When I crank up the tremolo on my new amp the speaker begins to throb - I can see the aluminum cap through the grill cloth pulsating as well. And it gives off an audible noise. Like a slight thump.

Is this normal? I'm new to the brownface tremolo game.

Yes it is.

Rev

Canadian Surf

http://www.urbansurfkings.com/

You should take a video.

Are you talking about how it throbs all the time? Even with no guitar plugged in?

Only throbs when the tremolo is engaged at a quick setting. When tremolo is disengaged it all sounds normal. Perhaps because I hadn't had the chance to really play it loud to drown it out - but at low volumes and when I'm sitting near the amp noodling guitar it does seem rather loud.

I had the same issue with my brown bandmaster. I fixed it with adding a high pass filter into the tremolo circuit. So these low frequencies from the tremolo oscillator are blocked. A tech could do that. It is fully reversible. It fixed my issue perfectly. I can run the intensity at 10 with no throbbing at all even with a cranked amp.
Cheers
LoeD

here comes the WEST SAMOA SURFER LEAGUE

Xavi, how do you like the Eminence Quilter Custom 15 compared to the JBL D130F soundwise?

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I guess I'm not understanding what exactly the problem is and scratching my head here reading these posts. People are modding amps and swapping speakers because the tremolo is causing the speaker to move when turned up? And it bugs you if you stare at it through the grill cloth? I've never owned a vintage amp, but have 5-6 clones with tremolo that all do this. And they sound fantastic. And when I'm playing through these amps, I definitely don't want my head to be close enough to notice a speaker throbbing. I'm just trying to wrap my head around what you guys are trying to accomplish here..

I have 2 Surfers, a Tahitian, a G-Reverb, Two Rock Studio Pro Plus 35 and a Bluetron Blueverb all with tremolos. The blackface style are indeed different, but the vibe style brownface trem I too see the same thing. And my D130 is running strong, and my 2x15 with Weber Chicagos is chugging along as well. I'm no expert either, but I wouldn't want to modify the brownface style trem to behave more like a blackface trem. They're tonally different and I like the variety. And I'd be willing to bet that if someone asked Dario he'd say it's supposed to be that way. The brownface trem to my ears reminds me a lot of a uni-vibe. I have a uni-vibe clone and now I'm tempted to plug it up when I get home to see if it does the same as well. But I honestly think what it's doing with the speaker is required to give you that sound. And I wouldn't change mine for any reason at all. If I started going through speakers weekly, well then I might look into it. But the fact that nobody here has had any problems occur, rather it seems to be an annoyance is why I don't understand the need to modify it. My $0.02.

Dear all,

this is going to be a longer post.
Once again, I had the problem with throbbing speakers with my 6G7A Bandmaster with the 2 1/2 Tube harmonic vibrato) To explain the reason I have to go a bit into detail...

Attached I have a schematic of the vibrato channel of the 6G7A. I highlighted the area where the Tremolo effect is created. There you have a tube on the far left side with a feedback loop that gets into self-oscillation. You can control the speed of the oscillation by changing the resonance frequency of that circuit and the amount of it forwarded to the mixing section (intensity). After that, the sine shaped signal coming out of the LFO (low frequency oscillator) is split in two sine waves that are out of phase with each other. This is the main trick of that circuit!
When we proceed to the far right end we see a mixing stage where both signals are mixed again with each other. (We do not focus on the guitar signal which is split into two frequency bands and then “tremoloed” individually (what makes that famous vibe like sound) before it gets mixed together again)

In idle operation we only have the two oscillating signals that are mixed together. What happens when we mix two identical but phase inverted signals? Both signals are cancelling themselves out.
This is what should happen in theory, but not always in real life and that´s why we are talking here...
What can be the reason for imperfect cancelling?
Both signals are not identical in amplitude or not exactly 180° out of phase or both do not have the exact same sine wave shape.
All that can happen as the parts used to create the sine waves are not perfect. We have slightly deviations of resistors, tubes, capacitors...
In the end we get a cancelling of the two phase inverted signals but only up to a certain amount. What remains is a part of the LFO signal. This goes through the power section and throbs your speaker.
Now we know why this happens.

Next step is to find a solution to fix it.
I first tried to get the sine waves into perfect shape and size by changing the amplification factor of the inverter stage, by changing the tube, coupling caps…
It didn´t work the way I wanted it to. I could reduce the throbbing, but only a bit.
So I focused on another approach.
If I cannot get perfect cancelling of the sine waves, I filter the remains out of the signal!
The LFO frequency is only a few Hz, (max. 10 Hz or so) so a high pass filter should be able to filter that out without affecting the guitar signal (starting at ca. 80Hz)
And that´s what I did: I selected a second order RC high pass filter that cuts around 30 Hz. I added one filter each on the anodes of the mixing stage tube.
I put all the Rs and Cs on a small tag board located next to the original eyelet board of the amp and hooked it up with wires.

Conclusion:
This is a minimal invasive operation and it is completely reversible. You only need to unsolder two connections on the eyelet board and attach the filter.
It does not change anything regarding sound of the amp or sound of the tremolo. It only eliminates the throbbing.

If you need more details, just ask!

Cheers
LoeD

here comes the WEST SAMOA SURFER LEAGUE

Thanks for the explanation LoeD. I think depending on how often do you use the tremolo and specially how deep, this mod can be very useful. Is not that I have broken several speakers due to this, is that I have been avoiding passing the 5-6 on the intensity of the tremolo because the fear of breaking the speaker.