Distortion Behind the Note Problem - Could use a little help

[Blind Lemon]

Kind of slow over at Amp Garage.

Trainwreck Rocket type, I've built about a dozen of these. First one I've heard this rumbling noise/distortion behind the note as the note decays. Thought it might be speaker rub but it is not, run it though a different box and same/same.

If you want to check out the thread:  http://www.ampgarage.com/forum/viewtopic.php?t=12215

But long story short, checked all resister values, changed tone caps, touched all suspect solder joints and changed to a know set of good tube out of an 18w Plexi.

Anybody run across this noise/distortion before? Seems to be freq dependent as F# on the G string is the worst note.

BL

[John]

Someone else on here had that problem recently, and IIRC it did turn out to be a bad joint. I think they found it by chopsticking. Good luck! :)

[Blind Lemon]

I can't believe you got Charlie Brown's pic  on there................I was just playing Linus and Lucy on acoustic.

I'll try that again.

BL

[RicharD]

>Seems to be freq dependent as F# on the G string is the worst note

Sounds like resonance to me.  I did some experiments with 6BG6 output tubes.  They resonated on about 1/2 a dozen different notes.  Check by trying some different output tubes.  The next step (since you've already chopsticked) would be to use a listening amp and try to isolate the source.  Does it do it at all volumes or just when loud?

[Blind Lemon]

Pretty much at all volumes and I've got the pre-amp cathode by pass caps on a switch also and it does it with them in or out ( high/low gain so to speak). Don't have a listening amp.........guess I should make one one of these days.......soon.

I use the old Saratov EL84s and I've got 4 quad on hand so I tried a different set to no avail.

BL

[RicharD]

I don't know what your schematic looks like.  Are the outputs cathode biased and is there a bypass cap?  If yes, try lifting the bypass cap and see if it goes away.

For a listening amp, I use what ever other "working" amp happens to be around.  My probe consists of:

The better 1/2 of a broken geetar cord
a .1uF cap
The better 1/2 of a clip lead
& some electrical tape

I built mine to troubleshoot a Hammond organ tone generator.

[labb]

Check all of your ground connections. A cold joint in a ground can cause that.

[Fresh_Start]

I'm 1/2 way between Butter and Hoffman's dedicated listening amp.  You can build a very simple box that let's you listen to your amp's signal at different points in the circuit.  Scroll down to the bottom here for Hoffman's listening amp box:
http://www.el34world.com/Hoffman/tools.htm

You just can't beat the listening amp for finding trouble spots quickly.  I've even used my signal generator to inject a tone part way through the circuit and then listen at some subsequent point.  Divide & Conquer!

Here's my take on the listening box:
http://www.el34world.com/Forum/index.php?topic=3950.0
I just plug it into a small, solid state amp I don't use anymore and keep in the shop.  Make sure you always turn the volume down to "0" on the box before turning things on though!

Here are Hoffman's tips on tracking down weird noises:
http://www.el34world.com/charts/fenderservice6.htm
Unfortunately it doesn't seem to address your particular type of noise.

I hate trying to track down what turns out to be a bad ground/solder joint.

Cheers,

Chip

[Geezer]

touched all suspect solder joints

Time to go for the "un-suspect" joints....IOW, suspect them all.

[FYL]

Anybody run across this noise/distortion before? Seems to be freq dependent as F# on the G string is the worst note.

Could be caused by a dry solder joint, a defective filtering cap, an amplifier mechanical resonance or a combination.

[jjasilli]

search this forum, google, and/or www.geofex.com for "parasitic oscillation":  can be mechanical, including bad joints, or tube as others have posted in this thread; lead dress; post-PI suppression cap needed.

[phsyconoodler]

I built a 5F6A bassman once that had the same issue.It had no grid stoppers in the drive lines.I added 1k resistors and it stopped the distortion as the note faded.Try upping yours til it stops.
  I should add that the original 5F6A did not have any grid stoppers.Some need then and some don't.But it sure sounds like the same problem.

[tubesornothing]

Parasitic oscillation.  Follow jj and psyco's advice.  If not corrected by the grid stoppers, then chopstick the hell out of it - including wires.

[Blind Lemon]

Not gone...just working on it. 

1.5k grid stoppers have been on it from the start.

I'll get back.

BL

[phsyconoodler]

Make em bigger.Try up to 50k.If it alters the tone,go smaller.

[jjasilli]

Also, I forgot to mention that if all else fails NFB, or more of it, might help.

[Blind Lemon]

I take it back.......I can still hear it.  I don't know if its me and I'm just listening hard for it, but I can still hear it. Guess I'll try putting larger grid stoppers on the power tubes tomorrow.

This sucks.....

BL

[jjasilli]

This sucks..... It's just part of the drill.  Happens to everyone sooner or later.

[ac427v]

My 5F6a Bassman did that too. It started about three years after I built it. I chopsticked the board and pots twice--nothing. Added grid stoppers. Changed tubes. Changed power supply caps--you can tell where this is going? Just by accident I touched a wire going to a power tube base and got a half second of static. Yup. A cold solder joint on the tube socket was the culprit. It only made ghost notes on G or G#. Two seconds with a hot iron and 20+ hours of troubleshooting was done. Good luck!

[terminalgs]

Butterylicious mentioned the cathode bypass cap on the output tubes.  This is was I thought of when I first read your problem.   do have one?  if so,  lift it, or replace it...

[Blind Lemon]

Completely took them out of the circuit and the noise was still there. Moved a couple of grounds and cleaned power tube pins and sockets. Seems when it was making the noise if I moved the two inside EL84s it would stop so I cleaned the tube sockets. Seemed to work.........we'll see.

BL

Thanks to all that made suggestions.

[dwp]

I've had this problem, recently. I fixed it. I removed the shielded cable on the input jack. for whatever reason, my shielding was creating slight oscillation. grounded at input jack end, not the tube, was careful about trimming and heat shrink, could not just cut the shield lose, removed it and replace with single conductor, routed carefully over top of board components, problem solved. I did it twice to make sure......was the shielded conductor....

[Blind Lemon]

Yes it was, I got it quieted down. I wish I had greater expectations for the noise to stay gone. We'll see.

BL