Orange AD200 Squealing

[nevetslab]

Finally have one that's for the moment, baffling me. Orange AD200 Bass Amp, stated to Squeal loudly when the input gain is turned up. By itself, the amp is nice and quiet. Low noise, turing everything up is tame and well behaved, so I plugged in my early 60's Fender P-Bass. Now it's squealing loudly. Starts with the Master and Gain controls half way up, turning Treble and Mid up past half way increases it. Turning the instrument volume half-way down kills it, as does turning the Tone on the bass CCW.

I'm now reading all the threads found when I use Squealing as a search parameter in our Archives.

Saw one from Juan (JM Fahey) suggesting using a 47nF/400V or higher cap, grounding one end with wire, other end as a probe to contact tube input grids from the Phase Inverter on back towards input stage to aid in finding where the problem is.

Any fast clues/hints?

AD200 Front Board Schematic.pdf
AD200 Main Board Schematic sheet 1.pdf
AD200 Main Board Schematic sheet 2.pdf

[nevetslab]

Also, plugging in an unterminated shielded phone plug sends it squealing (as well as lots of hum).

[mikeydee77]

Was there a fault with the amp, other than the squealing? Jus' asking.

Sometimes I have squealing with high gain amps, especially marshall, on my bench because of the way I connect them to my test speaker... and input switch.. think it is lead proximity and clutter related issue. When I connect directly they are fine.... Just a thought.

Obviously usual stuff, pre amp valves... flaky earth points etc

Does it do it in other instruments?

As Enzo always says.. got to isolate.

[mikeydee77]

Also, plugging in an unterminated shielded phone plug sends it squealing (as well as lots of hum).

weird jack socket voodoo?

[Enzo]

Unless I missed something, it sounds like your pickups. Amp is tame sitting there, but only squeals when the guitar is connected? And then only after turning it up past halfway?

Does turning side to side affect the squeal? That is to say aiming the guitar at the ;left wall or the right wall or turning your back to the amp? And does standing back 30 feet from the amp change it compared to standing 3 feet away?

High gain amps can squeal pickups even at low volumes.

Plug a keyboard or something into it, still squeal?

[Enzo]

An unterminated plug turns the input into an antenna.

[nevetslab]

Was there a fault with the amp, other than the squealing? Jus' asking.

Sometimes I have squealing with high gain amps, especially marshall, on my bench because of the way I connect them to my test speaker... and input switch.. think it is lead proximity and clutter related issue. When I connect directly they are fine.... Just a thought.

Obviously usual stuff, pre amp valves... flaky earth points etc

Does it do it in other instruments?

As Enzo always says.. got to isolate.

I hadn't done any work on this recently. I didn't check my database to see the last time it was in the shop, but, after removing the chassis from the cabinet, standing up on the Output Xfmr side, braced for stability, I found the source of the problem.

Orange, in their infinite wisdom-way of doing things, placed the Primary Lead breakout hole (of the output xfmr) right below the input tube stage, and, did nothing to dress the three leads down to the chassis. I was able to first remove the corner of the PCB from the standoff at that front input corner, lift the xfmr primary wires over the standoff so I could then route it along the chassis floor as tight as I could get it, then dressed it with the Secondary wire bundle, strapped it down first with a tie wrap hold-down block, but it actually needed to be taped down, so using 2" wide gaffer's tape, got as much of that taped down flush to the chassis.

That still wasn't enough. I could use body capacitance to absorb the field behind the input jack PCB, so I removed that, cut and punched a section of brass shim stock (5mil thick) so I could solder it to the bottom side of the input jack PCB, while providing clearance holes for the signal terminals, then with tape on the jack side of the shield, folded it up and over the jacks, tensioned that 'shield' to stay put, then re-mounted it to the front panel.

Still not enough to kill it. I then moved my aluminum shield plate I often use to set atop open amp chassis' on the service cradle to place it across the open bottom of the chassis. THAT additional shield killed the oscillation....which is WHAT it was. When the 'shrieking started, it was producing FULL Output Swing and into soft clipping, while the current draw from the mains went up as one would expect, seeing the amp clipping.

I got the dimensions of the chassis and the inside of the cabinet...19" inside width, and 9" deep, and with the first section of that 10" wide Aluminum flashing I had bought for lining the bottom of that Hiwatt DR201 Clone I just built, I cut and punched a shield panel, having now set the thru-holes to align with the chassis, and stapled it down to the insides of the cabinet.

Moved the chassis back into the cabinet with all those lead-dress mods made, and now sitting on a shield plane, it's nice and quiet, and NOT A HINT of Oscillation/shrieking.

I had also plugged in my shielded 100k input termination plug, instead of the unterminated plug.

[nevetslab]

Unless I missed something, it sounds like your pickups. Amp is tame sitting there, but only squeals when the guitar is connected? And then only after turning it up past halfway?

Does turning side to side affect the squeal? That is to say aiming the guitar at the ;left wall or the right wall or turning your back to the amp? And does standing back 30 feet from the amp change it compared to standing 3 feet away?

High gain amps can squeal pickups even at low volumes.

Plug a keyboard or something into it, still squeal?

I thought about that with the pickups, but wasn't getting any variation with the bass's orientation...apart from the florescent light fixtures 12 ft up on the ceiling. My shop's shell dimensions is 33ft long by 10ft wide, 12 ft ceilings. Check-out bench is about 5 feet from my shop speaker stack at the one end of the shop. Can't get very far, though with along cord, I can get out the door at the other end of the wall.

Nah....this was an engineering mistake from Orange, as I now see it, finding the source being the Primary of the Output Transformer exiting right below the input tube. So, at least I now have a 'fix'' for Orange AD200 amps. I didn't stop to take photos. Probably didn't need to make that input Jack PCB shield I added...that was an on-the-fly attempt, but my hand in place did more than that brass shield.

[Jon Snell]

Sounds like that was a pig of a job. (Pun intended, just had to)!

[nevetslab]

From what I had read on other member's pursuits into the cause and solution to eliminating the shrieking/feedback oscillation, I was expecting a lengthy battle. I really didn't want to have to pull that main PCB ups. But, seeing that three-wire primary twisted pair exiting from underneath the input tube area in the chassis, and exiting around the adjacent corner standoff, I began playing with those wires, and found the oscillation changing immediately. Using a small mirror to look underneath the PCB, and seeing those wires bulging up and limited by the PCB height, that seemed like the place to start. Very little room to get a small strip of gaffers tape in under that board, as it just kept sticking where it wanted to, not where I wanted to. There was little change at the power tube end of the wire bundle with regards to that oscillation sensitivity, which made sense.

I felt a bit helpless in the pursuit, finding the presence of my hand in that area of the input jacks, like it really wanted my hand left behind as a shield. Installing the Brass shield on the small phone jack pcb assy was a feeble attempt as it turned out. The PCB already had plenty of ground plane on both sides of the small board. I was surprised to find placing the large aluminum shield over the bottom of the chassis stopping the oscillation, but glad it did, as that led to a simple completing solution, cutting/installing the ground plane shield onto the floor of the cabinet.

[nevetslab]

Today, after getting side-tracked by the absence of the Rean Chrome Ferrule Ground input sleeve on the 8-ohm output jack on one of the other three AD200 amps in our rental inventory, I connected the shop test speaker, plugged in a shielded Switchcraft 280 Phone plug with a 100k termination resistor installed, turned the Master Volume up full, and after it warmed up, switched out of Standby. Turned up the Gain control, and sure enough.... this amp also oscillates like the first one did. Shut this amp off, rolled in the other two stacked on a road case, connected speaker and power to the next amp, and it too oscillated in the same fashion. The last one, interestingly.... did NOT squeal or oscillate.

I'll know more when I pull each one apart and take the (presumed) same steps as that first one to re-route the Primary wire bundle that probably also exits right under the input tube area. I'll have photos this time. Obviously, the lead breakout can't be directly under the tube socket, as that would defy physics. But close enough to be causing close-coupled field to invoke oscillation.

[nevetslab]

This is the first of the three remaining Orange AD200 amps from our inventory. Also a Mk 3 model. I took inside photos of this, having verified it oscillates with Master at full CW, Gain at full CW, Treble and Mid pots at full CW. The Output Xfmr primary hole breakout in the chassis on this one is different than the first one that took a lot of effort to defeat the oscillation threshold. On that, the Primary lead hole was underneath the main PCB, where this one there are two different holes for the Primary...lower one closer to the component area of the input tube stage.



I unplugged the Primary wire bundle (WHT, YEL, BLK) and unraveled the twist, then re-twisted the three wires. Unplugged the three Secondary wires (BRN, BLK, RED) from the PCB, and did the same. Twisted the Primary with Secondary wires, now in the same tighter twist and plugged the Secondary wires back onto the board. Lifted the left corner of the PCB up after removing the mtg screw, and slipped the Primary wires up over the standoff. Installed a small Tie Wrap hold-down block after snipping off two of the molded ridges, so the harness bundle could be strapped as close to the chassis floor as possible. Strapped it down. Installed a second larger block, and strapped that bundle of the Pri/Sec wires down. Finished routing the Primary wires back to their destination terminals.



Plugged the power cord back in, powered up. 100k Terminator plug still installed. Switched out of Standby, turned master fully CW, Gain fully CW, Trble and Mid fully CW. No Oscillation now. Fiddled with the lead dress of the O/T wires some, not finding any change in stability, though with all that gain, my hand did induce hum. And, on this one, I didn't need a bottom shield plate, as I did on the first one. Plugged my Fender P-Bass in, and still remains stable with these simple lead-dress revisions.

The previous AD200 where all this effort began, had much higher degree of instability. I'm guessing either a different Output Xfmr, or a revision in the end bells and chassis for the Primary lead breakout location. I have two more AD200's to go thru. One of them isn't oscillating at all.

Unrelated to this Oscillation instability of the amps, the Master Volume knob has a broken skirt. Not sure who the knob mfgr is, and whether I can find one outside of going to Orange for the replacement knob. These amps in our inventory may belong to Orange, so if that's the case, hopefully we can get mfgr parts support for those (as well as those Chrome Jack Gnd Ferrules).

Next amp coming shortly.

[nevetslab]

The second of the remaining three Orange AD200 Mk 3's looked much the same. This one had J/J 6550 power tubes instead of J/J KT88's installed. The Output Xfmr lead dress was similar in the lead dress, sloppy, and led to the instability when turning the Gain and Tone controls all the way up with the Master Volume up full.



As done with the previous AD200 Mk 3 chassis, I unplugged the Primary wire bundle (WHT, YEL, BLK) and unraveled the twist, then re-twisted the three wires. Unplugged the three Secondary wires (BRN, BLK, RED) from the PCB, and did the same. Twisted the Primary with Secondary wires, now in the same tighter twist and plugged the Secondary wires back onto the board. Lifted the left corner of the PCB up after removing the mtg screw, and slipped the Primary wires up over the standoff. Installed a small Tie Wrap hold-down block after snipping off two of the molded ridges, so the harness bundle could be strapped as close to the chassis floor as possible. Strapped it down. There wasn't enough Primary/Secondary wires in the bundle on this one to install a second hold-down block.



As I had found with this second AD200 Mk 3, after revising the O/T Primary/Secondary wires lead dress, there wasn't any more instability. So, essentially the same method used as on that second chassis.

The power tubes on this one...6550's, are NOT matched. On V4 and V5, that pair are sitting at 25mA and 23mA thru the 1 ohm cathode resistors, while V6 and V7, those are sitting lower at 16mA and 18.5mA thru the 1 ohm cathode resistors.

I don't yet know what tubes are in the third AD200 Mk 3 just yet. I know I DON'T have any J/J 6550's on hand. If that remaining AD200 also has J/J 6550's, perhaps I could juggle tubes between the two amps and come up with a better match...depending on what I find on it. I do have a pair of Sovtek 6550's that balanced up nice on the Test Fixture I just built...though that was with using both bias pots to land them. Here, Orange only uses a single bias pot, so you're more dependent on having matched power tubes in the set. And, as I'm pretty sure these aren't owned by CenterStaging, I don't have the option of hacking the main PCB to revise the bias circuit for split bias pots.

At least these two amps are now stable, and the last one didn't show any instability. Moving on to it.

[nevetslab]

I pulled the last Orange AD200 Mk 3 apart, finding it also had 6550's, but those were Sovtek's. And, after powering it up, found they were well matched, so that leaves just the one with the J/J 6550's that aren't so matched. I tried swapping two of the tubes around, having one higher transconductance tube with a lower one, but that further reduced the plate current on the two weaker tubes, so restored it as it was. I normally don't mix different mfgr's tubes together in a set. Maybe it doesn't matter....being the same type. Just makes me nervous not knowing it being a wise thing to do or not. Though, I do now have a proper test fixture to go digging thru the years of accumulated 6550's and KT88's to see what I can come up with.

Once I looked at this one, now on the bench and accessible, I nudged the Primary O/T lead wires a bit, and found it WOULD squeal. So, good reason to follow thru with the lead dress and anchoring as I just did on the previous two amps, so everything stays put and stable.



As done with the previous AD200 Mk 3 chassis, I unplugged the Primary wire bundle (WHT, YEL, BLK) and unraveled the twist, then re-twisted the three wires. Unplugged the three Secondary wires (BRN, BLK, RED) from the PCB, and did the same. Twisted the Primary with Secondary wires, now in the same tighter twist and plugged the Secondary wires back onto the board. Lifted the left corner of the PCB up after removing the mtg screw, and slipped the Primary wires up over the standoff. Installed a small Tie Wrap hold-down block after snipping off two of the molded ridges, so the harness bundle could be strapped as close to the chassis floor as possible. Strapped it down. There wasn't enough Primary/Secondary wires in the bundle on this one to install a second hold-down block.



This last amp is now solid and stable, and I can't nudge it into screeching and oscillating. So, this was a worthwhile exercise in Preventative Maintenance, as that first very unstable amp suggested there be gremlins about in your collection of quality tube amps.

The previous amp, I found the RF ID Tag had come off the insides of the cabinet, leaving the back side of that tag sticky, but not able to adhere to any surface. I had just picked up a pint of Weldwood Contact Cement, so mixed that up, applied a thin coat to the tag, after cleaning off the tacky surface, and did likewise with a patch of the inside cabinet surface. That's now been re-attached, and should be there forever now.