| Posted by Ricco | May 07, 2012 - 03:28 pm |
| It seems the current configuration is using the regulator much like a follower to provide about 1.7V more at the cathodes than at the slider of the bias pot. It's making the output stage act more like a shunt regulator. If the line voltage goes up, it'll tend to load the supply more, stabilizing the B+ some, while shifting the operating point. It lacks the usual feedback path to actually regulate the cathode voltage. In fact, if the B+ falls some with current drain, the falling reference will reduce cathode bias voltage, increasing the current a bit more - a slight positive feedback bias situation. Wouldn't it work out better to simply replace the 360K resistor with a smaller one from the output pin of the regulator? That would also avoid most of the waste in the resistor, permitting use of a 1/4 watt part. The circuit here is a big improvement over simple cathode bias, since the bias doesn't rise excessively when power goes up. But one could get more power output avoiding the voltage lost in bias the cathode by using conventional adjustable (negative) grid bias instead. Some may have thought that difficult without a lower voltage winding for the bias supply, but It's easy to develop that voltage without a special coil or tap. A non-polar capacitive divider from one side of the tapped h.v. secondary can provide reduced a.c. to the bias rectifier/regulator. Since caps won't pass d.c. current, a half-wave bias rectifier would need an additional diode from the divided a.c. to ground (cathode to ground) to provide current flow on the other half-cycle. That approach can be very efficient since a capacitive divider doesn't dissipate power. |
| Posted by Lydia | April 10, 2012 - 04:47 am |
| Hi Dave, I have a question about the possibilty of bi-wiring an Dynaco SCA-35. I am in the process of building a set of Zigmahornets, and I want to Bi-wire them. I have taken note that the output transformers, have (original ones) 8,16 ohm connections. I am going to use a tang band 4" full range and on top or the speaker a vifa slight horn loaded soft dome tweeter. the tweeter will be facing the back wall from about 2-feet and it is on a 45 deg. angle too. The vent and full range will face the front where you will be listening and I can assure that this set up is going to be nice! when broken in!!. I want to hook up the full range to the tang band FR at the 8 ohm tap. used the 16 ohm tap for the tweeter but I don't know is what capacitor or resistor to use in that series positive line to the tweeter. any suggestions would be appreciated. P.s. I will be using your EFB eventually. but I believe the drivers being separated by the transformer itself should lend to less frequency distortion and dedicating an ohm for each driver without a crossover!. hope I can have this..... Lydia |
| Posted by Dave | March 14, 2012 - 10:22 am |
| Hi Bill -- The solution you propose is excellent. The 560K resistor will in fact be dissipating the lion's share of the heat, but it is very little heat to begin with. The original components were well overrated so as to minimize any bias drift from any heating of these components. In your case, the 560K resistor will dissipate just under .25 watt, so the 1 watt rating will be just fine. If possible, metal film units will be best, again to address maximum stability. Good luck with your installation! Dave |
| Posted by Bill | March 11, 2012 - 09:26 pm |
| Sorry, I have to say one more thing: with unequal values in parallel, I'm not sure if the 2W rating will be maintained as would be the case of equal value resistors. I think I need help on this one. |
| Posted by Bill | March 11, 2012 - 08:19 pm |
| One more thing: I can get these resistors in metal film and carbon film (not carbon comp). I'm assuming the metal film might be better in this application? |
| Posted by Bill | March 11, 2012 - 08:15 pm |
| I have been working on an original ST-35 with EFB for some time now and over heated a 2W 360K Kiwame resistor. All other resistors work fine so maybe that was a bum sample. Anyhow, this value of resistor is hard to source and I don't have room for (2) 180K's in series, but I can do something in parallel. I can get a 1M and and a 560K (1W both) for a value of 358.97K 2W. I guess that would be close enough to 360K? Thanks, Bill |