Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dude on September 02, 2016, 06:00:47 pm
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I've been reading about diode rectification and can't seem to find an answer for:
Why is it that some full wave rectifiers have two diodes in series on each leg of the PT and some amps only have one diode on each leg?
al
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The practice (of doubling them up) doubles the peak voltage rating of the diodes used. There are sometimes odd conditions that result during turn on and turn-off of an amp that produce large voltage spikes and it's a piece of insurance using what are very cheap parts. Each add'l diode drops .7 volts which is nothing in 350-volt land. Some part of this is from copying Leo Fender's practice from his amps, maybe diodes weren't quite as trusted then as they are now. Don't overthink it.
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Thanks, I knew that diodes drop some voltage but nothing as you confirmed. I figure something like what you said as the diode just converts AC to DC. It's like e-caps in series doubles the voltage rating, just insurance.
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Used to be rects only went to 400V, and 200V rating was much cheaper.
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You can also add diodes in series to the vacuum rectifier's diodes for the same reasons
Franco
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I've been reading about diode rectification and can't seem to find an answer for:
Why is it that some full wave rectifiers have two diodes in series on each leg of the PT and some amps only have one diode on each leg?
al
Google "Peak Inverse Voltage rating for diodes" and see what you get. The PIV is important. Adding diodes in series increases the available PIV buffer.
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You can see a photo of the old Fender rectifier diodes here (http://i1191.photobucket.com/albums/z461/BMW2002Ti/biassupply_zps3fc8cd73.jpg). If you look closely at several of the diodes, you can piece together that the part number is "RCA 35604" as confirmed in this thread (http://el34world.com/Forum/index.php?topic=9642.msg88014#msg88014).
I didn't know 6 years ago (or didn't search) to find out the 35604 has a 400v peak inverse voltage rating (http://www.weisd.com/test/GenericParts_WEISD_view.php?editid1=35604). So 3 in series yields a 1200v PIV rating.
These days, common power supply diodes are 1000v PIV, so one might be enough, but two for 2kV PIV will certainly do the job. Everyone using 3 in series are simply copying the original Fender fiberboard and not really thinking it through...