2. tech-talk
  3. 168924-147977

Help me diagnose a terrible new noise from my tube amp!!!

https://photos.app.goo.gl/QCjYHvuMhkArQiH88  

This is the left monoblock of my 100wpc Wavestream Kinetics Push-Pull amplifier, made around 2010, but diagnostically checked by amp's designer when I purchased it earlier this year.  It uses KT88s driven by EL34/KT77s and 6922s for input tubes.  Each EL34 drives two KT88s.  Bias is manually adjusted with dedicated knobs and a central meter.  

This noise is transmitted through the left speaker and is accompanied by flashing power tubes to the rhythm of this noise and the bias meter for that tube bouncing with that same rhythm.  No blows fuses.  Noise happens even with amp disconnected from preamp.  

This sound started on the left monoblock during loud passages in music last night like a temporary clipping or distortion sound, then overtook the music when played at higher volumes.  I traced the problem (or so I thought) to a bad EL34 tube, so I replaced it (and its counterpart as well) with KT77.  This made the noise go away for a while.  However, when I check the bias of this new KT77, it was near zero.  When I attempted to bring it back to its spec of 40mA via dedicated knob, the horrible sound came back.  If I leave the bias at 20mA for that tube, noise goes away. 

What the hell?

Now, for context, I replaced the coupling caps on both of these monoblocks a week ago (with caps to designers spec) and I may have screwed up the soldering somehow.  But I played these amps for several days at normal bias without any issue until last night.
redwoodaudio
redwoodaudio OP
413 posts
 
Or, from same thread:

Motorboating can be caused by having resistors of too low a value in the B+ supply. I.e. if you have a 1K5 between two preamp stages’ filter caps, you may get motorboating that could go away if that R was increased to 10K.

It can also be caused by a power supply that is not decoupled enough (too many stages being fed by the same PSU node).
And 

One caveat, motorboating happens too fast to see in a tube as a rule. If the usual techniques don't help, I'd also look for a wrong connection in a gain stage or going into the inverter. That can also produce a popping but much slower. A quick check for that is adjusting a control will reduce or intensify it.

redwoodaudio OP
413 posts
 
Or alternately, from geofex.com:

Sound cuts out or squawks on loud notes

This is why there are professionals…
pauly
1,012 posts
 
Coupling capacitor vs. decoupling capacitor. Different things.

BTW, This is the first time I’ve heard of motor boating. Cool beans - I’ve just learned something new. 🙂
atmasphere's avatar
atmasphere
11,590 posts
 
One other way motorboating can occur is if the coupling caps are a larger value than they should be. If they exceed the frequency poles in the power supply, the amp will be able to modulate the supply and the result can be a low frequency oscillation- motorboating.


Whenever I replace coupling caps in something I'm servicing, I replace them with the same value.
redwoodaudio OP
413 posts
 
Interesting. Amp's designer told me to replace 0.22 caps with 0.1, so they're actually smaller value, unless there's a decimal point mixup somewhere. 
More to discover