trinityamps.com

The one I posted is IMO a simpler design. It really is just an inline volume control. No DC on the pot, no resisters needed, no dangers of failure etc.

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Les

http://www.soundclick.com/unimind

My suggestions was in case of failure. Your purely relying on the pot and it's qul=quality.

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http://pdfelectronics.com

So I was reading up on biasing the PI and I found this "Hot-to-centre biasing will encourage the power valves to distort more, before the phase inverter does, while colder biasing will reveal more phase-inverter distortion". After doing the math to set a colder bias I came up with 820R for the bias resistor and 75K for the tail. Now it seems to have reduced the buzz a little in the el84s. I used to have to set the PPIMV around 6-7 and now I can set it around 8-8.5 before I get the buzz so it did help a little. I also notice a bit more compressed clipping coming from the PI (which is expected when biasing colder). It actually sounds pretty good.

Question for Stephen or Phil, by running the PI with those values will there be any ill effects? Will I be hurting the PI tube in any way?

Thanks.

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Les

http://www.soundclick.com/unimind

Hey Les

I posted your buzzy clip over at ampgarage along with mine asking dopey questions - hope that is OK

http://ampgarage.com/forum/viewtopic.php?t=15332

Lindsay

I'm somewhat colourblind and therefore have a real hard time making sense of amp gut shots posted online. It's quite a handicap when you're into amp building. However, tell me one thing - what have you done with the extra unused 5k OT primary leads?? As I discovered in an amp I was fixing for someone, those extra leads have to stay *above* the chassis, and be taped off an wrapped up there. They must not be allowed to sit inside the chassis, where they can radiate a huge signal into the rest of the amp circuitry. There appears to be a long unshielded wire running off the Volume control pot as well - a big no no. All lomg wires to/from Gain or volume pots must be shielded. I would also say that for a high-gain amp, your wiring could maybe do with being a bit tidier.


Unimind's clip was far more than a buzz problem. It's a severe case of what sounds like blocking distortion.


No, all regular 18W amps drive the snot out of their power tube grids, to quote Mr Paul Ruby. The extra preamp gain just gives more sustain in the the Brown Plexi version. Unimind's clip was far more than a buzz problem. It's a severe case of what sounds like blocking distortion.


Unimind's sound clip doesn't sound remotely anything like the Paul Ruby buzz, which is more like something mechanical vibrating inside the amp.

Unimind's clip was far more than a buzz problem. It's a severe case of what sounds like blocking distortion.


It looks like they're doing a kind of conjunctive filter, which in some cases can help get rid of some buzziness in the tone. A zobel filter on the OT secondary also does the same thing. Neither has the slightest relationship to the Paul Ruby mod. A completely different issues are being addressed.


The EL84 design guide says to use a minimum value of 10k, although in most cases the regular 8.2k used in Marshall 18W amps seems to work fine. This resistor helps reduce spikes in grid current which cause blocking distortion. So using higher values can sometimes help reduce blocking distortion problems.


OK, you just contradicted yourself there. In reality, some 18Ws have this problem and some don't, and no one has yet really figured out why. Plenty of folks crank their 18W amps real hard, especially the lower gain Classic and Lite II versions, which you have to crank hard to get the distortion. So far a zobel or conjunctive filter on the OT has generally proved effective in even real bad cases of buzziness.


The PPIMV isn't actually fixing the problem, but rather acting as a band-aid.


As I said already, those resistors can really help tame the grid current spikes, which cause blocking, as well as help kill ultrasonic oscillation. So larger values could be a real good idea, maybe as high as 47k or 56k.


I assure you there's nothing wrong with the 18W PI. It's actually a lot less hot than a regular Fender/Marshall LTP-PI. And if you're still using 220k grid reference resistors on your power tubes, you're already driving your power tubes a lot less than a regular 18W, which uses 470k ones.

If your clip is accurate, then like I said, your amp has *severe* blocking distortion, which I suspect is caused by parasitic ultrasonic oscillation, overloading the tube grids and driving them into a blocking state. By dialling back on the postPI-MV you are simply reducing loop gain which causes the oscillations to die off. If that is the case, then the answer is to work on the layout and get rid of that oscillation, rather than apply band-aids which wills poil the amp's tone and character.


Try going up to 47k or 56k.


I've heard that quite a few times, and have yet to see an amp that has suffered any harm from pot failure, even with real old vintage amps older than me. Having said that, my personal preference is the regular 18watt.com post-PI MV, which uses a dual gang 1M pot in paralell with 1M grid reference resistors. ).01uF signal caps isolate the two DC-wise.


This is yet another band-aid and a poor one at that, IMO. I don't believe you are yet actually dealing with the real issue - which is that there is something seriously wrong with your amp. It's not a design problem.


I just hope that canvasing further opinions doesn't get you even more confused....

In case anyone wants to try a Zobel filter on the OT secondary, Randy Fay aka ontariomaximus recommends the following:


You can connect a filter of the appropriate values to either the 8 ohm or the 16 ohm tap and just leave them there, regardless of what speaker impedance you're using.

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Great tones come in small glass jars!

Great stuff Zaphod - lots to chew on and I appreciate you taking the time to address some of our questions so thoroughly.

I'm going to putz with a few of the suggestions you have and will post any results for future reference. My amp is exhibiting similar buzz (not as noticeable on the chord clip I posted, but if I play muted single notes it buzzes similarly to Uniminds amp - perhaps a little less) plus it is quite hissy. I'm hoping that once I go through the amp and add some shielded wire on the master gain that it will quiet down a bit

Lindsay

Thanks for all you input Phil. I also will be adding shielded wires and trying to tidy things up in the amp. I have already moved the unused 5k OT primary leads outside the chassis but the buzz remains so shielded wires are next.

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Les

http://www.soundclick.com/unimind

Thought I would check in - I tried 15k, 22k, and 47k grid stoppers today (did not have 56k to try) - did not eliminate the buzzies here unfortunately. Did not seem to clip the highs noticeably at 47k so perhaps I will try 56k as well.

Took a chance and emailed Paul Ruby some clips, and he was gracious enough to reply - here is what he said..

That sounds similar to what I resolved with the cross-over elimination but your's sounds particularly bad. You can try the exact circuit and values in the article you found from me. What OT did you use? Where did it come from?

- Paul


Lindsay

Paul is a great guy, and I have found his advice to be really useful. The example in his buzz article however, has unusually low voltages for an 18W, since it was based on an old Bogen amp. So the 9.1V zeners he use would be too low for use in a regular 18W.

I also have to say once again that the way your amp is crapping out, doesn't sound much like the "Paul Ruby buzz" I've heard in amp builds I've worked on.

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Great tones come in small glass jars!

interestingly he replied again when I sent a thank you and answered his question about my amp (that my transformer is a Mercury Magnetics 18 watt)

"That OT should not manifest that buzzy tone. Makes me think there are other problems... :/"

In any event I will be trying a few different things (probably his mod too just for kicks), and will carefully go over the whole build - I have a bunch of new resistors and caps coming in case I managed to hose something with all the changes and rookie soldering, additional shielded cable for the one gain pot .. its all good learning experience.

One thing I just noticed. With my attenuator dialed so the amp is very quiet, I swear I can hear the OT literally buzzing very faintly a bit as I play - is this unusual? Might this be relevant?

Paul Ruby sent me another more detailed reply - very cool of him to take the time

"Do try the zener diode fix. It's a simple thing to do.

But, I have found that all EL84 based amps are easy to overdrive because the EL84s are only biased with about 9V or so. If the PI pushes more than 9V, then you get grid conduction on the EL84s, which pushes the bias current down (charges the coupling caps to the grids) and is the root cause of the cross-over distortion.

The key point is that ALL EL84 guitar amps with a full PI are overdriving the EL84 when cranked and have cross-over distortion. The amazing thing is that some amps exhibit the audible buzz and some do not, despite an oscilloscope clearly showing the cross-over distortion. The blocking and actual cross over distortion is required to get the buzz sound but is not the reason you can hear it.

If you hook up a purely resitive load instead of a speaker and look at the output, you see a clean wave. It's overdriven, so is squared off (sharp kinks in the wave) but it is smooth. There is obvious clipping and cross over distortion, which is there with all EL84 amps being overdriven.

But, then hook up a speaker instead, which is not a resistive load. A speaker's impedance will shoot up suddenly at about 6khz. At high frequencies, there is effectively no load and those kinks from clipping and cross over distortion are all high frequency. The result is that we get inductive kick back from the OT on those kinks. The output waveform will now show very large voltage spikes at thos kinks instead of the smoth transitions of the resistive load. This also happens on all amps but we don't hear it on all amps.... I've never been able to really figure that out but the OT seems to be the component responsible.

On a particularly bad amp that I had, I added the zeners to counter the blocking distortion, which got rid of cross over distortion, so got rid of the kink in the waveform, got rid of the large voltage spike at the zero crossing and cleared up the buzz.

Another thing to try is to simply add an 8-ohm power resistor in parallel to your speaker. This will provide a resistive load at teh high frequencies and will get rid of the large voltage spikes. If this clears up the buzz then I think it proves you are on the right track. If not, then it's a different problem. This experiment is a subset of an alternative solution called a "conjunctive filter". If this solution works, then we can add a capacitor in series with the 8-ohm resistor and make this a permanent solution. The capacitor will allow only the high frequencies to get to the 8-ohm resistor instead of everything. That's a conjunctive filter.

Let me know what you find... Feel free to post any of this if you see fit.

- Paul"

I still strongly doubt that the way your amp is crapping out is related to what Paul Ruby is talking about. I know the 'Ruby buzz" problem, and I've used his fix to cure it in some amps as well. What you have is far more than the "audible buzz" that Paul mentions.


In a typical 18W that voltage is actually around 11V to 12V at idle. It then rises as the amp gets driven harder.


The 18W PI is also designed to compensate by not driving the power tube grids so hard, as a typical Marshall style LTP PI. So there's truth in what Paul is saying, but IMO what's actually happening is to a lesser extent than he's suggesting.

Also, are you still using 220k resistors on the EL84 grids instead of the usual 18W's 470k? AFAIK the Ruby buzz problem has never occurred (at least in an audible way) in the case of 18W amps with the Vox-style 220k grid reference resistors.


It's even been suggested by merlinb of Valve Wizard, that a small amount of crossover distortion adds some harmonic richness into the 18W tone.


This loudspeaker impedance effect is what a Zobel filter on the OT secondary deals with.


Only as an experiment, as you will lose power in the speaker that way.


Actually, it's called a Zobel filter when it's on the OT secondary, but the principle is basically as Paul describes.


I think Paul and I are both agreed on that.

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Great tones come in small glass jars!

I tried a few more things on my amp to address the buzz

1. I tried the Guitar Player mag snubber filter idea that I posted earlier in the thread as well - no effect on the buzz.

2. Tried Paul Ruby's idea to experiment with a resistor on the speaker out to see if a resistive load helps kill the buzz - no love

Dumb question though - I just noticed that my 4 ohm tap is not hooked to the impedance switch when hooking up the resistor to the speaker

It is tied off outside the chassis with the unused wires from the OT (must have been that way when I got the amp as I did not hook up the OT). Assuming I am not using a 4 ohm load, will not having it hooked up affect anything? (My current cab is 16ohm and that is what I am running the amp at)

Kind of stumped about the buzz at this point.. any other suggestions?? Zaphod - would you suggest an overhaul to reverse the position of v2 and v3 like you mentioned in the other thread (Ceria-tone vs Trinity layout)??

Is there enough difference in cause/effect of a Zobel filter or Ruby mod vs the resistor on speaker load experiment to bother trying either?

thanks!!

Why, did you build your amp to the Ceri@tone layout? If so, the Trinity layout *may* help improve things.


The Zobel filter actually works very differently to the Ruby buzz mod, and they're actually addressing two different issues, which unfortunately both get called "buzz". I've known cases, where the Zobel filter was the only mod that fixed buzziness in the sound of an 18W.

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Great tones come in small glass jars!

I actually have 2 amps - one is a Ceria-tone tmb that I modded following the Plexi Brown thread, and the 2nd is a Trinity kit.

The Ceria-tone has the hiss and buzz issue, the Trinity is not done yet but will be interesting to compare the two

I am very interested in that. Been following this andPHil has a handle on it. I want to build a Plexi Brown some time and experiment too. Just lot going on and not getting much time for R&D!

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

And the Ceri@tone probably uses the old-style TMB layout, with the PI tube in the middle of the preamp section.

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Great tones come in small glass jars!

Yes it did have the PI in the middle but in my quest to figure out how to kill the buzz and hiss I have completely rewired the Ceria-tone board, swapping the position to the Trinity style layout, taking more care with all the soldering and lead dress. I should be ready to fire it up in a day or two, and should also have the Trinity Plexi done soon as well. Should be interesting to see how they sound and whether I end up getting rid of some of the issues in the Ceria-tone mod

Very interesting development.. being the gearhead I am, I bought a Gibson Goldtone 30 watt head a couple days ago. 12ax7's in preamp, EL84 power section and the 2nd channel with high gain plus boost. Great sounding amp - FYI.

That said, I am pretty stoked that my 18 watt holds up very well in comparison - perhaps more "Marshally" than the Gibson. As a huge Marshall fan I think I like my amp better, though the Gibson is pretty slick with the channel switching, mixing, reverb etc.

But, guess what... if I dime it on the boost channel, I get some swirly, fizzy, buzz essentially identical to my brown mod 18 watt. Not quite as much, and not nearly as much hiss as my noisy TMB channel, but absolutely the same fizzy/buzzy/swirly sound. I'm starting to think that EL84's at very high gain may just do that to some extent when you really push them. I can replicate Unimind's "zorchy" aftertones when muting strings like in his clip as well - again, not as prominent as in his clips but that same sound.

Perhaps we are chasing a non-problem (or is my Gibson also sick)?