chinacave
Its Like a Jungle Sometimes; It makes me Wonder
Thanks Guys -
This has been a real learning experience, especially with the help and guidance of the amps' owner. Plus the comprehensive library he gave me of how to test, how to use test gear, maintaining audio gear
I have learned that very few DMM's can accurately measure AC voltage over the typical audio range of 20hz-20Khz. Most, including the Fluke #77, are limited to a range of around 40-400hz. I believe.
I did some guided research and selected the Fluke #87-V, with an ac range of accuracy from 20-20Khz. I have the 177, and thought the "True RMS" meant it would be accurate for audio use. I am still puzzled by this. Its pretty clear if you drill down into the specs on the Fluke website. (Look in Selecting a DMM Look for "Wide Bandwidth"
I believe that old VTVM's were made to have this wide band range of accuracy, but I have so far resisted the urge to restore the ones I have.
I verified this last night by comparing the .5vac out from my sig. generator (a General Radio tube unit that, once warmed up, is remarkably stable and the dial indicator is spot-on,) The .5v is the input needed to make rated output with the MC-40's. The #87-V held steady right out to beyond 20Khz. while the #77 and the C.C.C.* DVM/DMM's fell off between 400-4000hz. and became hopelessly inaccurate.
Having said that, I ran some tests on the rebuilt amp #2499 and at .5v in with sine and square waves, it was putting out 43-45 watts with NO sign of clipping and really nice square waves with a tiny (to me) bit of overshoot or ringing. On the low end, something is up either with the Sig Generator or my scope so that the square waves from 20-100hz. or more are tilted / distorted at the input, so I have little idea of the true shape at the output.
I am also not seeing the 80+/- vac between the chassis and house ground that I was before. I have NOT yet installed a 3-prong modern grounded cord to the unit, though some reading here and elsewhere strongly suggests I should - Any thoughts on this?
I also ran some initial tests on one of the two remaining to-be-rebuilt amps, #2500, with the tubes it has installed (tests on #2499 above were done with all new or NOS tubes.)
It seems to perform very well. It puts out 47-48 watts with pretty clean, though not as clean output traces as the rebuilt amp.
All tests were done with the voltage selector on the amp at the 117v setting, into 8.2 Ohm non-inductive 100W resistor with bolted on heat-sink (still got plenty hot!) at .5vac measured input with dual trace scope on input and output.
Pictures of 'scope traces coming.
Since taking pictures of the scope with a post-it note showing the frequency is a little....20th century, I am interested in what PC-based scopes / audio analyzers folks use or recommend. I really want to be one of the cool kids in this science fair. I snagged the circuit board for Pete Millets' sound card interface and will build it eventually Pete Millet Soundcard interface He uses AudioTester Web Site... but I don't have enough knowledge to judge its specs or features.
Would this program be a likely candidate to measure frequency response, distortion, THD, IMD, plot an output curve, and automate what I have been doing with much scope - sig. generator - meter - knob twiddling and sketchy digital camera work?
Anything would be better than what I am doing now.....
Edit - I read through the features description of the Audiotester and it seemed to answer one question I had - How to use the programs signal generator OUT function while simultaneously monitoring it on the programs scope or FFT analyzer - "Separate sound cards for in- and output possible." how does one implement this for the PC - gurus?
Thanks to all!
*Cheap Chinese Crap
This has been a real learning experience, especially with the help and guidance of the amps' owner. Plus the comprehensive library he gave me of how to test, how to use test gear, maintaining audio gear
I have learned that very few DMM's can accurately measure AC voltage over the typical audio range of 20hz-20Khz. Most, including the Fluke #77, are limited to a range of around 40-400hz. I believe.
I did some guided research and selected the Fluke #87-V, with an ac range of accuracy from 20-20Khz. I have the 177, and thought the "True RMS" meant it would be accurate for audio use. I am still puzzled by this. Its pretty clear if you drill down into the specs on the Fluke website. (Look in Selecting a DMM Look for "Wide Bandwidth"
I believe that old VTVM's were made to have this wide band range of accuracy, but I have so far resisted the urge to restore the ones I have.
I verified this last night by comparing the .5vac out from my sig. generator (a General Radio tube unit that, once warmed up, is remarkably stable and the dial indicator is spot-on,) The .5v is the input needed to make rated output with the MC-40's. The #87-V held steady right out to beyond 20Khz. while the #77 and the C.C.C.* DVM/DMM's fell off between 400-4000hz. and became hopelessly inaccurate.
Having said that, I ran some tests on the rebuilt amp #2499 and at .5v in with sine and square waves, it was putting out 43-45 watts with NO sign of clipping and really nice square waves with a tiny (to me) bit of overshoot or ringing. On the low end, something is up either with the Sig Generator or my scope so that the square waves from 20-100hz. or more are tilted / distorted at the input, so I have little idea of the true shape at the output.
I am also not seeing the 80+/- vac between the chassis and house ground that I was before. I have NOT yet installed a 3-prong modern grounded cord to the unit, though some reading here and elsewhere strongly suggests I should - Any thoughts on this?
I also ran some initial tests on one of the two remaining to-be-rebuilt amps, #2500, with the tubes it has installed (tests on #2499 above were done with all new or NOS tubes.)
It seems to perform very well. It puts out 47-48 watts with pretty clean, though not as clean output traces as the rebuilt amp.
All tests were done with the voltage selector on the amp at the 117v setting, into 8.2 Ohm non-inductive 100W resistor with bolted on heat-sink (still got plenty hot!) at .5vac measured input with dual trace scope on input and output.
Pictures of 'scope traces coming.
Since taking pictures of the scope with a post-it note showing the frequency is a little....20th century, I am interested in what PC-based scopes / audio analyzers folks use or recommend. I really want to be one of the cool kids in this science fair. I snagged the circuit board for Pete Millets' sound card interface and will build it eventually Pete Millet Soundcard interface He uses AudioTester Web Site... but I don't have enough knowledge to judge its specs or features.
Would this program be a likely candidate to measure frequency response, distortion, THD, IMD, plot an output curve, and automate what I have been doing with much scope - sig. generator - meter - knob twiddling and sketchy digital camera work?
Anything would be better than what I am doing now.....
Edit - I read through the features description of the Audiotester and it seemed to answer one question I had - How to use the programs signal generator OUT function while simultaneously monitoring it on the programs scope or FFT analyzer - "Separate sound cards for in- and output possible." how does one implement this for the PC - gurus?
Thanks to all!
*Cheap Chinese Crap
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