Advantage or disadvantage to have first filter before or after the standby switch

Muttlyboy

Gold Supporting Member
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3,093
I changed out the power transformer on my fixer-upper 1977-78 Deluxe Reverb, and see that on this amp, the first filter caps are wired after the standby switch.

It looks like all of the AB763 schematics show the first filter before the standby switch.

I'm wondering if if matters for any reason...Anyone have any thoughts?

Thanks
 

JJman

Member
Messages
994
Never noticed that change. The 77-78 would cause a much larger sudden draw from the rectifier when switching into "play" mode from an initial power up. Probably not a problem for a 5u4gb but I like the earlier approach. I think some amps have the standby between those 2 caps, so to speak.
 

pdf64

Member
Messages
8,910
Hot switching the reservoir cap onto an operational rectifier is poor practice, as it will likely result in excessive surge current (ie breaching the rectifier plate limiting value) when initially charging the cap.
 

Muttlyboy

Gold Supporting Member
Messages
3,093
Thanks for the advice.

I thought that I may have read about hot switching being a stress on the switch, but it seems like a more important issue regarding stress on the rectifier. (May be a bigger issue with a 5R4 with 44uf first cap)

I'm going to rewire, and probably change the filter caps to all 16uf
 

pdf64

Member
Messages
8,910
I may have read about hot switching being a stress on the switch
No, it's the other way (switch between reservoir and choke) that puts a massive inductive votlage spike on the switch.
I think Merlin explains that.
He notes some ways around that, but a simple one is to just put a suitable resistor across the switch, as per the 100k 1W in the PS400 http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-400PS-Schematic.pdf or the 47k 2W suggested by Merlin. Such a resistor also mitigates for the (probably miniscule) risk of cathode poisoning in standby, by providing a tiny bit of plate current.

If moving the standby switch in your amp, the reservoir caps will almost certainly need uprating from 450V, it may be that even 500V working/550V surge types won't quite be sufficient, and we're back to the likely reason that standby was introduced.
 
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