I tried posting this information to a different forum, and it caused a HUGE stir and got taken down very quickly, but I think this information is important to put out for public consumption and thankfully I saved a copy before it was removed.
OK, here we go! So a friend of mine brought one of these over just for me to check out casually along with a completely different project he wanted some help on. I would not have really considered even bench testing the amp, but when I picked it up, I nearly smacked myself in the face because I was expecting a lot more weight. I crinkled my brow a bit and got out my equipment to measure his amp, and the results were surprising. The results were unexpected enough that I decided not to publish them without collecting a little more data, so I asked around and managed to borrow a second Crimson 275 and I have a fresh quad of KT-120s from Jim McShane.
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Let's review the specs from Carver:
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These are really impressive. 75W at 20Hz and 1% THD is something I've only seen from much larger tube amps (The MFA M-120s I repaired somewhat recently could do that, but they were pushing out 130+ watts!).
Anyway, let's get into the meat of the post. What kind of power do these amps make? I'll measure these all with an 8 ohm dummy load, ARTA as the FFT, and an OWON SDS-7102 scope. These tools have accurately measured a dozen or so vintage Mac amps (which all do slighly better than their specs), as well as some larger amps over 100W.
For the unsigned amp with one channel driven:
Right Channel
0W at 20Hz 0.99% THD. At around 5.5W, the noise coming out of that poor output transformer is too unsettling to continue the test.
15W at 35Hz 0.99% THD. At this power level, the transformer rattling is just starting to happen.
16.8W at 1kHz 0.99% THD.
Left Channel
0W at 20Hz 0.99% THD. This output transformer did not exhibit the gnarly noises.
0.03W at 35Hz 0.99% THD. This is usually where I look for close to full power to quantify reasonable power bandwidth. (Note that a bad solder joint I later found and is pictured later contributed to this result and fixing that made the left and right channels almost the same)
15.7W at 1kHz 0.99% THD.
For the unsigned amp with both channels driven:
Right Channel
0W at 20Hz 0.99% THD.
11.7W at 35Hz 0.99% THD.
16.8W at 1Khz 0.99% THD.
Left Channel
0W at 20Hz 0.99% THD.
0.04W at 35Hz 0.99% THD.
15.7W at 1kHz 0.99% THD.
For the unsigned amp with brand new output tubes and the driver tubes from the other amp:
Right Channel
17.1W at 1kHz 0.99% THD.
Left Channel
16.8W at 1kHz 0.99% THD.
So tubes obviously aren't a problem here, but maybe this amp that doesn't have its face plate, is missing a signature on the cover, and has an output transformer that's prone to rattling just needs repairs and isn't a good sample? Well let's try the other one!
For the signed amp with one channel driven:
Right Channel
0W at 20Hz 0.99% THD. Unpleasant noises started happening around 9W.
17.4W at 35Hz 0.99% THD.
17W at 1kHz 0.99% THD.
Left Channel
0W at 20Hz 0.99% THD. Unpleasant noises also happening around 9W.
11.7W at 35Hz 0.99% THD.
16.8W at 1kHz 0.99% THD.
At this point I don't really think I need to try driving both channels on the second amp to see what/if anything improves.
Another question is what things look like if you push past 1% THD, and you can certainly get to 75W with this amp and THD is 2.5% at 1kHz. At 35Hz, this amp makes 37.5W at 55% THD hard clipped.
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Here's a scope trace at 32W/35Hz, THD here is 27%.
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Here's a scope shot at 8W/20Hz, THD here is around 22%.
I decided to also check the claim of the amp being flat from 20Hz to 20KHz and at 1W nominal the amp is -1dB at 16.5Hz and 20.2kHz.
Output impedance measured to 2.7 Ohms, not 1.7 Ohms.
At this point I was DYING to pull the covers off the iron on this thing, and so I did!
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Seeing this is a little relieving in terms of what I've measured, as my numbers and Edcor's numbers agree pretty well. In the first picture of the center OT, one of the white wires was loose and this was causing measurement problems in one channel. Upon resoldering them I found the channels were pretty well matched.
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I pulled the PT cover off too, and for the ~8A of heater current required for a 6.3V winding, 5.5A seems to be pushing things. Similarly, the power supply is a full wave doubler with around 110mA of DC current drawn between the output tubes and the driver stage. With 330uF doubler caps, estimated draw from the HV winding is 440mA AC. More current means more heat, and this is under a transformer shroud.
I have these loaners for a few more days, so if there are other measurements anyone is curious about, I'm happy to take them. I probably should evaluate output at 4 ohms, since the specifications also claim 75WPC is available in that configuration as well.