Orange Reverb Twin - issues

[E_S_D]

Hi,

I've been having problems with an 1977 Orange Reverb Twin. I bought this amp 4 or 5 years ago, and it has intermittantly suffered from a particular problem: at times, the amp emits a bad crackling sound, which happens whilst playing but also when just tapping the amp on its top or side.

I've tried the obvious - tapping the tubes with a pencil, disconnecting the reverb unit, fitting new tubes in there. The amp has been serviced by 3 different technicians who have found nothing wrong with it (the problem didn't manifest itself while the amp was in their care... )

Another important detail: this sound will happen even with all knobs, EQ and volume turned all the way down.

Here's a short video demonstrating the issue/sound: http://www.youtube.com/watch?v=u1y4h0Az ... e=youtu.be


More often than not, the amp has worked well and sounded great, but recently the issue has become more pronounced and the amp is currently unsuable...

Any ideas?

Thanks a lot in advance.

[Jondog]

I'm thinking there's a loose or bad wire/connection somewhere.

[Gladmarr]

This, unfortunately, could be lots of things. Someone is going to have to get in there and tap test every component inside the amp to try and localize the problem. I had an amp a few years back that had a similar problem. After replacing every capacitor in the amp (and wasting a lot of money doing so) it turned out to be a cracked resistor on the PCB. It was a 50¢ part, that caused almost $100 in wasted parts and even more in wasted time. It would also help to clean everything that could cause this problem. Clean the input jacks, clean the pots, etc.

[a.hun]

Clean the input jacks, clean the pots, etc.

My first thought. Surprising how often a bit of light tarnishing on the input jack socket contacts can cause havoc with your sound. A bit of contact cleaner on the jack plug, insert / remove a few times, allow to dry (seconds) and test. You never know...


Andy.

[Valvelust]

Clean the input jacks, clean the pots, etc.

My first thought. Surprising how often a bit of light tarnishing on the input jack socket contacts can cause havoc with your sound. A bit of contact cleaner on the jack plug, insert / remove a few times, allow to dry (seconds) and test. You never know...


Andy.

I so agree this has happened numerous amount of times with me, yes also check anything that has contacts.
Clean as much as you can!

[Jondog]

This, unfortunately, could be lots of things. Someone is going to have to get in there and tap test every component inside the amp to try and localize the problem. I had an amp a few years back that had a similar problem. After replacing every capacitor in the amp (and wasting a lot of money doing so) it turned out to be a cracked resistor on the PCB. It was a 50¢ part, that caused almost $100 in wasted parts and even more in wasted time. It would also help to clean everything that could cause this problem. Clean the input jacks, clean the pots, etc.

Just heard an amp tech talk about how a fractured resistor can cause this problem. People bang the top of the amp and get an intermittent sound from the amp each time they bang it.

[bclaire]

I'd say it's amp tech time...

[jontheid]

One thing you can do is pull out all the preamp tubes (not the power tubes) and turn the amp on.
If the noise persists then the fault is either in the power amp section or the power supply.
If the nose goes the fault is in the preamp/phase inverter section.

[Gladmarr]

One thing you can do is pull out all the preamp tubes (not the power tubes) and turn the amp on.
If the noise persists then the fault is either in the power amp section or the power supply.
If the nose goes the fault is in the preamp/phase inverter section.

You can even do that section-by-section to localize it to a particular preamp stage as well. If you know which tube is which in the preamp signal chain....

[a.hun]

One thing you can do is pull out all the preamp tubes (not the power tubes) and turn the amp on.
If the noise persists then the fault is either in the power amp section or the power supply.
If the nose goes the fault is in the preamp/phase inverter section.

You can even do that section-by-section to localize it to a particular preamp stage as well. If you know which tube is which in the preamp signal chain....

Actually MUCH BETTER to do it section by section. For that reason and also because pulling all preamp valves at once can be risky with many amps!

Take out all preamp tube and power tubes, leave the rectifier in, and turn the AC and DC power 'on'.

No, do NOT do that!

If you remove all the preamp tubes, no current will flow through the resistors in the B+ chain, and the voltage at the far end of the chain will rise to the full unloaded B+ at the rectifier end, which in many amps will be well over the filter cap rating - that's why the first cap stage (and sometimes the second) is done with two stacked caps.

If you have a modern amp with 500V caps throughout, you're likely to be OK, but not in many older amps with 450V caps further down the chain (and sometimes only 350s).

And in fact, with two of these particular amps with solid-state rectifiers and unloaded B+ of over 500V, it's a risk even with 500V caps.

If you're metering unloaded DC voltage with the tubes out, you must leave the standby switch open. Better is not to remove the preamp tubes, or certainly not V1.

If you've already done this and didn't blow a cap, you're lucky. I found out about this problem the hard way.

Pulling all the power valves would be safe but wouldn't tell you much.

Could well be another small component though, resistor, cap. I even had a choke go microphonic (in my OR120) many years back which lead to similar symptoms, along with a frustrating series of blown caps. Took a while (and at least 3 different techs as well!) to figure that one out. The problem simply didn't show up on a test bench, only when running the amp on a bass cab as the actual vibrations caused the microphonic choke to emit voltage spikes.


Andy.

[jontheid]

You can even do that section-by-section to localize it to a particular preamp stage as well. If you know which tube is which in the preamp signal chain....

Yes you can, but if you don't know which tube is which you could have a fault around V1, pull out the phase inverter tube, stop the noise and falsely localise the fault to around the PI.

[Gladmarr]

Oh yeah, no doubt about that. This process is much tougher with the Reverb Twin than any other vintage Orange just because of the six preamp tubes. You'd have to work from v1 forward thorough the signal chain, but if you don't know which is which, you're gonna have problems localizing the problem.

[Jondog]

I don't know what type of circuit board is used in the Orange Twin, but th tech I was listening to was saying this is common problem in Fenders because the boardabsorbs moisture and over time warps, and the old components will crack if they are above a warped spot.

[Gladmarr]

The board in the Reverb Twin is similar in shape and construction to the board in the pix-only OR120s. I think it was Orange's first attempt to get away from Mat's designs. It wouldn't likely have the same moisture-related problems as the old Fender boards, but it's a big board subject to a good bit of flex and vibration. Certainly could contribute to parts flexing/cracking.

[jontheid]

If you remove all the preamp tubes, no current will flow through the resistors in the B+ chain, and the voltage at the far end of the chain will rise to the full unloaded B+ at the rectifier end, which in many amps will be well over the filter cap rating - that's why the first cap stage (and sometimes the second) is done with two stacked caps.

This is a really interesting point. I've never run into trouble when running amps without any preamp tubes in, maybe I've just been lucky.

I've been reading around about electrolytic ratings and apparently you can run them 10% over rated voltage for short periods of time (no more than 30 seconds total every 5 minutes) - it is called the 'surge voltage'. Perhaps the reason I've been lucky is because I only switch on the amp for a short period of time with no preamp tubes.

I'm going to change my practise following Andy's advice and not run with all the preamp tubes out now (unless all the B+ caps are obviously able to handle it.)