| Posted by Dave | March 03, 2015 - 02:00 pm |
| Hi Joe -- Thanks for the kind words -- and I'm so glad you got your problem resolved! As for your tone control bypass modification, it can be place where the current tone controls reside (between the volume and balance controls). The series resistor would go between the wiper of the volume control and the "top" of the balance control, while the shunt resistor would go from the top of the balance control, to ground. Good luck with your project! Dave |
| Posted by Joe Janus | March 02, 2015 - 08:15 pm |
| Hi Dave, I wanted to let you know, and its taken several months, but I was able to solve my "howling" nfb problem. Thanks much for your on target direction in resolving this. Although I can't say I isolated the problem, I did manage to clear it up by undoing the output binding posts and replacing them with the original output terminal connections along with a few other associated connections from (Z-565, PC-10a and grounds..) Over the last few years I have read many of yours posts and responses on various sites and forums, and I must say you are worth your weight(10x fold) in gold plated, Teflon, paper in oil, silver hazed, etc components to guys like me. All the best Joe now if I could figure out where exactly the series and shunt resistors go (or preferred alternate method) when bypassing the original tone controls, then I could ask myself and my ears if this endeavor was all worth it.... and perhaps just relax and just listen to the music.... the final destination.... |
| Posted by Dave | September 28, 2014 - 07:29 am |
| Hi Stephen -- Congrats on your successful installation of EFB! Your perceptions have been shared by virtually everyone who has installed this modification in the small Dynaco amplifiers. The improvements are both readily audible, and easily demonstrated in the lab as well. The conversion to fixed bias operation changes the fundamental operation of the output stage, while EFB action basically makes it immune to any fluctuations in power supply operating voltages. With the improved match this creates between the output tubes and output transformer, more power can now be developed, and at notably less distortion as well. In addition, with the lower quiescent current your output tubes now operate at, they will last longer, and the unit will operate cooler as well. Adding EFB to the small Dynaco amplifiers really is one of those few modifications where the results belie the simplicity of the modification, and the improvements gained are had without compromise. Congrats on your success! Dave |
| Posted by Dave | September 28, 2014 - 07:12 am |
| Hi Joe -- From the symptoms you have now described, it is obvious that the negative feedback circuit is now producing positive feedback. There are only two things that could cause that to happen in this amplifier: 1. The leads from your Z-565 OPTs are no longer connected the way they used to be. This includes both the connections to the output tubes at pins# 7&9 (now pin# 8 with 100 ohm Screen Stability resistors installed), and the connections to the output terminal speaker connections. If ANY ONE of the leads from these transformers is not connected properly, it will cause the problem you are having. 2. The connection to pin #2 of the output tubes have become reversed from their original connecting points. This two will cause the problem. It is a virtual certainty that the problem will be found in one of these two areas. And whatever it is, has been duplicated in both channels. You said you have already verified the secondary connections of the OPT, and the proper grounding of the black leads, so I would now concentrate on the output tube socket connections. Dave |
| Posted by stephen zaneteas | September 28, 2014 - 04:24 am |
| Plugged the sca-35 into a variac and gradually brought up the juice. Next, plugged in the DMM and adjusted bias to .267 volts. Using a very loud and busy mix from an xbox360 racing game plugged into an 24bit/96k upsampler...well...the sound was awesome. S U P E R WIDE freq response. Basement solid bass with a sweet upper high end. Compared to the pre-mod Dynaco version there was significant reduction in distortion. Even at full power. Lots of space and air around the instruments. Superior channel tracking. I don't know why but I can discern the crappy audio channels from DISH and the superior ones.....Compression is SO obvious on many digital channels. They are not FULL DYNAMIC RANGE nor do they-in my opinion-compare to a true direct blu-ray playback. Popping in some 24bit transferred source material-ssimply stunning. Detail and a deep soundstage. Im happy......very......... |
| Posted by Joe Janus | September 28, 2014 - 12:00 am |
| Dave I would compare the noise to the sound of feed back, as in the day when a mic or guitar got too close to its amp, more like a howl ,, siren like without the rising in pitch.. and fairly loud thru both channels with the pc-10a inputs shorted. Substituting different tubes does not change this although pulling the 6U8's does quiet it completely. Joe |