A friend recently asked me the question, "are these OK to use ?"
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3084286355&category=4674 My long winded answer: No simple answer here. There is a school of thought that says tubes with indirectly heated cathodes need to be at temperature before the plate voltage is applied. That said a 5U4, 5Y3, or 5R4 will heat up, and supply B+ before the indirectly heated tubes warm up. Tektronix used solid state rectifiers in their old tube oscilloscopes, but they also used a time delay relay (abt. 30 seconds). The other issue here is the directly heated vacuum tube rectifiers had a fairly high series impedance, and the forward conducting diodes would drop a fair amount of voltage. In a capacitor input filter, this would lower the output B+ considerably from the peak value of the AC out of the HV secondary. If you stick in these solid state diodes, the B+ will be immediate, and likely 20-30% higher than before. This leaves a few options: a.. Switch the filter to choke input, and double up the output capacitor size. The B+ may be a little lower than before, but considering the higher AC line voltage these days, the B+ may come out just as the boat anchor designer intended. To get the AC ripple down to where it was before, you may need to increase the output capacitor to ~ 4X what it was before. *** b.. Do the above, but add a time delay relay. Put N.O. switch contacts from HV secondary CT to ground. **** c.. Stay with a Pi filter, but put a vacuum tube diode in series with the rectified B+ between the SS diodes, and the filter. A couple of options are the 6W4, and GZ-34. For the GZ, parallel the two sections. These are low drop indirectly heated diodes with a long warm up time (> 15 seconds). I like the 6W4 (dirt cheap TV damper diode) idea ran off the 5 Vac filament winding (longer warm up time). If the B+ is still a little high, add a little resistance in series with the tube diode (in 50 ohm increments). **** d.. Just switch the rectifier to a GZ-34, and add resistance to drop the B+ to where it was before. Don't overload the GZ-34. This is best in circuits that used 5Y3's. Russian GZ's cost about $12.00 each, and work really well. ***** e.. Leave well enough alone! ******************* Or would you rather I said, "sure, those would work great"! I didn't get into the SS diode reverse recovery issue (like what the Hex-Fred's try to address), PIV limiting (varistor across xfmr primary, R-C snubbers, etc.). -:) --- StripMime Report -- processed MIME parts --- multipart/alternative text/plain (text body -- kept) text/html The reason this message is shown is because the post was in HTML or had an attachment. Attachments are not allowed. To learn how to post in Plain-Text go to: http://www.expita.com/nomime.html ---
