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 Post subject: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 2:09 am 
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Joined: Nov Fri 15, 2013 10:17 pm
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Location: Haddon Heights NJ
I have been looking at several schematics of guitar amps and something on solid state rectifying circuits caught my attention. In circuits like the Bassman-50, fender use 3 diodes in series to for the full wave rectification. https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Bassman-50-Schematic.pdf. Is this done because at the time weren't diodes that can wist and high reverse polarity? Or was something else that make that selection better? Could those 3 diodes be change for one like a 1N4007.

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 Post subject: Re: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 2:17 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: San Jose, CA USA
A single 1N4007 in each leg of the rectifier would be somewhat marginal, although most likely it would survive. The transformer in the example shown is 660 VAC centertapped. The peak reverse voltage seen by each leg of the rectifier for that type of circuit is 1.414 x 660 VAC = 933 VDC. Voltage can be a little higher if your line voltage is higher than the 120 VAC specified on the schematic. So I'd recommend using two 1N4007 diodes in series for each leg of the rectifier in that type of circuit.

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 Post subject: Re: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 2:32 am 
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Location: La Mesa Califonia
On guitar amps there is a big surge in current draw from sitting idle to a full loud attack. If one diode shorts out the other two still work rectifying. Thus no fuse blowing or smoke to ruin the show. Also an outside chance the player might get fried on stage due to live AC in his hands.


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 Post subject: Re: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 2:43 am 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
It's not obvious to me what the original diodes are. Back in the late 60s, it was common to stock 400 and 600 PRV diodes in repair shops (worked at Dr. Bob Casey's TV Hospital). 1000 PRV diodes were pretty expensive... maybe over $1.00.

For 660VAC, three 1N4007 in series should be fine. In a 2-diode rectifier, the peak diode voltage is 2 x peak AC. 660 x 1.414 x 2 = 1866 V.

Image

For relative comparison, Zenith used to buy 212-74 (400 V) and 212-76 (600 V) about 300,000 pcs. per month in 1970. The -74 went for about 9 cents and the -76 for about 11 cents in 100K lots. The nearest equivalent, 1N5054 @ 1000 PRV fetched about 50 cents in volume.

Rich


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 Post subject: Re: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 2:46 am 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
I made a mistake. The PRV of the diode in the CT version is 2X but the AC voltage is half of the secondary voltage. So two !N4007s should work fine.

https://www.elprocus.com/full-wave-brid ... rectifier/

Rich


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 Post subject: Re: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 7:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Seattle WA US
Nice chart, but to get those ripple frequencies you need to visit someplace with 50 Hz power mains.
-Chuck


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 Post subject: Re: Series Rectifying Diodes
PostPosted: Dec Sun 23, 2018 3:43 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
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Location: South Jersey East of Philly
Quote:
Is this done because at the time weren't diodes that can wist and high reverse polarity?
This amp is from the early '70's, so diode choice wasn't limited by then.


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