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In Reply to: RE: Steve look at this. posted by Triode_Kingdom on June 12, 2016 at 22:18:30
Jack
I think of the old MAC amps like an LS motor with these kind of numbers.They are detuned at the factory but easy to make lots of power.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Follow Ups:
Michael, if I'm not mistaken, these amps are driven into AB2 at the output powers you're measuring. Mac's service lit says B+ on the 6L6s is +440 V at idle, but it drops to +375 at 30W output. AFAIK, that's not nearly enough voltage to generate 50W unless the grids are being driven positive. Would you agree with this assessment?
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Buy Chinese. Bury freedom.
Jack
Notice in my first post I said I was using the GZ34 rect which adds 40vdc to the plate but keep in mind that the choke along with the film cap supply will not let the B+ sag much even with the 5U4. I think we hit AB1 but AB2 only on peak transients.Doc Hoyer was getting 60 watts using a SS bridge along with the .32hy choke at 10 ohms.Original post: " The direct power reading is slightly shy of 50 watts with the GZ34 installed.This verifies the 19.9 or 20v reading in the 1st photo and if you square the 20v and divided by the 8 ohm load,you get 50 watts.
This test was done with the GZ34 installed running a strong pair of RCA 6L6gc black plates..Yes,the GZ34 does drive up the B+ but the amp is fine with it so long as you proportionally adjust the neg bias supply resistor.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 06/13/16 06/13/16 06/13/16
"I think we hit AB1 but AB2 only on peak transients."50W RMS out isn't a peak transient. Even with the GZ34, it doesn't appear to me that you have enough voltage to achieve that level in AB1. You didn't say what the exact voltage is at full output, but I assume you've measured it. I want to point out that the output tubes are being driven directly from the cathodes of the 'AX7s. Again, is there any reason to believe output power above roughly 30W isn't due entirely to the ability of this amp to operate in AB2? I do like the DC-coupled driver topology, wasn't aware Mac had used it.
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Buy Chinese. Bury freedom.
Edits: 06/15/16
No
It isn't a peak transient..I will take a B+ reading but I don't think two different analyzers are going to lie to us..Here is the thing..When using the GZ34,we are running about 473v on the second cap.Running that kind of B+ you can make 48 watts at low distortion with no sweat..I could even go higher but the distortion goes out of sight if you go over 50 watts.
The CJ MV45 amp made 45 wpc with a pair of EL34s.The Macs are monos running 6L6gcs with a boosted B+ and more stable power supply so 48 watts isn't out of the ordinary. Here is the MV50 which is similar to the mv45.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 06/15/16 06/15/16 06/16/16
Michael, are you saying that B+ under full load is 473V? That would be 100V higher than the factory number. Just to be clear, I'm not questioning your power measurements. I'm just trying to understand how the amp is getting there.
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Buy Chinese. Bury freedom.
Here is how it's happening...The 150 ohm resistor was taken out and a 30 ohm choke of .5hy was put in its place..Like the 10 ohm .32hy choke,it didn't really raise the static B+ voltage at that point but what did raise it was putting in the GZ34 which gained us an additional 40vdc. I didn't measure the B+ under full power,but I have to assume that the lower resistance choke is helping maintain the B+ at or near full power levels.
We then also have an amp that was designed to run at 117vac and my AC voltage runs at 121vac on a typical day.That also plays into the equation.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 06/17/16
All I know is what the HP and Tektronix analyzers both told me and both read identical power and distortion readings. I can tell you from experience that any Mac mono amp will beat power and distortion spec when it's running right with fresh stock value components.Just like the stereo CJ MV50 I posted that runs a pair of EL34s makes 45wpc and maybe more on measurement,a Mac Mono can running a pair of 6L6gc black plates with a higher B+ can do that same thing..I will take a dc measurement at the 48 watt rating
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 06/16/16 06/16/16
So, at what power level does grid current begin to flow? This would be across the peaks of course, so at what instantaneous power is grid current starting to flow?
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
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