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In Reply to: RE: Well all the parts finally got here...... posted by vinnie2 on December 02, 2016 at 15:14:05
Well, we have good news and bad news..... I got the right channel all converted to AC using the halogen transformers on the 813's and a standard filament trans on the 26. I installed a 100uf cap across one pair of diodes on each halo because that is what I had. See pic. The good news is the hum is quite low with the caps in place. The bad news is even though it doesn't sound too bad, it has a distinct edge to it, a bit on the harsh side. Not sure why that would be. If anyone has any ideas I would sure like to hear them.
The next step is to do the left channel with dc and see if comparing the two channels with a mono source confirms what I think I am hearing now. I hope it can be figured out because as you can see from the pic the total package with the halo trans has a pretty small foot print. I know on past amps I have built I always thought they sounded better with ac on the filaments, but then I wasn't using honker transmitter tubes then either.
Question for Ray P: First thanks for the procedure you outlined in that post, it worked great and was not too difficult at all. I am also wondering how you came up with the 560uf for the value of the cap you used and if it was intended only for reducing mains ripple or if it had some other purpose as well. Thanks again!
Edits: 12/05/16Follow Ups:
The 560 uf was a recommendation from Alex, see link below. You have to scroll down a ways to get to the section on AC heating.
'270 to 330uF/400V cap (for European 230-240V AC mains) '
For 115 - 120 AC mains, he recommended twice the uf value and 200 volts. I happened to pick 560 uf.
ray
Thanks for the link. I have ordered some 560 units and will give one a try when they get here. I will be curious to see what difference I can hear.
Edits: 12/05/16
There is one further issue one might want/need to worry about, if one modifies the operation of the halogen switcher by putting a smoothing capacitor on the DC side of the bridge rectifier that is connected to the mains supply. To the extent that the smoothing capacitor actually smooths the rectified AC to make it DC, this will mean that the time average of the voltage seen by the switching transistors will be considerably greater. That is to say, instead of them seeing a pure rectified sinewave, they will be seeing a steadier continuous voltage that is close to the peak voltage of the pure rectified sinewave.
The upshot from this would be that, rather than producing a strongly modulated square-wave output that reaches, say, 35V peak-to-peak at the maximum of the mains voltage, the unit will now instead be producing more nearly a steady and unmodulated 35V peak-to-peak square wave. This will mean the true rms voltage of the output will be substantially increased, relative to what it would have been with the unfiltered rectified mains feeding the transistors.
Now it seems to me that 100uF would still be giving quite a significant amount of ripple on the rectified supply to the transistors. But probably there will be a fairly significant increase in the rms voltage on the output side. And a larger smoothing capacitor on the rectifier would give a much steadier DC voltage and hence a further increase in the rms output voltage from the unit.
I'm looking forward to seeing the scope trace of the modulated output from the switcher (preferably first without the capacitor modification, and then with it).
Chris
"The upshot from this would be that, rather than producing a strongly modulated square-wave output that reaches, say, 35V peak-to-peak at the maximum of the mains voltage, the unit will now instead be producing more nearly a steady and unmodulated 35V peak-to-peak square wave. This will mean the true rms voltage of the output will be substantially increased, relative to what it would have been with the unfiltered rectified mains feeding the transistors."
That's an interesting point. The earlier units I purchased didn't output such a high-value square wave. I'm wondering now if the manufacturer has increased it in order to accommodate the 120Hz envelope and produce a more correct RMS value. I never worried about it because I was filtering the DC. The amplitude of the square wave itself was representative of RMS output. Thanks for bringing this up!
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Buy Chinese. Bury freedom.
Hey Vinnie
If you have a 50 uf to 100uf plastic or oil cap my try it instead of electrolytic cap. Those things can sometimes make things sound nasty???
Tom
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