agedhorse

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.

Your wire size examples are for long runs of wire and based on a combination of voltage drop, temperature rise and continuous duty cycle. They are generally accepted sizes based on installed building applications with common insulation types. Components and component interconnections are sized differently for each application. For example I work with 50A transistors that have leads roughly equivalent to 2.5mm dia wire. It’s because the leads are short and the parts are used in <50% duty cycle applications.

The peak power of ANY 600 watt rms amp IS 1200 watts. It’s just two ways of describing exactly the same thing. It’s just a math conversion between units.

The Diesel is based on the tuning concepts in the TL cabinet, using the EVM-15L for the driver. The internal volume is pretty close to the stock EV recommendation (actually EV recommended a range of internal volumes, a range of porting versus internal volume and thus a range of tunings). In that box, the EVM-15L is good for right about 200 watts RMS down to ~60Hz and does so with very high sensitivity. These were calcs/plots that I did on the Diesel 115 w/ EVM-15L when designing the Subway speakers, in order to understand the legacy products and why they were as successful as they were. In this case, there was very little driver performance left on the table and because of the sensitivity (at the expense of power handling, it's only a 200 watt RMS driver regardless of the 400 watt numbers applied to the proline series), it will generally take about twice the power of most suggested "upgrade" drivers to achieve what this cabinet achieves with 200 watts. unfortunately both the driver and the box are very heavy which was a non-starter for the new product line.

Most of the Mesa Diesel 115 cabinets were loaded with EVM-15L drivers. This was specifically for the mid voicing. The EVM 15L/B drivers are good for about 200-250 watts RMS mechanically above 50Hz. The series 2 and proline drivers are mechanically the same, the only difference is a coating on the VC which helps bind the coil to the bobbin. The pro line drivers have pretty much the same mechanical power handling as the series 2, the way they were rated changed some.

Which also drops the mechanical power power handling on a driver with already limited power handling.

Because the vast majority of players are concerned with the ability to drive the lower impedance phones, something many headphone amps can not do well. We will look at updating the manuals at the next revision cycle to change this to include higher impedance phones too. I had to design a headphone amp for another product (guitar related) recently and the big issue was to be sure the new headphone amp could drive low impedance phones because more and more are going this direction. Many modern digital devices have limited voltage swing therefore can’t drive high impedance phones to more than a low volume.

Yes, the amp will drive 250 ohm headphones without any difficulty, but the vast majority of players use lower impedance phones and by far the greatest number of questions (and issues) is with the lower impedance phones. To refresh my memory, I checked the top 30 selling headphones and ~90% were 60 ohms and less. There were just a couple over 100 ohms. Just beware that there are quite a few high impedance phones that do not get as loud at low frequencies, the headphone out will drive some of them beyond their capability without them getting as loud as players expect.

To clarify, a first order filter is 6dB/octave and a second order filter is 12dB/octave.

A tweeter is typically about 10dB more sensitive than the low frequency section, so with the tweeter level control fully up (assuming that's the only attenuation), there will be a 10dB bump in the high end compared with the rest of the speaker's response.

Mid 90's. Be sure that you set your speaker to 8 ohm mode (assuming it's switchable between 4 and 8 ohms) I too would recommend a second speaker identical to your first.