Dual Rect 3-Ch Solo Head, Ch 1 Oscillation Pushed mode

[nevetslab]

I have a client's Dual Rectifier 3-Ch Solo Amp head on the bench, and besides it having a 15 Hz Oscillation in SS Rect Mode, which for the moment, I've removed the rectifiers (old version, 5U4GB's ALWAYS in circuit), Channel 1 in Pushed mode can be dialed up with a wide range of self-oscillation conditions...NOT just that mentioned in the manual (Gain at max, don't go past 2:30 on the Treble). Gain, Treble, Presence, Mid as well as Output & Effects Send Level controls all can affect affect this self-oscillation. Oscillation freq ranges from low-mid freq (200Hz) to beyond 20kHz. Nothing plugged in, 4 ohm speaker connected, Effects Loop In, controls at Normal, though it can also occur with Effects Loop Bypassed.

Swapped out V1 input tube, V4 Effects Loop tube, V2 2nd stage post-EQ tube. Switching out of Pushed mode seems to render it safe. I haven't pulled the main board yet. Looking for other posts from folks having had similar issues.

With regards to the LF oscillation associated with the 5U4GB's installed, if either Ch 2 or Ch 3 are selected, AND the voicing switch is changed to MODERN, it will kill the LF oscillation. Switch back out of it (Vintage, RAW), the LF Oscillation resumes.

Wierd!

 

[afu]

I have a client's Dual Rectifier 3-Ch Solo Amp head on the bench, and besides it having a 15 Hz Oscillation in SS Rect Mode, which for the moment, I've removed the rectifiers (old version, 5U4GB's ALWAYS in circuit), Channel 1 in Pushed mode can be dialed up with a wide range of self-oscillation conditions...NOT just that mentioned in the manual (Gain at max, don't go past 2:30 on the Treble). Gain, Treble, Presence, Mid as well as Output & Effects Send Level controls all can affect affect this self-oscillation. Oscillation freq ranges from low-mid freq (200Hz) to beyond 20kHz. Nothing plugged in, 4 ohm speaker connected, Effects Loop In, controls at Normal, though it can also occur with Effects Loop Bypassed.

Swapped out V1 input tube, V4 Effects Loop tube, V2 2nd stage post-EQ tube. Switching out of Pushed mode seems to render it safe. I haven't pulled the main board yet. Looking for other posts from folks having had similar issues.

With regards to the LF oscillation associated with the 5U4GB's installed, if either Ch 2 or Ch 3 are selected, AND the voicing switch is changed to MODERN, it will kill the LF oscillation. Switch back out of it (Vintage, RAW), the LF Oscillation resumes.

Wierd!

You could call Mesa. They're normally very helpful.

The oscillation could be from the PI at V5. Modern mode and Channel 1 have no negative feedback. I'm not exactly sure how or what is the problem, but a failure in the PI could cause LFO and if the feedback to V5a is not working properly, it could make it worse.

I'm only a hobby amp guy and my cognition is affected from rheumatoid arthritis, so I'm not feeling like the best source, but the PI was my first thought. Then I looked and saw this: http://vintageamps.com/plexiboard/viewtopic.php?f=28&t=98658&start=15

Also the schematic is available all over the place, including www.schems.com .

Good luck.

 

[nevetslab]

After having spent the past 3 months moving, I finally got the Dual Rectifier 3-Ch Solo head back up onto the bench yesterday. Where I left off in December, this amp has a 13Hz LF Oscillation when in SS Rectifier mode, about 3.5V RMS output, with the Output Level control being unable to change it at all. Switching to Tube Rectifier mode OR Ch 2 or Ch 3 with the Voicing Switches set to MODERN will kill the LF oscillation. Otherwise, switching out of Standby, it will start up within a few seconds. What you hear is the 39Hz 3rd harmonic, down around 24dB. But, enough to make you feel a little seasick, and add dissonant notes to your playing.

I didn't see it yesterday, having it on the bench, connected to my 8 ohm dummy load. This morning, I fired it up, and changed to my 4 ohm test speaker (Ampeg BXT-115HL4 cabinet), plugged into the 4 ohm jack. LF oscillation started right up (about 3.5V RMS) coming out of S/B. When I patched the speaker into the 8 ohm jack, LF Oscillation was gone. Tried an 8 ohm speaker, still NO LF Oscillation. Patched that back into 4 ohm output, LF Oscillation is back.

I had previously swapped rectifier tubes with no change. It was an earlier revision, where the Rectifier Mode switch was a SPST toggle, with the SS rectifiers ALWAYS connected to the anodes of the 5U4GB anodes.....and only switching in across the tubes in SS Mode. I had replaced that switch with a DPDT, wiring them with the cathodes connected, switching the HV taps to either the tube or SS diode anodes. That didn't make any difference with this LF oscillation. With the 5U4GB tubes removed, NO LF Oscillation.

So, this LF problem persists. I had looked at the Dual Rectifier 3-Ch Solo head in our rental inventory (CenterStating, LLC) in December, and it did the same thing.....though I haven't verified it only oscillates using the 4 ohm tap. That was today's finding.

 

[nevetslab]

More fuel for the fire. My test speakers are at the end of the long 33' x 10' shop I have here (used to be a cluster of closets, that later had the walls removed for a larger odd-shaped storage room. So, my test bench being at around mid-point in the room, I've lived with a 20 ft cable to attach my test speaker to the subjects under scrutiny.

What happens under 'normal' conditions, amp atop local speaker, short 2-3 foot cable? Hah! NO LF OSCILLATION!! Plugged it into an old Crate amp that's been holding up coffee cups, spray cans and such next to my chair. Connected it to it's 8 ohm 12" speaker, and all I get from it is the other problem.....HF oscillation. Though that you have to get radical with the controls cranked to nearly max before it comes unglued....not normal settings. It was a lot easier to get the HF oscillation out of it with the 20 ft cable. So, adding additional LCR to the speaker and you get more of a tuned circuit, which this Mesa apparently likes for LF sustained oscillation.

It also WILL NOT oscillate in resistive loads, even with a 20 ft cable attached. Another reason they're called 'dummy loads'!

I may get this amp out of the shop yet. It now seems to be left with the criteria one of Mesa's staff commented about: Richard Duvall @ Mesa puts it like this..."We design the amps to have as much gain as the customer can get in trouble with"

 

[domct203]

It now seems to be left with the criteria one of Mesa's staff commented about: Richard Duvall @ Mesa puts it like this..."We design the amps to have as much gain as the customer can get in trouble with"

That's for sure!

I love my high-gain metal tones, but in 14 years of playing Mesa's, mostly Rectos, 2:00-2:30 is as far as I ever go with the gain knob.

I think once you wrap your head around the treble knob it all falls into place.

Dom