Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. The sunn still shines online!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  
previous: TheElectricMoron Need some UL help -- 10/29/2003 6:01 PM View Thread

Here Y'are

10/30/2003 4:16 PM
TheElectricMoronHere Y'are
I have these two Fender UL amps with the cement resistors and the funny hard-to-find filter caps. One of them already feeds the screens from the rail with the UL taps disconnected. And they both have the full-wave bridge rectifiers.  
Soooo, if I yank those cement resistors, and the funny caps then I can put in an appropriate choke and the "normal" rail resistors and those pretty blue normal Spragues, right? The choke/regular filtercaps will kill all the ripple those resistors and BIG caps had to deal with will it not. I just want to remove some complicated layout/wiring and lead a simple life.  
And, even tho I do not anticipate doing this, can I elimininate the bridge rectifier by driving a 5U4GB off of each end of the PT secondary? A cool dual rectumfrier sorta setup? Just curious on that one.  
But I'm real serious about stuffing in the choke and the rail/cap mods. I appreciate your thoughts on this.  
Well, the intent is to clean the thing up and make it like a BF/SF. It semed to me that a good way to start would be to make the power supply like the earlier versions. Since the preamps and PI's are BF'd, why not the power end of it?  
Removing the existing filter caps and rail resistors and replacing them with a proper choke, appropriate resistors and filter cap compliment seems a relatively simple mod. Drawing from what I have read here the typical UL power layout is less than desirable with no proper choke and the older version is what is used in pretty much any other amp.  
Removing the UL taps from the screens and feeding them from the existing rail was no big deal, tho I did not detect any change. I suppose that the amp may have lost some small output but I couldn't hear it.  
What else can I add to that?  
I have to admit that in my ignorance I blew off all the admonitions about how these 70watters can't be BF'd and did it anyway. That and some extensive lead dress and snipping. And it all made a difference that was well worth the trouble. Is it a BF? Prolly no, but it sounds pretty damn good and is mucho better than it was. Will swapping the PT make it a BF? Prolly no, and likely would make no discernable dif in the sound by itself. Want sag?, I can put in a sagging copper cap for a rectifier too. I cannot see any dif in rectified dcv (less sag of course which has nothing to do with a decent PT), but how it is dealt with from that point, choked, filtered, dropped etc has the possibility of discernable change. That is what I expect to explore in this. I can check voltages and swap the resistors on the rail to get the voltages that get me in BF territory. If the plates and screens can tell the dif in PT's after those volt particles are mashed and mangled thru rectifiers and caps and chokes and resistors I would be crazy to know how that would be.  
I read it all again too and it does not seem that obscure to me, tho I admit that I sometimes leave a lot out.  
How about R&R post-rectifier B+ componentry, removing typical Fender UL circuit and replacing with typical Fender SF/BF circuit.  
1: remove non-choke fitted UL typical cement power resistors, and filter caps  
2: replace with Atoms, choke, and 2watt resistors on the rail for proper vdc at plates and screens.  
anybody got a simplier way to state this without circles, X's and O's?? I know that I don't ;)  

SpeedRacer OT is different, but can work. As y... -- 10/30/2003 5:07 PM