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In Reply to: RE: Here is the whole build manual and schematic for the heathkit AA-100 posted by burwoodc on January 24, 2015 at 13:48:54
"However I want to use my heath aa-100 outputs and el84 so reduced voltage in the circuit."
What is the primary Z of the AA-100 OPTs? Nearly all 7591 amplifiers use OPTs with a 6.6K plate-to-plate primary. Unless the AA-100 in unusual in this respect, those transformers may not be a good match for EL84s in pentode mode.
"EFB" is an acronym for "Enhanced Fixed Bias." The name is trademarked, and I think the designer sells PCBs. The basic idea is that amplifiers with cathode bias, which are at the mercy of varying tube current to create bias voltage, can be easily changed to a more stable bias system. The author uses the LM337 voltage regulator for this. In addition to stabilizing the bias voltage with respect to tube current, the regulator works from resistively-divided B+. This means cathode bias will track changes in B+ voltage. Pretty clever, actually, and the net effect in terms of bias stability is similar to amplifiers using fixed negative bias, but at less cost. The only downside I see (from an outside perspective) is that the circuit elevates the cathodes above ground. In comparison with fixed negative bias, less B+ is available to the output tubes, and that reduces output power. Of course, there's no such penalty when the circuit is compared to amplifiers using cathode bias, and it's as an improvement to those amplifiers that the author intended EFB to be used.
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Buy Chinese. Bury freedom.
Follow Ups:
"What is the primary Z of the AA-100 OPTs?"
I don't know? I just happened to come across the thread by this Dave fellow who said the transformers were excellent candidates for a Fisher sa-100 clone. I do not own a variac and have really no way to test my transformers.
However It seems he has used them successfully.
http://www.audiokarma.org/forums/showthread.php?t=466229
Thank you for explaining the EFB
"What is the primary Z of the AA-100 OPTs?"
I don't know? I just happened to come across the thread by this Dave fellow who said the transformers were excellent candidates for a Fisher sa-100 clone. I do not own a variac and have really no way to test my transformers.
However It seems he has used them successfully.
http://www.audiokarma.org/forums/showthread.php?t=466229
Well, if you're intent on building an amplifier like this from scratch (and it's no easy job), you should want to know this. It's one of the fundamental specifications, no different than matching the power transformer to the voltage you want from the supply. Maybe the guy who built the other amp will tell you if you ask. Incidentally, based on the numbers reported in the thread, he's running the EL84s *extremely* hard. I'm skeptical as to the benefits of that particular marriage.
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Buy Chinese. Bury freedom.
Thanks for your opinion. I will tread slowly and learn what I can about this
clone project. I actually have not digested everything in that thread and as I said, I have not seen his amended schematic. I can't open any of the images he posted. I'm waiting to be given clearance as a member there still.
...Attwood states 8.9K P-P, SAMS states 7.5K and I meas 7.5K. I think Atwood is in error.
Thanks for the info Steve-ohere is a quote from the thread made by Dave below,
I have conducted numerous tests on both of the SA-100's OP transformers from the unit I have here, and can say without a doubt that Sams has done it again. In particular, the transformers on the SA-100 are in fact 7800 ohm units (7831.47 ohms to be exact) AS MEASURED USING THE FULL SECONDARY winding. This is the ONLY way to measure the primary impedance of a given OPT, as it is the full secondary winding that forms the basis of a transformer's design to begin with. The 4 & 8 ohm taps then are simply just that -- taps on the full 16 ohm winding configuration. The 16 ohm winding is NOT a derivative of the 4 or 8 ohm windings. Therefore, it is always the full secondary winding that is used to determine primary impedance.
Because of the multiple windings and highly interleaved winding process used in high quality OPTs, the 8 ohm tap cannot always be placed at exactly 8 ohms, but is a close approximation. This usually results in the 8 ohm tap reflecting a slightly lower impedance, and the Fisher transformers used in the SA-100 are no exception. In this case, the 8 Ohm tap reflects 6961.3 ohms from an 8 ohm load, which may be the tap that Sams used to generate their data from -- which in any event is wrong for the reasons I have indicated. Either way, their data is "a bit off" regarding these transformers.
Using the 4 ohm tap for measurement usually produces far less error, as the 4 ohm tap simply uses 50 % of the windings used to make up the full 16 ohm configuration. Therefore, any error present then is usually quite small. In this case, the Fisher SA-100 transformers reflect 7901.23 ohms back to the output tube plates with a 4 ohm load placed on the 4 ohm tap. This is within 1% of the impedance reflected from the full 16 ohm winding with a 16 ohm load applied, while the 8 ohm tap only reflects 88.9% of the full secondary impedance.
In any event, since the primary impedance of the X-101 transformers is very close to this impedance, they would in fact work from an impedance standpoint, although with the LF limitations previously noted. And (as I know you know!), the NFB network would require some alterations as well.
If you are interested in building your own clone of the SA-100, I would recommend that you use the transformers I did, and have recommended to others to use -- who have now used them with success as well. My transformers came from a Heath AA-100 amplifier, and have proved to be every bit the equal of the original Fisher transformers -- in nearly every way. First, their impedance is so close as to be insignificantly different. Second, their lam stack is IDENTICAL in size to that of the SA-100 transformers (although they mount up differently). Third, they come from a pentode based 7591 amplifier rated for 25 watts RMS per channel, and finally, the measured performance of my clone shows them in fact to perform at least at the level that the SA-100's transformers will.
As an added bonus, if you follow the basics of my clone version, you also have all the NFB issues for these transformers worked out for you, eliminating that element of concern.
Once EFB II is installed in the SA-100 and its performance is improved, then we'll know for sure. But every indication is that whatever the SA-100's transformers could do, the Heath (Stancor) transformers can do just as well.
Edits: 01/25/15
One quirk of vintage OPTs is that the 8 ohm tap often isn't really 8 ohms, it's 9 ohms. When I measure an OPT I always base impedance on full sec winding, usually 16 ohm. I also check secondary ratios by applying 4V to 16 ohm tap. Doing this results in 4V at 16 tap, 2V at 4 tap and 3V at 8 tap if really 9 and 2.83V if truly 8. This situation is often overlooked. Also, my meas is based on turns ratios and doesn't include DC resistance of primary and secondary reflected to primary. This adds ~200ohm to the 7500 I noted.Edit: I just cranked the #s DG mentions he got for the Fisher OPT. If you take the 6961 ohms from 8 ohm tap, divide by 8 and multiply by 9 you get the 7830 ohms he measured. So apparently the 8 ohm taps on the Fisher OPTs are actually 9 ohms, just like the Heath OPTs.
Edits: 01/25/15 01/25/15
Per this site, the Heathkit transformers produce a superior 10 KHz square wave. http://www.one-electron.com/trans_p1.pdf
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