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Old 21st August 2013, 04:56 PM   #311
tomchr is offline tomchr  United States
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Quote:
Originally Posted by tubelab.com View Post
My mother had me tested, but the results were "inconclusive" so I was sent to a special school for the 5th and 6th grades.....50 years later the "inconclusive" results were obvious!
LOL. Love you George! Don't ever change.

~Tom
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Old 21st August 2013, 09:01 PM   #312
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Quote:
Don't ever change.
I hope not. My wife calls me the 60 year old kid.

It's been a while due to life's little interruptions, but I am finally back to playing with electricity again. I saw my 8903A hit 299.9 watts a couple of days ago....and this was from a single pair of sweep tubes. As the Mythbusters used to say....It's safe to assume that something's going to go boom!
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Old 21st August 2013, 09:19 PM   #313
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Quote:
Originally Posted by tubelab.com View Post
I hope not. My wife calls me the 60 year old kid.

It's been a while due to life's little interruptions, but I am finally back to playing with electricity again. I saw my 8903A hit 299.9 watts a couple of days ago....and this was from a single pair of sweep tubes. As the Mythbusters used to say....It's safe to assume that something's going to go boom!
oh no!
you and myth-busters in the same post!

i better get my fire extinguishers.

i think you should guest host that show.

anyone else think they got a dream job?
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Old 23rd August 2013, 12:25 AM   #314
Magz is offline Magz  United States
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Just a snapshot of the mirror-imaged drilled side panels for the left and right amp. The larger hole is for the RCA jack. I have low profile aluminum heat sinks on each where the Mosfets will attach to the chassis, to help with the heat dissipation (40+ watts from the one in the 833 cathode and another 6 from the one in the gyrator).

I tried to hang as much of the heavy stuff on the extruded aluminum sides as I could in order to reduce somewhat the weight that the top plate has to carry (still a LOT). I still need to do the other sides of each box and once I get the other parts back from FPE, they will go off to Landfall Systems for anodizing.

By the way, if you haven't found Heatsinkusa on the web yet, they are AWESOME! They have a wide variety of aluminum heatsinks, they will cut the lengths and widths exactly to your specs, they ship within a day, and their prices are very reasonable. That's where I got those low profile sinks from.

Extruded Aluminum Heatsinks from HeatSinkUSA

No affiliation, just a happy customer.
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Old 25th August 2013, 11:01 PM   #315
Magz is offline Magz  United States
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I was playing around with LTSpice FFT functions today, and found some interesting things regarding the harmonic spectrum in class A1 versus class A2. Thought you all might be interested.

As I've mentioned earlier in this thread, running the B+ at 2300V lets me keep the amp in A1 up to 40W output or so, before any grid current starts flowing in the 833. The first picture below is the distortion spectrum at 38W output into 4 ohms. As you can see, distortion is predominantly second harmonic, with the 2H at .45%. Below this level, the spectrum is consistently 2H dominant.

Once the amp goes into A2, however, the distortion spectrum changes, with much less 2H and more 3H. The second picture is the spectrum at 84W output into 4 ohms. Distortion is now predominantly 3H, with the 3H at .76%. This is what was seen in all analyses I ran with the amp in A2.

Anecdotally, I've read reports that A2 sounds a little edgier and a little brighter than A1, and those FFT results seem to reinforce that. In any case, most listening will be in A1, except for crescendos and perhaps drunken volume cranking...


Note: in order to get consistent, believable results from this LTSpice analysis, I had to disable all compression, force double precision math, and set max timestep to 1uS. Otherwise, there's a lot of hash and false peaks along the baseline.
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Old 26th August 2013, 12:07 AM   #316
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1u second huh?

is that the same as multisim time step? where it takes forever to simulate?

that is realy cool data. i am trying to soak all this tube data in a bit at a time and, your posts are helping thanks.
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Old 26th August 2013, 12:34 AM   #317
Magz is offline Magz  United States
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It definitely took longer to simulate with those settings, but the data made a lot more sense. Still it was only a couple minutes total.

Funny how what we think "forever" is keeps changing.

Showing my age, I remember sitting around for hours waiting for FORTRAN programs on a shared mainframe to run in school. Invariably they came back with "error on line..." messages, then it was back to the terminal (sans display screen). Of course, the older guys said I was lucky to have such modern technology...then told me their punchcard horror stories.

Now, I get impatient when my phone doesn't pull up a webpage in 3 seconds or less.
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Old 26th August 2013, 01:05 AM   #318
rmb is offline rmb  United States
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Magz...Remember how long a FFT (written in FORTRAN) took to run on a VAX11/780 around 1980 or so? Same code compiled under Intel FORTRAN runs within a few seconds on a Intel Core processor... Also remember running VMSSpice years ago...making up the run file sometimes took hours...Very interesting transition between A1 and A2 operation on your LTSpice...Haven't played with SE amplifiers much...would be interesting to see the FFT display on a push-pull AB1 type amp at various power levels...
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Old 26th August 2013, 01:28 AM   #319
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good point, i worked on a buddys pc, and my phone was faster and had more ram.

but it was fast enough for him.
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Old 26th August 2013, 10:06 PM   #320
tomchr is offline tomchr  United States
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Beware that the simulation is only as good as the models. If your tube model accurately models the tube characteristics and grid current in A2 mode, then you probably get good results. Personally, I've never been able to get accurate prediction of the THD of a tube circuit in LTspice. Especially when the circuit behavior near clipping (or A2) is simulated.

For DC and small-signal AC sims, I generally get a very good match between sims and reality.

~Tom
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