Rectoverb resonant frequency problem

[94Tremoverb]

I'm getting a serious resonance problem with a Rectoverb combo (series 2) - when I play a palm-muted C power chord on the red channel at anything above medium volume, there is a loud after-resonance 'bloom'. It's not a microphonic tube, or anything else in the amp section - eliminated by running the chassis out of the combo with a long speaker cable. The trigger frequency changes to E when the combo is run with the back off, but the after-bloom sounds the same (C). When run with the back on, you can feel a huge blast of air coming out of the port when it happens, and not at any other time, so clearly the speaker is working massively too hard at that frequency.

I've also tried replacing the speaker, both with another Black Shadow 90, and also with a Classic Lead 80 which is closely related but I hoped might be just different enough to either cure it or at least change the problem frequency, and at worst prove it was the speaker... no change. I also tried running the amp with EL34s just in case... no change. And it's nothing to do with the reverb - it's turned off.

Has anyone else found this? Is it a problem with the speaker type, or some sort of bad interaction between the power stage and the speaker? I haven't tried other speakers yet, but I don't want to if I can avoid it because it sounds so good otherwise. I also don't want to change my settings! It sounds exactly how I want when it's not doing this.

 

[fluff191]

Change your power tubes. It still could be a microphonic power, as the tubes take alot of abuse in the combo form being right behind the speakers.

In addition, I would also change the power tubes to see if the low end is shifted or reduced. Maybe the set just has alot of low end, ya know? If that doesnt work I would then start looking at the preamp tubes.

Good luck.

 

[94Tremoverb]

Thanks for the quick reply! I did change the power tubes - I even replaced the 6L6s with EL34s to be sure it's not a characteristic of the 6L6s. It's *definitely* not tubes. I've tried running the amp section well away from the combo, done a tap test on every tube, and on all the components inside too, using a plastic screwdriver handle. Nothing, apart from the usual (and very different-sounding) clang on V1.

 

[NoGlassNoClass]

...trigger frequency changes to E when the combo is run with the back off, but the after-bloom sounds the same (C). When run with the back on, you can feel a huge blast of air coming out of the port when it happens, and not at any other time, so clearly the speaker is working massively too hard at that frequency.

Sounds like an interaction between the speaker and enclosure. I'm not really familiar with the ROV combo, but I am assuming from your post it is closed-back front-ported, correct? Is the port just a slot cut out of the baffle board or does it have a plastic tube going into the box? If it has the tube, try cutting the end off at an angle. This way the port will not be "tuned" to any particular frequency.

 

[94Tremoverb]

No, the port is a rectangular hole in the back panel, directly behind the speaker and about 10" x 4", much smaller than the usual opening in the back of a combo. Removing the back does seem to change what chord starts it, but the final pitch of the resonance is the same, so I don't think it's directly a result of the port.

I think it's some sort of electrical resonance between the amp and the speaker - it still does it, though slightly less, when I run the amp section through the speakers of my Tremoverb (Vintage 30s). The Tremoverb doesn't do it through either speakers.

 

[94Tremoverb]

Got it... damn. It's the negative feedback being switched off in Modern High-Gain mode. It doesn't do it in the other modes no matter what gain and volume settings, and if I temporarily bypass the switch in the Modern mode, it stops too - although then it sounds almost identical to the Vintage mode. It's obviously allowing the speaker resonance get out of control when the damping from the NFB is removed. Of course, it's exactly the lack of damping that makes the amp sound so huge in this mode!

I wonder if I can adapt to FX mix control (which is disconnected since I made the loop series) into a resonance control for the Modern mode, and find a happy medium...

 

[NoGlassNoClass]

You could try a speaker with longer travel like an EV or similar...should be able to handle the reduced damping factor of the modern mode.

As far as the variable NFB mod, sounds like a good idea but afraid I cant help with the how-to.

 

[94Tremoverb]

No worries, I already worked out how to do the variable NFB a couple of hours ago when I did the trial relay bypass . You just need to find the two resistors either side of the relay and link those points via the pot. I've just finished doing it. It works, very nicely - although I had to use a 1Megohm pot instead of the stock 25K to get the right range - but of course as you dial back in the NFB, the amp starts to lose that massive thump at the low-end... which is exactly what's so good about the sound, when it's not resonating.

At house volume it seems like there is a usable compromise, but if it doesn't work at band volume I had considered trying an EV, too - a friend has one spare. The problem is that it will change the tone, as well as adding quite a bit of weight to the amp!

What really puzzles me is that I haven't found any mention of this problem anywhere - I came here to the forum expecting to find it to be commonly known, since it's clearly a design characteristic of the amp. Or maybe this one is just unusually unstable?