Fender Power Supply Q

[Justin Thomas]

Hey Everyone,

I just bought a Fender Concert 2 amp & have owned or own currently several other late seventies Fenders. They all have a B.A.R. (Big-Azz Resistor) in the power supply & I'm wondering if their purpose is as a voltage divider to drop B+ or something else? Concert 2 schematic segment attached. I'm looking at that 30K@20W after the 2nd or 3rd filter node in this particular example. Is this necessary?

Thanks,
Jusrin

[The Dude]

It's a divider resistor of sorts, yes. I think it was put in this amp to keep voltage from exceeding cap ratings in case of no tubes, open filaments, etc. Some of these had pretty high B+. Under certain conditions, downstream voltage could exceed cap voltage ratings.

[Justin Thomas]

Makes sense. I'm using series caps in the first 2 stages but using 2x47uF instead of their 70/20 combo. Otherwise it's get it to good stock working order before tweaking, as I believe it lived in a garage for many previous years... Thanks Dude!

Jusrin

[Chuck H]

I'm using series caps in the first 2 stages but using 2x47uF instead of their 70/20 combo.

FWIW when using equal value caps you should also have equal value resistors when doing the series cap arrangement in order to balance voltage across the caps equally. You probably already know this but I just figured I'd say it out loud anyway. I'd probably just go with 100k for both rather than the 120k/22k. Value chosen arbitrarily. Even though I've read about how and why these balancing resistor values are chosen it went over my head then and I can't remember it anyway.

[Justin Thomas]

Chuck,

Gotcha on that. I like symmetry, and I hate math (actually the biggest impediment to my amp skills probably). I remember reading something about it here too, but since I'm a single dude not counting every single penny, I just spend the extra $.50 on parts to avoid these shenanigans.

Horror of horrors, I also use those "cheap" "crappy" "modern caps" that cost me $.25-.50 indtead of $5 Spragues for cathode bypass & bias supplies. Haven't had any failures yet, so... Though I still like F&T for filters; they just look nice & are still less than half the price of Atoms. I thank Juan for that philosophy.

Jusrin

[Chuck H]

I stopped using Atoms over a decade ago. But not because of price. I actually had performance issues with them. Though I suspected stale caps from the vendor (Which was Mouser!) I just decided to look into actual performance specs for available brands. That's when I found that Sprague/Atom didn't have many published performance specs like others did. Someone here once mentioned that if a component manufacturer omits specs it's usually because they aren't good specs and they don't want you to know them.

When I first started doing mods I didn't know about favored brands for things like power supply filters and used a bunch of cheap Xicon caps in several projects. Never had any problem at all with those. But I did have problems with Atoms. So there you go.

I've used Nichicon radials for the last few amps I built and no problems so far. Way less expensive than Atoms, the specs look good and they don't omit any. Basically the Sprague/Atom caps lost my trust.

[Justin Thomas]

I use all of the above. Also been using the SunTans tubedepot sells. Generally, for my vintage Fenders it's Mallory & F&T; builds are "whatever." I also don't use ODs, cuz they're just too big. Huh, must be a Sprague thing. I ain't got space in a projector chassis for oversized caps...

Jusrin