Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on February 14, 2024, 07:00:46 am
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This Fender schematic has the standby switch prior to the first filter cap on the B+ rail.
Fender_deluxereverb_a1172.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxereverb_a1172.pdf)
This Fender schematic has the standby switch after the first filter cap on the B+ rail.
Fender_deluxe_ab763_schem.pdf (el34world.com) (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_ab763_schem.pdf)
Is one approach "better" than the other? Would a solid state rectifier make a difference as to which approach might be "better"?
With respect, Tubenit
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Looking at all the different trains of thought out there over time, I kind of landed on the thinking that a SB switch between the reservoir cap & the second cap was the the preferred location as it kept the largest caps charged, as to limit in-rush current.
It is probably moot with a SS rectifier & beyond that opens the door to discussion about using a SB switch at all.
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i'm in the not at all group. unless you leave the amp on for hours n hours, simply power on, wait a minutes, strum til "oh ya" Used the $12 in savings to buy a loaf of bread
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I just built a mini Silver Jubilee clone and it was after the first cap (SS rectifier but not sure it matters). I would agree with above that it's probably to keep first cap charged. Maybe easier duty on the standby switch? Red-lined schematic is all I have.
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After the first cap seems to be the consensus if one needs to be used at all. Rationales are explained better elsewhere but the short version is hot swapping a cap onto the output of a warmed up rectifier tube is a good way to shorten its life.
I asked a more nuanced version of this question but didn't really get a good answer. Plates and screens are the two highest current draws. The plates are powered off in the typical "after the reservoir cap" standby implementation. Additionally, the physical switches used for standby are not rated for the DC voltage they will see in this application. Presumably inrush current is related to the steady state current after the transient. By installing the standby switch after the B+ (OT CT) at the near side of the choke the plates will be active when on standby. This reduces the steady state current switched by the standby and (in my mind) reduces the stress on the switch itself and accomplishes the same function of muting the amp and keeping the tubes warm.
I'll probably have to get a good high frequency daq and compare the two installations directly to get a good answer to if this is really an improvement.
I've attached a schematic Sluckey drew to better convey this idea.
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i'm in the not at all group. unless you leave the amp on for hours n hours, simply power on, wait a minutes, strum til "oh ya" Used the $12 in savings to buy a loaf of bread
+1
I don't include them either unless it's an existing amp &/or I am using a "traditional" backplate that already has the hole & label. Sometimes not even then.
I DO use the mute switch on my bass rig often; makes more sense to me, it's on the front panel, etc.
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Back when all of this was new territory to me, I implemented standby on my Eico ST70 by switching the 5VAC line to the rectifier. The stock 5AR4 rect. tubes were not as readily available then, so I swapped in a 5U4GB for the failed original part. It also tolerated the extra 100uF photoflash cap I'd wired in to the 1st pole.. ;-)
The ST70 is still running today with the same 5U4, Afaik.I've recently moved into the "not at all" camp, at least in the majority of cases.
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I don't have a standby on any of my 3 amps including the one with solid state rectification.
However, this amp is for a friend who wanted a standby switch. I did mine like Sluckey's.
Thanks for the replies! With respect, Tubenit