, very interesting. The coupling caps to the power tubes - only - are now 0.01 uf and no fizzz. C8, C9 ?
Under all conditions then? This is good to know if it comes up again!

download/file.php?id=449&mode=view

Otherwise, schematic is the same after all this?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Correct. C8 and C9 are 0.01uF.Additional changes to that schematic are:
R18 and R19 are now 100K
R42 is now 1M
Edit:
R5 is now 2.7K

Latest clip:
Bright ch only (no cascade), everything dimed (even the PPIMV now as well )
http://www.unimind.us/PlexiNoodle2.mp3

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Les

http://www.soundclick.com/unimind

"even the PPIMV now"

What PPIMV design did you put in there and how did you fit it? Did you use the MV hole so now the MV controls both channels?

Nice clips too! What guitar is that?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Docs updated in resource section.

Schematic download/file.php?id=575&mode=view

Let me know if it is correct per your latest build, and how well the PPIMV works.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Here is the PPIMV design I used: download/file.php?id=396&mode=view
I drilled a hole on the back next to the impendence selector as seen in this pic: download/file.php?id=476&mode=viewThanks. The guitar is my Ibanez 560 with a Duncan distortion pup in the bridge.Schematic looks good.
The PPIMV works very well. I have been using it for months now without any issues.

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Les

http://www.soundclick.com/unimind

AARRRRGG!!!
OK so someone please tell me why biasing near 100% dissipation helps reduce the fizz???
I realized today that I was set on fixed bias but I do not remember the last time I checked the bias. I checked and found it at 100% dissipation.
So I re-biased to 70% and the FIZZ CAME BACK.
I made some clips to be sure.
At 98% the fizz is just slightly there but at 70% you can really hear it (high pitched phase-like overtone).
http://www.unimind.us/Bias70Percent.mp3
http://www.unimind.us/Bias98Percent.mp3

So Coco, false alarm on the coupling caps fixing this.
Back to the drawing board as they say.

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Les

http://www.soundclick.com/unimind

Damn, that PlexiNoodle clip sounded sooooo good too. I was really liking the tone I was getting. I wonder how long it will take the power tubes to fail if I just run them at 98% all the time.

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Les

http://www.soundclick.com/unimind

Can you try it cathode biased again?

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

The fizz is there as well. I will try to make a quick clip.

Edit, Here is the cathode bias clip:
http://www.unimind.us/CathodeBias.mp3

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Les

http://www.soundclick.com/unimind

Get some EL84M or military grade equivalent from Russia! Take a licking and keep on ticking.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps