agedhorse

With a low powered speaker, a strong motor and just a couple mm of VC overhang, it’s possible to make a very efficient driver using a 4” coil and a tighter than average gap.

Be sure to use SpeakOn cables with genuine Neutrik plugs. Some knock-offs can damage the amp’s jacks.

It’s not due to the difference between class AB versus D. That’s a false premise to start with.

Turn the gain/volume up!

Correct, it’s the most common configuration, and a player would have a reasonable expectation that the head can be operated on top of a cabinet.

Not much safer given the plate voltages are fairly close.

Yup, this was also suggested Saturday on your TalkBass thread asking for help on the same amp... post #19. You could at least have the courtesy of following up there with your resolution IMO.

Maybe not, no way to know without a little more troubleshooting. The good news is that we do still have the newest PH 10" driver (8 ohms) in stock.

At this point, we don't know if any of the speakers are damaged yet. We do know that one is (very) incorrect.

Sorry, I wasn’t feeling well for a couple of days and missed this. First, a professional service technician that made that kind of mistake (substituting a 4 ohm driver for an 8 ohm driver in a 410 needs to hand in any credentials he may (or may not) have. That’s a bone-head error in judgement, just like choosing a BP-102 which is about the worst driver I can think of for that application. The original vendor for the earlier PH cabinets went out of business, the parts for those drivers are no longer available. The closest match to the drivers in your cabinet is the Legend CA-10 (8 ohm), and the drivers are wired series-parallel. Note that wiring so the polarities are correct is essential. If you need a diagram, message me and I will get it for you. This is why I always recommend verifying that the cabinet you are buying used is really what you think it is, I see folks get burned all the time.

A piece of heavy carpet might help, but some of this is going to be caused by the acoustic coupling between the speaker’s acoustic output and the top diaphragm of the riser. A high pass filter is one tool that may help, a parametric eq is another tool, turning down a bit might help too.

The GX-5 is an easy amp to repair for a qualified authorized service center. They have also proven to be extremely reliable over the past ~20 years or so. I don't recall any of the ~100 GX series amps that I had installed ever failing, they were at least as reliable as the RMX amps, but in a smaller (shallower) package. That said, the reduced cost of manufacture of some new amps can make them less costly to purchase in some cases.

Which model?

Why wasn't it repairable? Any authorized QSC service center should be able to service this. It's certainly not a self-repair type device, but it was never intended to be a DIY service device with the technology involved.

How are the secondaries attached to the bridge rectifier? Theres a lot of sloppy wiring and terminations showing in the picture.

With that wiring, I would be wary of intermittent short circuits that load down the secondary.

Not necessarily, it depends on the designer's intent and ability.

How would you propose to change the impedance matching by re-biasing? Bias does nothing to the high reflected impedance LxdV/dT effect.

Hold on a minute, 5A is plenty more than enough for an 800 watt amp (especially if it's SMPS/class D). If you were to do then math, with an 80% efficient amp (typical for a quality SMPS/class D), the IEC safety regulations calculate the input power as: (rated audio power x 1.2) x 0.125 duty cycle, so for an 800 watt amp this would be 120 watts mains input. Now if you wanted to use a higher duty cycle than the minimum (I typically use between 33% and 40% myself) to account for overdriving the power amp or high levels of compression, the calculation would be (800W x 1.2) x 0.40 = 384 watts input. Now divide by 230V and you get 1.67 amps which is EASILY handled by a 5A cordset (with the correct fuse for protecting the cord).

Well stated.

Not necessarily, some tube amps are fairly sensitive to output stage loading. A 50% mismatch could result in double the L(dI/dT) voltage which can over time (or immediately) break down insulation in the output stage.

No, some tube amps are particularly sensitive to a higher impedance load, because the reflected impedance to the tubes an primary of the OT can cause ringing and larger than expected voltage spikes which can damage OT’s, tubes and in some cases arcing between the pins of tubes.

There are companies that have no problem supporting 15-20 year old class D amps as well, making them non-disposable. it depends on the company and not the technology used ime.

The current in an electrical power circuit is generally considered to be continuous (there are a few exceptions, every region is slightly different) and the distances are much longer than what is being dealt with on a PCB or interconnection wiring. 50 amps of building power circuit would require at least 8 times the copper area as an audio circuit or internal power wiring for an audio device). This is because the audio circuits are designed around a minimum 12.5% duty cycle, and the voltage drop is based or wire cross-sectional area its length and the current in the wire. Power electrical current is based on 100% duty cycle (80% in the US for non-continuous loads).

Actually, it depends very much on the amp. Tube amps reflect the speaker’s impedance back through the output transformer to the plate circuit. This output matching transformer maintains the voltage (and current) ratio so that the high impedance plates transfer the energy effectively to the low impedance load. When the load is mis-matched to the plates, the voltages can increase to unsafe levels, especially when the output stage is overdriven. This effect is generally worse when the load is higher than the impedance tap is set for. some amps are more forgiving of mismatches than others, though the cost of being wrong can become awfully expensive.