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Old 9th April 2008, 09:35 PM   #1
Trout is offline Trout  United States
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Default Diodes On Rectifier Question

I was just looking over the service manual diagram for the Re-issue 1957 5E3 Deluxe and saw something I do not recall ever seeing before.

A pair of avalanche rectifier diodes inline with the 5Y3 rectifier tube?

BYD33V data sheet

What would be the purpose of these diodes in this location?

Trout
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Old 9th April 2008, 10:16 PM   #2
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Back EMF protection? Bizarre though, as the 5Y3 itself is a diode, but perhaps a pre-emptive strike in case of arcing from a cap further down the line (unlikely or what!! But flashover at >300V can't be nice at any time) and not to fry the trafo?
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Old 9th April 2008, 10:54 PM   #3
Trout is offline Trout  United States
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I found it odd, because the 1950's units of course did not have these, and many are still functioning perfectly after 50+ years.

I have built a few 5E3 amps from kits and from scratch and the only thing I see different is the diodes in the new units.

another noteworthy item would be the original units did not have a standby switch.
On all of the ones I built it is common to get a POP when flipping the standby switch.

On the Re-Issue unit they interrupt the HV CT, I have tried that setup as well as interrupting the B+ lead on pin 8 of the 5Y3, both techniques pop when entering standby unless you put a .05/630V cap across the switch.

Maybe they felt the diode was a cheaper way to protect the PT instead of a fuse?

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Old 10th April 2008, 02:24 AM   #4
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Quote:
On the Re-Issue unit they interrupt the HV CT
You have found the reason for the avalanche rated diodes. Some users had FRED diodes short out when flipping the standby switch on the SimpleSE amplifiers. One user had a 5AR4 self destruct. Some glitch capturing with a storage scope revealed some serious transients generated form the collapsing magnetic field in the power transformer. It is transformer dependent, and spikes of over 3KV have been seen on some Hammond transformers. The avalanche rated diodes allow most of the transient to be dissipated before becoming a lightning bolt inside the 5Y3.
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Old 10th April 2008, 03:31 AM   #5
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Sweeet, this might actually be useful information.

I have on several occasions seen arcs in the rectifer tubes on these old amps after adding standby switches. I'll have to order a few and try it out.

Thanks for the scoop.

Trout
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Old 10th April 2008, 04:10 AM   #6
Trout is offline Trout  United States
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Hmmmm,
anyone have a possible source for these diodes? I am not seeing them at my usual sources like Mouser, or Allied.

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Old 10th April 2008, 07:02 AM   #7
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radionics or farnell?
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Old 10th April 2008, 07:31 PM   #8
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Quote:
anyone have a possible source for these diodes? I am not seeing them at my usual sources like Mouser, or Allied.
Can't find those, but here are some other diodes with similar characteristics. I have not tested them yet.

Fairchild ISL9R8120P2 (got some from a SMPS project)
BYT52
BY228
1N5062
IXYS DSA2-1600 (too expensive)
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Old 10th April 2008, 07:58 PM   #9
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Add MUR4100E to that list _ I think Mouser may have those. 'E' suffix is avalanche rated for ONSemi (formerly Motorola Semiconductor).
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Old 10th April 2008, 11:16 PM   #10
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BYV96E is a very nice diode that I use a lot... Ebay is the best place I know to find it.

Datasheet:

http://www.ortodoxism.ro/datasheets/eic/BYV95C.pdf

Ebay:

http://cgi.ebay.com/Recitifier-Silic...2em118Q2el1247

(if this is totally not what you're talking about, I apologize in advance)
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