My OR120 MV Is squealing like mad.

[Frank_Jeeves]

Also it is a loud screech. It's the kind of screech one would get if you line up 2 overdrive pedals and set them to max and turn them on. Only it stops screeching when I lower volume on my guitar but if i just hold the strings it still screeches which is unlike normal feedback I'm use too. I'll look to see if the connection to the NFB is there

[fiveightandten]

You need to determine definitively if the feedback is coming from the amp or from something outside the amp.

Sources from the amp:
-A microphonic tube
-An oscillation (this usually happens with higher gain and treble settings)

Sources from outside the amp:
-Microphonics from the guitar pickups
-Noise from a pedal

There are 3 distinct possibilities here:
1) It's something inside the amp that will make noise without external influence.
2) It's something other than the amp that will make noise with other amps (guitar, guitar/pedal combination).
3) It's something inside the amp that only makes noise with a certain condition of external influence.

Be advised that using too much gain, or using pedals (which introduce too much gain) can bring about the feedback. You have to do some investigating to determine what the source is. This will involve using the amp with and without pedals, and with the gain and volume both low and high to try and see when it will feedback.

Tube test... with the back off the amp, *gently* use a pencil to tap on the tubes with the volume and gain (if it's an overdrive model) up. If you hit a tube that rings or squeals, that's your problem.

Determine definitively if we're looking at scenario 1, 2, or 3 here and report back.


A few more things:
-If the OT was wired backwards, you'd know it immediately. The negative feedback loop would be positive and the amp would put out a deafening hum at full volume immediately upon powering it on. The OT primary legs are wired properly if the amp is making sound normally.

-The negative feedback loop in a 90's reissue is a green wire that runs to the back of the board, and the turret is marked "NF". If this wasn't reconnected, it will make the amp less stable and it can oscillate. If the NFB loop is disconnected, the amp will have a response that's notably more loose and spongy. I ran my OR-120 without a NFB loop for a while. It sounded like a Vox that had been through a long period of binge drinking and a really bad day.

-OR-120's are prone to oscillations and will oscillate if the lead dress on an OT install isn't nice and neat. I've added some pics below so you can see what it should look like. 90's amps are pretty stable. 70's amps are wired up on the sloppy side and every one i've been inside could benefit from some cleanup work. They weren't the neatest thing from the factory.

-The TH100 OT will likely sound different than the OR-120 OT did. You're the only person i've heard of with a TH100 iron in an OR-120, so we can't really say what the differences should sound like. Ideally, he would've used a Mercury Magnetics iron in the amp. It's a drop in replacement and sounds like it should. I'm not sure why he didn't go this route. Though, due to the lack of info about a TH100 iron in an OR-120, we can't say whether it worth it for you to pay for another OT and another install.


Anyways, here stock secondary wiring for a 90's OR-120:


Here is wiring from a replacement OT I put in this amp (belonged to a forum member) Note that everything is nice and neat. The NFB wire can be seen on the impedance selector, the small light green wire. Yours should be in the same place. That's tied to the 16 ohm tap, which is a thicker green wire):


Here is an overview, showing the stock primary wiring (bottom part of the screen, green and red wires against the chassis):


Here's my primary wiring with the replacement OT (you'll also note that I replaced a tube socket and a few parts that went up in smoke, along with some new can caps):



Note how the location of the leads didn't change, things are twisted together and tied neatly into place, and the lead dress on the solder joints is nice and clean. Your amp should look like this inside. If it doesn't, the work should be cleaned up.

Hope that helps. Do some testing and report back.

-Nick

[bclaire]

I'd think tube before everything else...

[Frank_Jeeves]

Tube's tested. Non microphoic. Tested my Fender mustang I think the bridge pickup is slightly microphonic. I have to get within two feet with my jazzmaster before it squeals. The neck of the mustang doesn't until it's close and the bridge starts at about twice the distance. I think it's the mustang bridge pickup.

I must say though it didn't use to be this sensitive. The sound is a bit off from when I first had it. The output transformer is smaller. I'll take a look inside once I've giving it 2 days to disapate the voltage. I don't have a voltage meter and i'd rather not be messing with the inside for at least awhile.

[jontheid]

I see this amp is up for sale on ebay:

http://www.ebay.co.uk/itm/Orange-Overdr ... 3a8812ebec" onclick="window.open(this.href);return false;

In the description it states :

I just recently switched out one of the output transformer as the old one gassed out. It was switched too an orange TH100 transformer. It also has a fresh pair of tubes and it gets alot gain/distortion if you put a driver pedal behind it. It sounds amazing and I hardly can tell the difference from the old transformer.

Well, if you don't mind me saying, that is quite a different story to that given in this thread.
I assume then that you got this squealing sorted and the amp is now working perfectly.
How did you cure the squealing in the end?

Cheers
Jon

[Frank_Jeeves]

Turns out it was my pickup on my mustang the entire time. I just checked the insides and everything is pretty neat. The New OT is actually wired cleaner then the old IT or OT was so it's not the amp. It has been rebias to normal bias. Before it was biased cold and that might be why the sound change and why it didn't feedback as much before the OT blew. So it really isn't the amp. It's the guitar that was the problem.

[Frank_Jeeves]

WAIT I may have found something.

The wire on the 16 ohm switch is NOT wired with the green wire. It's Wired with the RED wire.

Let me see if I can show a picture.



If you look carefully the other green wire is just another green wire. But this green is connected to the red. Would the bare any significance on the amps operation?

The playabilty sound is pretty close to what it once was. And the bias is hotter then before. It was cold. But the amp does tend to feedback more easily when there is a pedal. But it doesn't when there isn't a pedal. I just tried it a few days ago full blast and no noticeable feed back.

[jontheid]

Hey Frank, the picture does work but doesn't embed properly.

Does you o/p transformer look like this?



If so it would follow that green should be 16 ohm tap, red should be 8 ohm tap and brown should be 4 ohm tap.
The brown wire would not have been used originally I think, as the TH100 doesn't have a 4 ohm output . . .

[Frank_Jeeves]

Right click the picture and open in new tab or copy the Html and paste it should come up after that.

My transfer is very similar to the one pictured. and the wiring hook up to the ohm selector seems right. it's just the grounding wire to the circuit board labeled NF is wired to the 8 ohm wire rather then the 16 ohm. I was wonder if that would make a difference is why it feedbacks more sensitively then before. Or if it's simply because it's no longer bias cold.

[jontheid]

The NF is the negative feedback connection and should be connected to the 16 ohm tap, not the 8 ohm tap.
If it is connected to the 8 ohm tap the overall gain of the power section is increased and could well be responsible for the symptoms you describe.

[Frank_Jeeves]

Well that solves it. I'm going to call the guy who repaired it last to see if he's willing to repair it for free otherwise I'm patronizing the guy closer.

[Frank_Jeeves]

My amp tech said it would only change the presence if I switch the wire

[jontheid]

Sorry to be blunt, but your amp tech doesn't know what he is talking about I am afraid.
Yes, the presence control will be affected. The greater the amount of negative feedback, the more effect the presence control has. Having the negative feedback connection tapped off the 16 ohm output (where it should be) will give more negative feedback and will make the presence control do more.

But, and this is a massive but - even with the presence control on zero, the amp will be affected by moving the negative feedback to the correct position.

Negative feedback smooths out the frequency response of the amplifier - the greater the amount of feedback, the more the smoothing effect. It also reduces the gain of any stage it is applied across.

Thus moving the negative feedback to the correct position on your amp will tame the output stage somewhat.
It may cure the problems you are having, it may only slightly improve things, but it is definitely worth trying.

Some amps use negative feedback and don't even have a presence control - the Rocker 30 for example.

Is this the same amp tech that fitted the TH100 transformer?

I'm happy to try to answer any questions you may have regarding your amp.

Happy Christmas Frank, and I hope you get this sorted.

[a.hun]

Lose the tech!!!

Not knowing something but finding it out would be acceptable. BS-ing customers with wrong info to defend doing bad work just isn't...


Andy.

[Frank_Jeeves]

The Amp tech offered to do it for free. Should I get rid of him?