Mesa Dual Recfifier 3-Ch Solo Head, Ch 1 Oacillation problems

[nevetslab]

This is the same Dual Rectifier 3-Ch Solo head that has the LF Oscillation problem (tube rectifiers ALWAYS in circuit). But, that same amp also has an unrelated problem with Ch 1 being able to be create LOUD audio range oscillation by a variety of controls, NOT JUST THE GAIN @ MAX & TREBLE SET BEYOND 2:30 O'CLOCK. If the Effects Loop is engaged, all these controls (Voicing SW set to PUSHED, Gain, Master, Treble, Presence, Mid, Effects Loop Send, Output) can inadvertently cause the amp to burst into loud oscillation, ranging from 100Hz to beyond 20kHz! It will also oscillate with the Effects Loop bypassed. Usually the oscillation is between 20-30kHz, which emits a loud thump when it starts or stops (adusting Master, Gain, Effects Loop Send or Output).

When I had a reliable oscillation dialed in, and could start/sop it with the voicing switch or the Output control, I swapped out V1 tube (input and Ch 1 Master Output tube), let it warm up and switched back out of S/B. While that killed the oscillation with those settings, readjusting the controls dialed in a new oscillation! So, replaced V4 (effects loop tube). That didn't change anything, still oscillated. Replacing V2 (Post Gain/Ch 1 Tone Stack tube) didn't change anything. I just found I could get it to oscillate in the CLEAN Voicing Mode as well.

I haven't yet dug in to pull the main board up....very tedious task! Most of the component designation numbers are underneath the parts, and I don't have a PCB layout silkscreen to aid in this.

Has anyone run into this problem with Ch 1?

boogie_dualrectifier_3ch_solo_head.pdf

added note:

With enough tubes in place to make the amp run (removed V7, V8 power tubes, both 5U4GB rectifier tubes), when I dial the oscillation in....about 20kHz in this instance, it's not there at the plate of V1A input tube, but is there on V2A plate (2nd stage for Ch 1's tone & master volume), there at V1B, there on V4A & V4B (effects loop tube), and on thru V5 LTPI & power stage.

Removing V5 kills the oscillation all the way thru the chain. I replaced V5 with another 12AX7, oscillation restored. I hope this isn't in the PCB layout. That's about as far as I got today.

[nevetslab]

...We design the amps to have as much gain as the customer can get in trouble with.

During my search thru threads regarding other members attempts to tame these DR 3-Ch Solo amps when they take off on ya, I came upon this classic statement from Mars Amp Repair's post from 7-26-08: "FYI, Richard Duvall @ Mesa puts it like this...We design the amps to have as much gain as the customer can get in trouble with."

The Effects Loop gain stage following the Return Jack has about 32dB gain, and with the Loop engaged, even with the Send & Return pots set to Normal, this amp at present will break into oscillation quite easily in the 20kHz+ region. While you can't hear it, it's effects cause LF thumps, hum, and modulation of your signal. As I'm trying to cure this with no signal applied, the Master control is very sensitive to starting and stopping the oscillation.

Thus far, I've lowered the gain of V1b gain stage (ahead of the Master) by 6dB (tacked in 100k across the 100k plate resistor), did the same on V4b (Effects Return stage, tacking 120k across it's 120k plate resistor), and also lowered the gain on the input stage V1a in the same fashion. The result was, with the Effects Loop bypassed, I can no longer dial in oscillation with any of the controls of Ch 1. But, with the loop engaged, turning up the Send level, it will still take off and sustain oscillation.

I was looking to put a grid stopper resistor in on V4a, though my options there physically limited to either lifting up the grid side of the 100nF input cap, or finding the two voltage divider resistors, lifting both of them up, re-connecting them and inserting a suitable value resistor between that and the grid. I spotted a 100 ohm resistor following the Relay RY2b (selects either Ch 1 or Ch 2/3's Master Volume controls, routes them to either the effects loop input or to the input of the LTPI circuit). Found the 100 ohm resistor, removed it and tried a 100k resistor. Doubted this would work, since it's on the wrong side of the 100nF DC blocking cap ahead of V4a's input. Tried it anyway.

The result was it was much easier to set off the oscillation. Lowered the value a couple times before restoring it to 100 ohms, with no sucess. Then, looking at the 220k resistor following V1b, which feeds the top of the Ch 1 Master pot, I increased it to 470k. No success there either.

There's a relay that opens up the Feedback circuit of the power amp. Having already discovered removing V5 LTPI tube stops the oscillation all together (no presence of the oscillation on any of the preamp stages with it removed), I checked to see what the power amp gain was. Input my oscillator signal into the Effects Loop Return jack, set the Output control max CW, Return Jack to 100%, and checked the difference between input signal and output (4 ohm tap, resistive loading). 47.5dB gain. Checked the gain of the Effects Loop Return, 32db, so it's 15.5dB gain. Then, checking what the gain is when Relay RY7b opens, which requires selecting Ch 2 or Ch 3, and selecting 'MODERN' with the Voicing switch, I had an overall gain then of 54.5dB, so with the feedback open, the power amp gain is 22.5dB gain.

But, with Ch 1 selected, the power amp isn't in the open loop state, so that isn't a factor with this oscillation.

I stopped at that point, bypassed the Effects Loop, re-connected the test speaker (had resistive load in place for the gain measurements), and plugged my bass in, since it appeared I couldn't dial in oscillation in that mode. It sounded awful....murky, non-distinct & fluttery sounds on the E and A strings, from Low D on down. Low power, being modulated by something. And, saw a lot of ripple on the scope while playing & hammering low notes......I heard this sound the first time I had plugged in 3 days ago before I began the diagnostics. I already had the two rectifier tubes removed, since I was getting low frequency oscillation with them in SS Rectifier Mode. Having measured what looked like clean 90W, I didn't think there was a power supply filter problem, but after hearing this again, I decided to pull the main P/S filter caps....the pair of stacked 220uF/300V buss caps. I only had enough time to remove them.....well, make that fight with the bloody things, having found them firmly glued to the PCB!!!!! (still left the PCB's installed in the chassis). What was it member 'drewl' said in that same post I quoted Mars Amp Service......Damn this thing to hell! Amen to that!

I looked at the clock, and found I had to pack it in to go catch my buses & trains thru LA, so I don't yet know if all of these problems are related to bad power supply caps. So, this adventure continues.......

[nevetslab]

Tried another DR 3-Ch head from our rental inventory. Easy to make it oscillate too!

After being sidetracked by our kitchen's conveyer/toaster oven tripping the GFCI circuit and effecting repairs, I finally dragged over another Mesa Dual Rectifier 3-Ch Amp head. Later model....front panel channels have a 50/100W switch, rear panel has SS/Tube Rectifier switches for each Ch, and only an Effects Send pot for the Effects Loop. Set all the rectifier switches for SS Rect, remembering on my client's amp, I could get LF oscillation going in that mode. Set all the channels for 100W (vs 50W).

Powered up, switched out of Standby, no bass plugged in....wanted to see if I could dial in oscillation with Ch 1 controls. By golly, easier then the dickens to make it oscillate!! I didn't have the scope or instrumentation attached to see it, though I did power up thru my power analyzer. When you have HF/ultrasonic oscillation on an amp like these, which are sensitive to control settings, any control that will set it off will emit a LF thump, and once oscillating, the control will sound scratchy, a little like you have DC on the pot, but not exactly. And, it draws more power, so I could also see it's effect on the power analyzer. I began in the Ch 1 Pushed Voicing mode, and with the Output control turned up (2o'clock), Gain at 12 o'clock, Treble @ 12 o'clock, turning the Ch 1 Master full CW set it off. Once there, I could control the oscillation with the Treble, Presence, Gain, Master & Output contorls, as well as the Effects Send....just like my client's amp. I also switched out of Pushed back to Clean Voicing, and got it to oscillate in that mode too.

And, unlike the client's amp, I was able to get LF Oscillation on Ch 1. That would stop by going back to 50W, and, I'm assuming also by switching back to Tube Rect, but didn't try that. I was more amused by having another amp behave the same stupid way!!!!

I next switched over to Ch 2 & Ch 3. Turned the Master and Gain controls full CW (off), set for 100W, RAW voicing. Output control was turned up. Sure enough, here comes the big truck rumbling down the alleyway, just like the client's amp. And, just like that, switching into MODERN voicing, it killed the LF oscillation.

Same ol, same ol.......... never even got around to plugging in the bass. That's next. If my bass sounds just as non-distinct, murky, fluffy and modulated, like you're playing a couple notes together, but dissonant, then I don't know WHAT Mesa has hidden inside.

On the client's amp, I looked at the ripple yesterday, as well as the voltage levels on the taps to the various stages. There was around 25V of 120Hz ripple on the main filter stack at full output.....quite typical of any of our tube amps, actually. Following the choke, where it feeds the screens of the output stage off a 30uF/500V buss cap, the ripple there was about 1.7V P-P at full power, so the choke is doing it's job. At no sig level, 5V P-P ripple on the main filters, and under 100mV ripple on any of the voltage taps south of the choke.



In the test set-up on the Client's amp (sitting on the lab jacks supporting the chassis on the test bench) I injected my 120Hz sine wave into the Effects Return jack (rear panel), with the Effects Loop on, bypassing all the preamp circuits, just dealing with the power amp. Changing those filter caps didn't buy me anything but aggravation.

I did order a high current DPDT toggle switch, with the intention of wiring up the PT's HV Sec leads to the poles, and switching to either the 5U4GB Anodes or the SS Rectifier Anodes, as I saw in a later version of the Dual Rectifier, and NOT let the tube rectifier remain powered up (apart from the heater/cathode). Which also makes me wonder if that heater being lit up and always connected with the SS rectifier's cathodes, will this still produce the 15Hz fundamental oscillation?

I also didn't appreciate Mesa's teeny tiny solder pads all over their board. Why flush all that copper down the drain during the etch? Don't their layout people KNOW you want substantial copper & adhesive on the PCB for servicing?? At least the hole sizes allowed the component leads to be removed from top side. I was just waiting for one lead to be folded over too much, and kill a solder pad, but that didn't happen.

As for dialing in audible oscillation, that takes a lot more effort. One that will have a potential to kill woofers is when you get an oscillation that's just right, and the Low beat frequency from it wants to launch your speaker cones!!

[Jazz P Bass]

Nice test bench.
(reminds me of the old movies' mad scientist)
(I don't see a Van de Graff generator, though.)

The 'too small copper pad' thing has always irked me. (any mfg.)

Gallien Krueger's bass amps take it way too far small wise.

Either they do not want a serviceable unit or the board layout guy just graduated from Vo-Tech.

[Gregg]

I also had this high frequency oscillation problem (with the Presence having most impact on it) on ch 1 (on an amp that came for retubing and no complaints about oscillation) but what I noticed is when the loop was engaged (with the Send level somewhere around 10 o'clock) it cured the problem. My guess was those SA22 TVS diodes which have a capacitance of around 1,2nF each were taking care of the problem however I wouldn't bet on it because I didn't have enough time to dim more into this. Apparently this problem is not an exception. One thing is for sure - when the loop is engaged the return stage needs a signal of around -10dB to drive the power amp to full power so my advice is to dial carefully your Master and Send levels.

[nevetslab]

I also had this high frequency oscillation problem on ch 1 but what I noticed is when the loop was engaged (with the Send level somewhere around 10 o'clock) it cured the problem. My guess was those SA22 TVS diodes which have a capacitance of around 1,2nF each were taking care of the problem however I wouldn't bet on it because I didn't have enough time to dim more into this. Apparently this problem is not an exception. One thing is for sure - when the loop is engaged the return stage needs a signal of around -10dB to drive the power amp to full power so my advice is to dial carefully your Master and Send levels.

I haven't yet looked at those TVS diodes....good point. The Send side of the loop is a cathode follower, coming from a log taper pot, so the typical value leaving the pot is 10-15% of the signal feeding it. The Rtn side, stock has gobs of gain, and the loop I found really noisy, besides adding it's hand to the oscillation. I haven't yet FOUND the source of it, even though the power amp, as a function block, has a big hand in it. After I had tamed down the gain on the client's amp, bumping up the Send level (with the loop active) set it off again. I think the logic behind the gain structure in the loop is to accommodate anyone's favorite 9V floor pedal.

Just finishing up the wiring on a replacement Rectifier Select switch, which I hope stops this 15Hz LF oscillation. That one is just wierd.


Added note: The results of switching the tube & SS diode anodes, while keeping the cathodes common is on the other post for the Mesa Dual Rectifier 3-Ch Solo Amp head with LF Oscillation. Outcome was IT DIDN'T CORRECT THE LF OSCILLATION IN SS RECT MODE.

[Gregg]

The other reason for this oscillation I can think of is the way the main PCB was designed along with all those flying leads coming out from the pots all over the place.