Replying here because I have one of these on my bench right now. Virtually part for part, exactly this amp.
So the following are just some observations and thoughts about the WT550 on my bench, circa 2004 build date based on stickers, circuit conformance to schematic and factory mod. notes, etc.
1. Age of amp and circuit board dates - re: someone mentioned in this thread that they thought it was built in 2001 because of "board dates". Be aware this is the date of board DESIGN not the build date for your amp. The original power supply board shows a 2001 design date. This does not mean the amp is that old. Look to other features to determine your amp's aproximate build. One clue is what kind of regulator transistors are found at Q54/55 on the power supply board. You'll have to lift the Preamp board to figure it out, as Eden foolishly hid them under there. It is not too hard, but annoying inaccessible for such a failure prone part. If they are TIP32 transistors at Q54/55, then your amp is pre-2003. If they are TIP42, your amp is post 2003. Does your power supply board have 18V Zener diodes across the ouput header pins of the bottom of the power supply board? Look at the schematic. That'll tell you the date those mods were added. The available shematics I have indicate those factory "improvements" were added in 2003 and 2004. The amp I repaired had them done as modifications, with factory tags and rather one-off cuts through circuit traces, etc.
WHY THIS FEATURE CHANGE? The TIP32 regulator originally designed in there by Eden in 2001 is barely sufficient, running close to the valley of death, and therefore prone to failure. Q55 is especially prone to failure, as it appears that they used the -15V rail as a filament supply for the tube. That's about half the total practical output of the negative side of the bipolar 15V supply's total reliable output spent on the filament current, when using a TIP32 driver transistor. No wonder they upgraded to the TIP42 in 2003. They use a large dropping resistor get filament voltage down to 12.6V. rather inelegant, given the weak sister from which they are drawing the current. Oh well, it works.
2. Power transformer. The earlier era WT550 amp like the one in this thread, and the one I'm working on used a linear supply with a conventional E/I core transformer. Later WT550 models I have seen appear to use a torroidal mains transformer, but still a linear power supply. Let's look at why this transformer choice may matter.
3. Output Power. This is marketed as a 550 watt amp, right? I mean it is called a WT550! Well, maybe. Or maybe it is 900 watts, or maybe 300 watts...or maybe it is 78 watts. From my bench it appears to depend on the load and the duty cycle you select. The one I just fixed can achieve 306 watts into 4 ohm resistive load...with a 25% duty cycle. With 100% duty cycle (a sine wave and a measured output of 306 watts) the fan quickly comes on and she starts to heat up...like any amplifier, transistor or tube. with 100% duty cycle it is doing good to carry 100 watts!
If I keep pushing her at 100% duty cycle, she will eventually give in and trigger the built in over-heat shut down protective feature, again when pushed hard with 100% duty cycle. This is normal. See, amps don't typically have ratings measured in 100% duty cycles, and 4 ohm resistive loads do not represent reality for amplifier's typical useful load. HINT: Your loudspeaker is not 4 ohms, (or 2 ohms) o.k? Those spaces you leave between the notes are low duty cycle moments for your amp. Those places other than the peak of the musical waveform are low duty cycle moments too. those high notes way up on the neck are also lower duty cycle moments. During all of these frequency changes caused by playing music, the impedances of your speakers vary wildly, depending on the note you are reproducing. Please don't shoot me. I am just the mailman. I don't write the letters, I just deliver them. FWIW, the amp I repaired and measured thus plugs into a normal cabinet and plays regular electric bass content just fine and without interruption at normal volumes all day long, and mostly without the fan coming on even for a minute. Like I said "DUTY CYCLE".
4. The ultimate power limitation of this design is the power supply. As I mentioned in point 2, I believe the E/I cored design was replaced with a better torroidal transformer design later on - not sure exactly when this happened, but it was after 2004. Why does this matter? I have observed a torroidal transformer equipped WT550 sustaining 376 watts into a 4 ohm load with no apparent trouble. I believe the E/I core transformer and associated power supply design are the power limitation for this amp. The E/I cored WT550 on my bench claims a maximum power draw of 960 watts. It does not blow an 8A fuse when under full load and clipping. Average efficiency was calculated at 23.4% at 100W output. Do the math yourself. 100w = 20V out into 4 ohms, with a measured primary current draw of 3.55 amps @ 119.6 line volts. (Sencore 157 measured, with voltages measured using a Tektronix 2445B backed up by an Hewlett Packard 3446) So, that's ~426 watts in to make 100 watts out. That's 23.4% gross efficiency. Efficiency might go UP a bit at higher powers, but it's not going to go to 60%, which is what it would take to get near 550 watts with a power supply limited by transformer design to 960 watts input.
5. Where do I get "960 watts"? The maximum power draw of the amp is stated on the back panel to be "960 watts" you can see that at max un-ciipped output power (that I observed) of 306 watts, it was using all 960 watts input. It did not blow the 8 amp factory fuse doing this, no matter how long I ran it. So, it is a fair deduction to surmise that the power supply's inherent limits are the output power limitation.
6. Torroidal transformers are more efficient (if used correctly). The extra 70 watts output that I saw demonstrated by the torroidal equipped WT550 generating suggests to me that the torroidal transformer equipped unit is a better design, from a power efficiency standpoint. Not saying the overall AMP is better, but the power supply apparently is, and we all know CoolBlueGlow's favorite maxim "AN AMPLIFIER IS A MODULATED POWER SUPPLY" (haha)
7. Two Ohm. I don't care what Eden says on the back. I would not run this amp at 2 ohms. I just don't think it has the PSU oomph to really do it, and what's the point? Your results may vary, but this looks like marketing hooey some non-engineer demanded so they could "compete with other products".
8. Fan - The fan is barely adequate, and not well implimented, IMO. It cannot quite supply sufficient makeup cooling air as built. It is on/off, instead of variable speed, etc. When it gets behind the eight ball, it takes a long time to catch up. A better idea might be a variable speed fan to get ahead of the heat curve...but that's just my opinion.
On the plus side, I note that the design favors a "blow in" arrangment. I've heard via internet scuttlebutt that this is how Eden built them, and it makes sense, but I am basing this blow-in conclusion of direct observation of the design features, not internet scuttlebutt. There is no good scenario where I can imagine the heat of the primary heat sink should be drawn into the chassis and across the primary power supply board and transformer assembly and then out the fan side of the amp. That is exactly what a blow-out design would do. It would "suck" the biggest heat source's output right into the amplifier and right across the PSU board capacitors. No designer would do this if she could help it. The placement of the chassis cooling holes demonstrates that Eden designers intended for air to blow INTO the chassis, then across the PSU board, then finally last of all under and up through the fin assembly of the output transistor heat sink and out of the amp. That is why the amplifier assembly is lifted up off the floor of the chassis. This makes electronic and thermal sense, since the PSU board is a minor heat contributor, and carries many heat sensitive capacitors. Meanwhile, the output devices are the major energy wasters (23-30% efficiency, remember?) and by far output the most waste heat.
Just some thoughts about the Eden WT550 I had on my bench. Your result may vary. I make no particular claims of wisdom about Eden amps. I'm just observing what my bench tools told me about this one amp.
YMMV
Cheers,
cooblueglow
