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Old 25th March 2011, 02:56 AM   #491
labjr is offline labjr  United States
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Quote:
Originally Posted by CanAm Man View Post
6L6, Jim (and Woody, etc)......

I dug a litte deeper into Antek's published specs for there 18-0-18 VAC torroid core transformer families. All data I used is from the Antek website. I've plotted the data in the attached. The info for the smaller transformers (100, 200, 300 VA) is mostly FYI. Please note that for each VA model I've plotted the three data points in the Antek specs, except for the 500 VA model, where the third data point (for the most heavy current load) was clearly erroneous.

Some interesting observations. The 500 VA model, based upon Antek's own data, is not a strong candidate for the F5, considering it's cave-in under loads; a little surprising for a claimed 500 VA model. Note that the Antek data suggests the 400 VA and 600 VA models are ALMOST IDENTICAL in VAC/current specs, to include voltage sag under load, up through the "expected" operational range for a straight-forward F5. (I'm certain some will chose to debate what "expected" operational range ought to be, but I'm considing that to be about 10 amps/rail, and close to the stated 18 VAC secondary. For those who want to debate, please feel free to start a new thread).

Also note the "blue dot", at about 18 VAC and ~3 Amps--that's where my curent 400VA transformer is working......pretty far below the Antek data for that model.

In any case, enjoy. All, FYI..... Again, all the "hard data" is from Antek itself. I'd be interested in others' thoughts........

Ken

I've been criticized for questioning the qualiy of the Antek transformers. All I can say is, I'm glad mine is Plitron.
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Old 25th March 2011, 02:59 AM   #492
6L6 is online now 6L6  United States
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Well, the 600va is fine. I am quite lucky that I bought one that was "too big". No worries, no wondering if it's going to work. It's quiet, it's cool.

Last edited by 6L6; 25th March 2011 at 03:22 AM.
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Old 25th March 2011, 03:06 AM   #493
Lerg is offline Lerg  Canada
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Isn't each output device biased to 1.3A? Doesn't that mean that you are drawing more than 5.2A from the transformer?
In the F5 manual the amp as per Firstwatt a stereo F5 pulls about 180W from the wall, so for fun lets assume that the transformer has to provide about 160VA with a 1.3A bias per output device due to losses and such. That means you are drawing around 9A from the transformer at stock bias.
Measuring actual current in your circuit would tell you what you are really drawing in your amp, but I would say that it is far more than 3A.
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Old 25th March 2011, 03:07 AM   #494
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The Antek 300VA is a hefty chunk of core. It should do the
job nicely unless you're planning something special.

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Old 25th March 2011, 03:42 AM   #495
6L6 is online now 6L6  United States
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Here are the photos of the initial powerup and the final tweaking/tuning.

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The powercord with series lightbulb. Although everything was ok, I'm glad that now I have this in my toolbox.

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This is the photo taken before flipping the switch the first time... just in case something blows up...

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Previously I mentioned that one channel was not connected properly to the PSU, the wire on the pad just to the right of the screwdriver needed to be moved further to the right.

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I am extremely happy that I chose to use spades connecting the wires to the rectifiers - without that, it would have been extremely difficult to get the board back up where it could be soldered.

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WhooHoo!!!!! YeeHah! This is going great! Sure, I am pretty much at the end of the potentiometer's adjustment, but look at those numbers! Let's go to the other side and make it look the same...

...

... And the results are nothing short of disaster. It's so bad that I don't even take a photo. The other channel is will not bias to more than .2v with zero offset, and the most bias I can get is only .45v or so, and that's with .4v offset! Ugh.

So the question is posed to the Gurus, and they say to increase the value of resistor paralleled with the pot. (R3, R4)

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So out comes one lead.

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In goes another resistor.

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Twist

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Solder and trim. Easy! (Well, desoldering something off a PCB is never fun, but no damage was done to the board. Whew!)

Ok! Let's look at the numbers!

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Hmmm... That's it? .5v?? And this is at the end of the range of adjustment of the pot...

Well, the good news is that I did the resistor mod to both channels, and the 'good' one still biased up nicely, now with lots of adjustment left in the pot.

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This time I really do not want to solder the board again, so I'm going to cheat and add another resistor to the ones already there. So the recently twisted leads are de-soldered and un-twisted...

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And another added...

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And trimmed. Yes, it's not that pretty. Oh well.


...But it biases up beautifully! Hooray!!

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Here it is, all buttoned up and done.

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And the back.



And as NP said, "...And there you have it!!"



Oh, the Hiaku -

Building the F5
In solder fumes I smile
heat and balance sings


Or, if you are into 3-5-3

Pass amps are
in their special way
his and mine

Last edited by 6L6; 25th March 2011 at 03:55 AM. Reason: 494/30,554
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Old 25th March 2011, 04:40 AM   #496
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6L6, Jim....

Nicely done, and the finished amp looks great....!

(But the haiku could use a little work....!)

Ken
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Old 25th March 2011, 06:12 AM   #497
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Very nice, indeed, I hope to build a few Nelson Pass design amplifiers as I have a few 1.5 kva transformers from a DEC PDP 11/70 mainframe computer just sitting around wanting to be used for something. jer
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Old 25th March 2011, 08:10 AM   #498
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Congrats! Everything ended just beautifull

It's been fun and and interesting following your thread. I have picked up a few things handy. Just in case I'll be using connectors on everything possible before I solder stuff up. Seems wise to have a few resistors at hand, even if you don't know when they could be used, so I need to get an assortment. Troubled with adjusting the bias, are one of the things putting me off this build, and others for that matter. Also the lightbulb thing looks like a nifty lil' gadget. I would appreciate if you could either post here or drop me a mail with the "how-to"

Again, congrats with a nice build
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Old 25th March 2011, 12:24 PM   #499
6L6 is online now 6L6  United States
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Courtesy of Decibel Dungeon -

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Use a incandescent lightbulb, obviously...
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Old 25th March 2011, 12:42 PM   #500
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Pretty obvious maybe, but a practical thing is to mount an oldfashioned lampholder and some sockets on a piece of board, wood or otherwise.
Safest setup and at hands' reach for the next one.

Example pic : http://www.instructables.com/image/F...-Bulb-Load.jpg
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