trinityamps.com

I finally have some time to work on the amp now so I want to try and get rid of this fizzie buzzing when the master volume is cranked up.

http://www.unimind.us/fizz1.mp3

It is in the PI or the PA and I think it is because I might be driving it too hard. Has anyone else experienced this? Is there anything I can do to fix this?

Thanks.

_________________
Les

http://www.soundclick.com/unimind

That's not anything even slightly resembling the kind of top end fizzyness some 18Ws can suffer from when driven too hard. I believe there's a fault in your amp. Your amp uses lower value grid reference resistors, as well as a 56k PI tail resistor, so it's also very unlikely that it's getting driven too hard.


Sounds like a bad tube or tubes somewhere in the amp. Might also be a bad solder joint or a faulty cap.

_________________
Great tones come in small glass jars!

My sIII used to do that when I first buit it (it was my first build). It was never this bad though. It would happen as the note decayed but not this quickly or pronounced.

I later re-built it and the problem is pretty much gone, unless I really wind out the master & gain/volume. Even then it's nowhere near as bad as yours. It's not all that noticable at all really. I suspect it was a lead dress issue as my rebuild is a LOT tidier than my first attempt.

Now here's an interesting tidbit. When I re-built my sIII I converted it to a sIIIv6. This little quirk only happens in EL84 mode. In 6V6 mode, even with the vol & gain dimed and the boost kicked in this doesn't happen. Maybe it has something to do with EL84's in concert with lead dress. And I LOVE the amp cranked in 6V6 mode, BTW. Sounds much more like a cranked high powered Marshall (to my ears) than the EL84's.

And also interesting, this same thing happens with my 2204 running EL34's (again, subtle, not like your clip), but not as much with the KT-77's I have in there now.

And here's another interesting tidbit. I've gotten into a band called "Dixie Witch" the last few years and on some of their recordings you can hear this artifact in JT's guitar tone at times. It's more noticable on the first two albums ("Into the Sun" and especially "One Bird, Two Stones") - his tone is very different after those two. I'm not sure what amps he used, but it sounds like Marshall (or Marshall based) to my ears, and it also sounds like he's got them cranked. Great band, BTW. If you like old school rock their stuff may speak to you.

_________________
SG + Brit tube head = Nirvana

Well I checked my voltages and caps and resistors, all were normal. I checked for bad solder joints, could not really find any. I did touch some up since I was in there. Tried a different tube in the PI, no change. Added back the PPIMV so now I have main volume to drive the PI and the PPIMV to drive the PA. It is not in the PA at all. In fact I can not tell any difference at all in drive/gain when adjusting the PPIMV. If I reduce the bass it cleans up the buzzy/fizz a little. Also when the NFB is off it is even worse.

Any other thoughts?

_________________
Les

http://www.soundclick.com/unimind

After further analysis, it is the EL84s (thanks Emohawk). When I set the PPIMV at 1, the fizz is gone. I still have some flubbiness but no fizz. When I turn up the PPIMV I get the fizz. I will post some clips later.

_________________
Les

http://www.soundclick.com/unimind

So here are some clips.

Here it is with the PPIMV set at 1. There is no fizz.
http://www.unimind.us/PPIMV_1_No_Fizz.mp3

Here it is with the PPIMV set at 10. You can hear the fizz.
http://www.unimind.us/PPIMV_10_Fizz.mp3

Now I played around a bit to find the right spot for the PPIMV and found that with just the bright ch dimed and no cascade, if I set the PPIMV at around 8 it sounds great.
http://www.unimind.us/Little_Dreamer.mp3

_________________
Les

http://www.soundclick.com/unimind

I found some info that might interest you on the music electronics forum

My amp also does kind of a weird fizz on notes or chords when it is pushed after the brown tweaks- with the stock MV in mine it is noticeable over 5 a little bit and more noticeable when dimed - seemingly worsening as I push the power tubes as you surmised with your amp. I did not really notice it much until I got my attenuator as the amp was so loud before it was getting kind of drowned out. But now with the attenuator I seem to notice it a fair bit because I can play it at sane volumes - clip below. See if you notice what I mean with kind of a high end fizz riding on top of the chords particularly as they decay - sort of similar to your issue

http://snowboardmaterials.com/tmb_fizzz.mp3

In terms of addressing it - below is a cut and paste of a post from one of the gurus there and a link to the thread following that - the guy posting it is not a huge fan of EL84's or low watt amps in general so the thread is kind of funny cause the guy is a real basher. Nonetheless he posts some interesting ideas

"There ARE ways to make EL84's behave. The biggest reason I know of for EL84's buzzy/fizzy reputation is that they grid load very easily causing crossover distortion. Second to that is the fact that most of these amps are cathode biased. So as the amp pushes harder the bias cools. If things aren't set up just right this causes even more crossover distortion. Combine this with high gain and the need to crank them for volume and it's a mess really. The other option is to NOT push the power tubes that hard and generate most of your distortion in the preamp. Either way you get thin, buzzy/fizzy tone.

Bias on the hot side. EL84's don't seem to mind.

Use the "Paul Ruby" mod to stop grid loading.

Don't run plate volts excessively high.

Use a zener across the cathode resistor to "fix" the bias at the onset of clipping.

If needed you can use a small conjuctive filter to pull down excessive overshoot spikes on OD. With an 8k priZ a 10k resistor and a 1500pf cap only effects the spike and won't dull your tone.

Use an OT with some meat on it.

Since you can now smash the PI and power tubes without fear of crossover distortion or excessive overshoot spikes there is no need for excessive preamp overdrive.

A good efficient speaker can add more volume than trying to push the power tubes into more watts.

Ta Daaaa... Now your "pair of EL84's" amp won't offend Bruce

Uh oh... Did I give something "secret" away like Randy Fay?

Chuck"

http://music-electronics-forum.com/t19503/

Thanks for the replies guys. I will read up and see if anything there helps.

However with the PPIMV backed off a bit this thing sounds killer right now.

http://www.unimind.us/Mean_Street.mp3

_________________
Les

http://www.soundclick.com/unimind

Guys, I've now been working with 18W amps for more years than I can remember. There have been instances of people finding a kind of fizziness with heavily overdriven EL84s. A conjunctive filter or Zobel filter on the OT is a good way of fixing that small issue. The Paul Ruby fix or having a zener on the cathode will only work in amps that don't have a VVR/VRM or other voltage-based power scaling. If anyone does want to try it, the zener should e chosen to be a couple of volts higher than the cathode voltage at idle. In any case the Paul Ruby fix is for a completely different problem, which sounds mechanical like something loose rattling inside the amp. And one more thing for the record, the vintage-style 18W OTs, such as provided by Trinity, are oversized and certainly qualify as "an OT with some meat on it".

OK, having said all that, Unimind's amp's problem, going from that MP3, is something far more severe than anything that's been mentioned here. It is in fact a sick amp. Like I said before, one possibility is a faulty tube somewhere in the chain. However, Emohawk's post led me to mention one other possibility, which I have also seen occasionally. That clip sounds very much like a case of extreme blocking distortion. If the tubes are OK, this kind of severe blocking can be caused by parasitic ultrasonic oscillation overloading the grids of certain stages in the amp. The way the problem goes away when the MV is reduced, would also tend to fit that possibility.

The cure is to work on the lead dress, use shielded wire in strategic places, and also take steps to reduce signal being radiated from the OT into the rest of the amp circuitry.

PS: That arrogant comment from Bruce Collins is absolutely outrageous. I've suddenly lost a lot of respect for the guy. There is also quite a bit-more stupidity and mis-information in that thread.

_________________
Great tones come in small glass jars!

It is a pretty sick amp isn't it.

That's what I think as well based on what I have been reading. I have checked everything in the amp now. Voltages are fine (in fixed bias the bias voltage was a little low so I re-biased it), resistor and cap values are fine, solder joints are fine and tubes appear to be fine (but I do not have any other el84s to check). I thought my lead dress was pretty good. I can post some pics and maybe you could point out where potential problems may be or where I can improve things?

_________________
Les

http://www.soundclick.com/unimind

Here are the updated pics of my amp.

Front: http://www.unimind.us/images/DSCN7604.JPG
Preamp side: http://www.unimind.us/images/DSCN7605.JPG
Power side: http://www.unimind.us/images/DSCN7606.JPG

_________________
Les

http://www.soundclick.com/unimind

Hey Zaphod - my amp pretty closely replicates Unimind's buzzing sound in spite of not using the same PT and OT - mine actually does it slightly on the Normal channel too when it is cranked. Could very well be shitty soldering or lead dress in my amp as I am pretty green (my amp is quite hissy as well), but none the less if I play muted strings like he did it has that same buzz trailing the notes (more noticeable when it is attenuated).

I'm curious if perhaps all the brown component changes, with boost on, every control dimed we are perhaps just pushing something too hard that is a little abnormal for a typical 18 watt (seemingly in the power section judging from Unimind's ability to dial out the buzz with the PPIMV)

After finding that thread I posted above yesterday I thought it might be relevant. I researched the Ruby mod looking for a total noob layout or instructions i.e. "hook this value diode with this value resistor to this pin etc..." - something a moron who can solder could follow - i.e. me - but was unable to find an idiots guide to the Ruby mod

Found a couple schematics but seemingly always with the caveat "check with your scope for clipping", or a description of the mod in a thread where the reader is intended to have a better understanding of the electronics than I do.

So, if you can help me with complete noob instructions and parts you think would suffice I would happily guinea pig the additional mod. I have also ordered parts for a PPIMV too just to see if I can also dial out the buzz as Unimind did.

One more dumb question - this month's guitar player features an amp mod the author calls a "snubber" that sounds like it is intended to perform similarly to the Ruby mod - "creates a filter on the primary side of the OT to remove unwanted high frequencies while leaving the rest of the frequencies untouched" - lists two 3kilo ohm 5 watt resistors, two .0047uF capacitors, and two 8.2 megaohm resistors wired as in the picture below attached "to the OT primary leads that connect to the tube sockets"

see attached photo/article

Whats your 2 cents on this vs ruby mod?

Lindsay

Zaphod, you had mentioned in the brown mods thread that the grid stoppers on el84s should be 10k and that you were thinking of trying 15k or 22k. Would that help maybe tame the signal a little on the input to the el84s? Effectively the same as dialing back the PPIMV a little?

Thanks

_________________
Les

http://www.soundclick.com/unimind

I was also thinking it was the brown mods that caused this but when I went back and listened to my clips of the amp before the mods, the buzz was already there. I think most people just do not push their 18 watts this hard and therefore do not hear it. I have found lots of cases on the net where someone is asking for help with buzz in their 18 watt when they crank it all the way up. Unfortunately I have not found any real solutions to it.
If you are interested, here is the PPIMV I implemented except I use a 1M dual pot.

_________________
Les

http://www.soundclick.com/unimind

Thanks for the link - I already ordered a pot etc and have a few examples of Lar/mar and other PPIMV types that I bookmarked as well so I am confident I can hook it up.

what do you think about the Ruby mod or the link I posted from GP mag? Both seem possibly relevant to the issue we are having. In my case I just am a little too green to figure out the particular resistors and diodes necessary to hook a Ruby mod up hence my request for help. The guitar player article is pretty simplified and even I could do that and may try hooking it up just to see how it sounds and what it does

You have more electronics knowledge than I .. maybe try hooking one or both of them up to see if they help (or help me with a layman's Ruby mod parts list and idiot proof layout so I can try it - ha ha)

No,
You probably wont notice any difference at all. All those resistors do is provide stability (lots included in that statement) and at high values (I'd say higher than 22K) roll off high end.

_________________
http://pdfelectronics.com

Try the PPIV MV with 2M2 resistors strapped from lugs 2 to 1 on both pots. Changes the taper slightly and if the pot fails the resistors prevent the tubes from running too hot and then redplating and doing other nasty stuff.

_________________
http://pdfelectronics.com

What lead me to that instead of the Larmar is that it is DC isolated.
I guess from what I read the Ruby mod would work but I don't know enough to figure out the values needed for the diodes. The other problem I have is that I have switchable bias from fixed to cathode bias so I think that complicates things.

Not sure about the Stubber mod from GP but from what I read it is like a damper on the highs like putting a blanket over your speakers. Doesn't sound good to me.
I am not sure about that. I have only been learning about amps for about 8 mos to a year and only built one.

Other things I have read is to tame the output of the PI to not drive the PA tubes so hard. Effectively what the PPIMV is doing. So I am leaning more towards making adjustments in the PI section as a fix. Still researching right now.

_________________
Les

http://www.soundclick.com/unimind

Yeah I tried it up to 22k. Had no effect.

_________________
Les

http://www.soundclick.com/unimind