Fuzzy tone

[Leevi]

I'm wondering a fuzzy non-mechanical tone that comes from the amp when playing with bass strings.
The tone is related to signal level and disappears when the signal drops under certain level. The amp is
a single-ended amp with 12AX7 and EL84. Any ideas what could be the reason for that?
/Leevi

[SirElwood]

Blocking distortion?

[Leevi]

Yes, I'm thinking that the PT cannot supply enough power for the power tube which causes
this problem.
/Leevi

[tubesornothing]

Solder joint/poor connection.

[loogie]

I was working on a Silvertone 1482 last night.  It made a fuzzy sound on low frequency notes as well.  A bit worse than fuzzy maybe.  It turned out to be one of the cheap pressed sheet metal nuts on one of the input jacks was loose.  Probably not your problem, but exclude nothing.  Great sounding amp, but flimsy as hell.

[phsyconoodler]

I always recheck my work before I condemn a part like a power transformer.It doesn't take much of a power transformer to support a single EL84.
  I have also had a bad speaker voice coil cause a trailing distorted noise as the note decays.Especially on bass notes.Drove me nuts finding that one.
  What about a grid stopper resistor?Do you have one?
 

[tubenit]

a fuzzy non-mechanical tone

How did you determine this was a non-mechanical tone? Did you use another cab and speaker isolated from the chassis?  Or was that conclusion based on how it sounded?

The reason I mention that, is that I've had a couple of amps where I heard a weird noise that happened on bass notes ( or a specific bass note) that turned out to be some mechanical issue when I thought I'd ruled that out.

Maybe none of these apply, but I thought I'd share this hoping it might help someone out?

One example was that wire braid looking thing on a speaker was too big/long of a loop and on bass notes would rub the speaker cone. I unsoldered it, shortened it and resoldered and never had a problem since.

Another one was the sheet metal cover on a stereo amp was vibrating in the same room with bass notes, so when I tried another speaker and cab hooked up, it still did it & so I initialy concluded it was electrical & it wasn't. Moving the guitar amp to the other side of the room completely stopped the noise. Had a hollow bedroom door on a previous home vibrate once also kind of like playing an amp next to a snare drum. I had a vibrating reverb tank once that buzzed with bass notes being bolted down only on one side. Adding double stick tape and mounting screws on both sides resolved it completely.

I usually can figure out if it's mechanical or electrical but sometimes it's not been as easy for me as I would have expected. Some wood cabs can buzz also particularly with plywood or poor wood joints.

With respect, Tubenit

[Leevi]

Yes, I have a 1.5K grid stopper on EL84. I have even tried with 100K without success. I have reduced coupling caps, set bigger resistors on the signal way. Of course I have changed the tubes too. The tone comes with low notes only and last quite a long time when volume is high, it's like oscillation.
/Leevi

[Leevi]

Tubenit
I have tried this with three different speakers and one of them was outside of the box. I believe the source for this tone is in the amp. It is very much like bass resonance ("fart") which can be heard even with a 10" speaker. The more bass and volume the longer time it lasts. I have built several amps of this type but never met this kind of phenom.
/Leevi

[tubesornothing]

Did you chopstick the board?  Press on each solder joint?  Also check the jacks.

[Leevi]

Yes, I have done that. I have re-soldered each contact. I also have tried a more powerful PT.
I put in low frequency signal with a signal generator and noticed that the buzz tone does not start at all
if the bass is totally off. I used chopstick when the tone was on but didn't find anything.

I think the problem is related to blocking distortion where the low notes i.e. a low frequency together with strong signal level
cause the phenom. There is on the Aiken amp pages a description about that:

http://www.aikenamps.com/BlockingDistortion.html

Leevi

[Heinz]

What you are hearing is most likely parasitic oscillation. Since you ruled out the usual suspects I'd try the following:

• Use high value (e.g. 470k) grid stopper resistors for the preamp tubes. Blocking distortion can occur in every stage, not only in the power amp
• Make sure that the supply voltages of all amp stages are sufficiently decoupled, otherwise you get a certain amount of feedback through your power rail
• Try a different location for the OT
• Solder a 2nF cap between cathode and plate of the power tube to suppress the oscillation. Use a high voltage cap e.g. 600V or higher

Good luck!

[phsyconoodler]

Does that 2nf cap alter the tone much?If not,I bet it would help with a lot of fizzy EL84 amps.

[Heinz]

I have used it on PP and SE amps and could not hear a difference at all. I think Leevi used it in one of his builds, too.
2nF is just a guess. I tried several caps and that was the smallest value that did the trick without altering the sound.

Using a larger cap (and a resistor) takes away some of the harshness. Search for "conjunctive filter" for more details.
Here's a good page with sound samples: http://www.cs.cmu.edu/~ehn/ax84/

[Leevi]

Thanks for the tips Heinz,
So far I have tried 2.2n cap which didn't help. I'll come back later in the evening.
Leevi

[SirElwood]

This is a little off topic, but badcat minicat II has a 1.2nF cap between anode and g2 on EL84. What does that do?

[Leevi]

g2(pin9) = screen or g1 (pin2)?
/Leevi

[Leevi]

Seems to be g2 as you stated:
Leevi

http://www.schematicheaven.com/newamps/badcat_minicat.pdf

[Leevi]

I tried grid stoppers but unfortunately they didn´t help.
/Leevi

[phsyconoodler]

What about bias?What are your plate voltages and cathode voltage across the cathode resistor?
  Are you cooking the crap out of this tube?

[Leevi]

The cathode voltage is 4.7V with 100ohms resistor which means 47mA current.
the tube operates on its maximum limit. That is not the problem. I have changed the tube and reduced the cathode current by
increasing the resistance without any success.

I'm thinking that the problem could simply be that the tone stack lets too much low frequencies through and when the amplification
is high the result is this sort of bass resonance. The corrective actions should then be done in the tone stack.

The schematic I'm using:
http://www.blackhearttalk.com/pdf/BH5_schematic%20Rev%2020070611.pdf

/Leevi

[jojokeo]

In researching conjuctive filters and smoothing I came acoss this thread. Did Levi ever get to the bottom of this issue and get it figured out???

[firemedic]

I am following with interest as my 6v6 SE has what sounds like the exact same problem. I'm tweak-fatigued by now & can't get rid of this %#@&^ fuzz. I may try the conjunctive filter too.

[John]

As a very broad generalization, are SE amps more prone to this sort of thing than PP?

[Leevi]

Did Levi ever get to the bottom of this issue and get it figured out???

Not really. Now afterwards I have been thinking that the cause could be a bad capacitor.
I have met some problems with Silver Mica caps in couple of my amps.
/Leevi

[firemedic]

Doesn't a PP design tends to cancel out noise to some degree?

I want to try a conjunctive filter- Leevi did you try one? Some sources suggest a plate-to-ground C-R vs. the plate-to-cathode cap suggested here. Merlin has a grid-to-ground C-R but only in the setting of global NFB. Which I do not have.

I'm tempted to dial in a local C-R plate-to-grid NFBL on the 5879 stage to abolish the noise but doing so may kill the pentode chime I was trying for in the 1st place.

As usual the more I research the less I know.

Then it may be a bad cap....

[Leevi]

I want to try a conjunctive filter- Leevi did you try one?

No, I did't. Unfortunately I don't have the amp anymore. I'm curious to know if that had any effect.
/Leevi

[firemedic]

Well before I do any more mods I'll check my 250p silver mica tone cap- come to think of it I think my treble pot was a little scratchy. Thanks for the reality check!

[jojokeo]

A bad cap was something I was considering being an issue over the last few days thinking about this. Also other poss was the tube's pdiss being exceeded and also the grid leak resistor/bias issue and finally the overall other tube's voltages not being over or under-exceeded and finally maybe too high of a signal level being feed into the grid?  Curious on this issue being remedied and it's cause?

[firemedic]

AAAAAAAAGH!
One little 250pF cap being bad caused me to redesign my entire amp for no reason at all.
Short version: my fuzzy tone is fixed. 

[Willabe]

I've seen that a few times now, here, bad little pF silver mica cap. I wonder if there bad from the factory or if guys are over heating them with the iron?

One little 250pF cap being bad caused me to redesign my entire amp for no reason at all.
Short version: my fuzzy tone is fixed. 

Yeah, but you did gain new experience by changing it around.        Thats not a bad thing.


            Brad         

[jojokeo]

Another mystery solved - (saved to mental hard drive)

[firemedic]

I probably overheated it.
And the new version is more chimey. But still.