unintentional consequence

[shooter]

I had 3 gigging guitar guys test out my last build, LeGourmet’  SE 6550,  All three asked how I got that “tremolo sound”, knowing there was no trem circuit.
I had no answer besides “I designed it that way”. 

I ran across this link by K and referenced part of the article.  Is it possible the 1-5HZ is somehow “modulating” the tubes beam enough to modulate the audio signal? 

It is a very low-level nuanced trem sound, but all 3 guys defiantly hear it and also say it’s a good sound.
Nothing I’m gonna troubleshoot, just a curiosity, short of killing the cat


http://el34world.com/Forum/index.php?topic=14138.msg133084#msg133084

“In vacuum, electrons flow without audible noise of course, but some small electrical  noise may result from it sometimes,  sounding like "pt...pt...pt" at 1...5 Hz from the loudspeakers.  That's because the blue glow path's resistance is dynamic and has a negative resistance.   Negative resistance means,  the higher the current,  the lower the resistance gets.  So it changes it's path, resistance and appearance all of the time.  The blue glow may move and flicker from it.  With most tube there is no noise coming from this. “

[PRR]

That makes no sense.

A subsonic wobble is probably incipient motorboating. Insufficient B+ filtering between stages. It may be OK today but it will shift as the caps age, and probably toward outright MOTOboating.

[shooter]

Thanks PRR,  I was looking for "normal" blue glow vs gassy blue glow when I found the 1-5hz inside the glass and ....wandered!  I do only have 2 B+ taps, plate 100uF, everyone else,200uf,  so i'll tack in a 3rd for the pre and see.

[jjasilli]

To my ears a Bassman 5F6A also seems to do what you say.  If there is a tremolo-like affect then there must be a scientific explanation.  Something is causing LFO - low frequency oscillation -- either of volume (tremolo); or of signal phase (vibrato).  Amps are prone to oscillate. In fact it could be said that amps are natural oscillators and we need to stop, or at least damp, them. 


Amps are built of circuits each of which has its own natural oscillating frequency (Resonance).  Once they start processing a signal, they may also set-off oscillating at their own resonance frequency.  If the oscillation is only about a few times per second, it itself will not be audible (not until it reaches about 20 Hz).  But that LFO may cause a modulation in signal strength, hence volume modulation: tremolo.  Or possibly cause a modulating phase shift:  vibrato. 


Does your amp have negative feedback?  Speaker damping factors coupled with a global negative feedback loop can cause low frequency oscillation of the bias circuit.  IOW, the equivalent of bias-vary tremolo might spontaneously arise.  This is a problem in hi-fi, but might make a guitar amp seem to "bloom".

[shooter]

Does your amp have negative feedback?

Nope, except for my 5C1 build I have done all my SE & PSE without NFB, figuring the cathode biasing sorta-kinda acts as a NFB.
The musicians all distinctly said tremolo, I tried scoping the signal path but didn't find anything that stood out and the "effect" really only shows itself with actual playing, using a sig-gen, even sweeping the audio range didn't get me anything.  and like I said it's very subtle.  I've used different speakers.

I think PRR might be right about motorboating, Sluckey commented about the way I set up my PS also.  Originally plate, grid, and all the pre's came off the same tap C>R>C>R>tap.  Then I tweaked it for more B+ and moved the plate back after the 1st R.  No musicians have played since this final? version, but I can still hear it, mostly when I play at "bedroom" levels, but the amp is still 80% dimed, I just roll back the guitar to 1 or 2.

[jjasilli]

Yes, could be "pre-motorboating".

A pulse may produce a more noticeable affect than a steady signal.  Maybe hook-up a voltmeter to the bias circuit.  An analog meter might be nicer than a digital readout.  Use a low-end note.  Play a lo-end 16th note on the guitar, or use a momentary SW on a generated signal to make a short pulse of signal.  See if the bias voltage oscillates for a couple of seconds. 


Another test:  play a steady signal through the amp from a signal generator.  Monitor the amp's output AC signal voltage across the speaker (or dummy load) with a meter or scope.  While that's going on, simultaneously play a short lo-note guitar pulse through the amp.  See if the amp's output voltage of the signal generator's signal wobbles for a couple of seconds after the pulse of the guitar note. 


These two tests might help indicate where the warble is coming from.





 

[shooter]

I'm in the middle of finishing up an amp now and the snows gone!  might be a week or 2 before I'll check.  I'll post what-if-anything I find.  No analog meters left but my scope is pretty good, although 1-5hz low level might get lost in the weeds, we'll see.
thanks
dave

[darryl]

I have a question about the low frequency oscillation we have traditionally called "motorboating". Do any modern motor boats still make that distinctive "putt-putt-putt" sound?   

[shooter]

putt-putt-putt

Not a big fan of motors on boats, a hiked sail, a good breeze, the sound is more a swish..ssshhh kinda sound.  I did play put put though

[Willabe]

Not a big fan of motors on boats, a hiked sail, a good breeze, the sound is more a swish..ssshhh kinda sound.  I did play put put though

Yeah, yeah, yeah, can't catch fish that way.


                Brad     

[shooter]

can't catch fish that way

Dynamite off the port side, swing back around with net, problem solved 

[Willabe]

Dynamite off the port side, swing back around with net, problem solved 

 

[sluckey]

I've caught a few fish on my catamaran. 

[shooter]

I spent a lot of time floundering around when I first got my snipe, does that count? 

[jjasilli]

putt-putt-putt

Not a big fan of motors on boats, a hiked sail, a good breeze, the sound is more a swish..ssshhh kinda sound.  I did play put put though

With modern 4-stoke & jet boat engines, maybe we'll have to start calling it sail-flapping.

[PRR]

> Do any modern motor boats still make that distinctive "putt-putt-putt" sound?

No modern engines, that you can meet, run that slow.

You can make significant Power with a large engine run slow, or a small engine run fast. When materials and methods are crude, the big/slow engine is less trouble. As engine techniques improved, the small/fast engine became cheaper, also smoother and more thrifty (up to a point).

If I want to see a putt-putt I have to go to an Antique Engine Show.

There's always a hit-n-miss engine. When running free they go POW - - - - - - POW - - - - - - POW, many seconds between firings, and I have seen them turn near 130RPM (they do not fire every cycle when light-loaded).

The rock-guy has a factory engine with flywheel 9' (3m) diameter, piston the size of a trash-can. Runs very slow and probably has the power of a Ford Flathead.

The trend to higher speeds goes well back into the 1920s. The Allison of ~~1930 is a spectacular example of not-slow thinking, with 12 jugs and 4 camsticks and 4 valves per cylinder. It didn't sell well in boats, under-priced by cruder 4-bangers like truck engines, which were already as spiffy as car engines but speed limited for good life near full output.

While you would think there would be a putt-putt dingy in every old harbor, apparently when they give trouble nobody knows what to do, it is easier to get a used outboard motor and take the ton of cast-iron to the scrapyard.

There is an English/Indian Diesel, widely used for pumping, which is probably as near a putt-putt as you can buy new today. http://www.vidhataindia.com/lister.htm - http://en.wikipedia.org/wiki/R_A_Lister_and_Company