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In Reply to: Dynaco ST-70 rectification question posted by airtime25@comcast.net on November 10, 2002 at 14:29:59:
My recommendation would be to buy a plug in diode rectifier and give it a try. They can be had for about $10 or you can make one yourself for the cost of two diodes.Your B+ will shoot up a bit and you'll hit max voltage very quickly unlike with a tube rectifier. I tried it and really, really, really didn't like it.
John
Follow Ups:
Charles,I've seen many guitar amps which had a couple of SS diodes soldered to the rectifier's lugs AND the tube recitifer still in place. I suppose this is to get some of the benefits of both worlds. To me, the tube rectifier's "spongy" attack or sag is lost---so why not just replace it with a complete SS recitification, if more bass or loudness is what you what?
For playing six-string guitar, you kinda want the amp to pump or "breathe" which the tube rectifier allows. But, for use in hi-fi perhaps the SS-rectifier is better, since there is less sag and therefore less overall distortion.
BTW---all of the amps I've seen with SS diodes replacing OEM tube types have had blown filter caps. Like John said the B+ (and other VDC) will go up and be instantaneous---so probably best to change the first filter caps perhaps to two caps, in parallel, with higher capacitance at a healthy voltage rating (like 400 volts).
Gerald Weber suggests two 220 mfd, 350 volts caps in parallel to replace the OEM two 70 mfd caps, in a tweed Fender Bassman. 220 K, 2 watt resistor across the two caps to divide the voltage evenly. Then, the first set of caps would have a total filtering capacity of 110 mfd with a 700 volt rating.
As others have already stated (or implied), replacing the 5AR4 with SS rect will raise the PS output voltage considerably. In the case of the ST70, you will most likely exceed the safe operating voltage of the filter caps. This is especially true considering that in most localities in the US at least, the mains voltage is now routinely 120+VAC compared to the nominal 117VAC when the ST70 was current. If you want to experiment with SS rect you need to up the voltage rating of the PS caps. One way of doing this is to put two identical caps in SERIES, not parallel as stated above (a slip of the keyboard, I'm sure). The cap will divide by 2 in value while the voltage rating will double. The resistors can be placed in parallel across each cap in series to equalize voltage drop. Another way to do this is to get the SDS labs cap board kit from triodeel, a sponsor.BTW I'm not steven oda but yet another Steve O.
Steve,Thanks, for the clarification.
steve
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