| 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 |
| Posted by Dave | September 26, 2014 - 08:51 pm |
| Joe -- It would be helpful to have more detailed information. For example, is the noise in both channels or just one? Is it loud or very soft in nature? You call it a high pitched hum, which is somewhat confusing: Hum implies a low frequency signal, but high pitched is a very wide open description. More accurate information then would be a big help. Normally, connecting the 16 ohm feedback connection would not cause the production of any noise, unless the leads at pins 7 and 9 of the output tubes are connected incorrectly. If your modifications included disturbing those, you might check for accuracy of these connections. The only other item would be lead dress. Input and output connections that come too close together can cause feedback problems as well. As a test, place a direct short across the inputs to your PC-10A boards. If the noise is still present, then at lease it has been confined to the power amplifier section. Dave |
| Posted by Joe Janus | September 26, 2014 - 05:50 pm |
| After redoing the ground and 16 ohm connections, the oscilating hight pitched hum remains. It would be nice to have a 2nd sca35 to compare readings at various undocumented points... but... it will teach me not to make to many changes at once... which in this case i did...(replace origial selector switch and mod to pc11, new output binding posts, 3 prong ground which I removed) but even with input to pc10 removed and tubes changed it persists... wondering if I did not cause harm with 3 prong AC attempt.. the hum does disappear when pc10 connections to 16 ohm output taps are removed which leads me to suspect the pc10 board components. ;Perhaps there are some readings and or method for isolating the fault(yea mine) here rather than by disconnecting and testing individual pieces and/or purchasing and substituting a new pc10.... If not, then so be it and let the work begin... |