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  #31  
Old 02-26-2012, 09:56 AM
neteraser neteraser is offline
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Man! These Jupiters look good. I'm going to get some soon.
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  #32  
Old 02-26-2012, 04:55 PM
Gearaddict Gearaddict is offline
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Was it the the Gold Lions? Or was it preamp tube? I'm having hiss issues with my amp after a retube.
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  #33  
Old 02-29-2012, 08:45 PM
guiltless guiltless is offline
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With using the "chill" spray, I found a few noisy carbon comps in the preamp sections and when I pulled V1 a majority of the noise disappeared. A few of the pots were also noisy when sprayed (even one of the new CTS's was noisy). So I have a few 12ax7's on the way, I ordered a compliment of Alpha Pots and some key Carbon Film resistors and come contact cleaner.

We'll see how it goes.
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  #34  
Old 03-07-2012, 06:14 PM
guiltless guiltless is offline
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Just a little update. I changed out all of the cathode resistors, except on the PI, with CF's, changed the 68k and 1meg input resistors with CF as they were noisy when tapped or cooled. Changed out all of the pots for Alpha 1/2 watt pots (much nicer feel).

In the end, the hiss dropped dramatically. There is still some, but everything has to be dimed to hear it. And, at that point I hear more hum than hiss.

I realized that when I did the "star grounding" I did not isolate the input jacks or the output jacks... I basically made the chassis in to one huge ground loop. So, I am going to go back and alter the ground scheme to something more like http://www.el34world.com/charts/grounds.htm tonight. My only issue is that the PT is a Voltage Doubling type so the HT Center Tap does not go directly to ground as you can see here. So that will be interesting. I'll let you all know how that turns out.
I am also looking into picking up a 20mhz scope for as cheap as I can (while still getting a working, calibrated one) so I can chase down any other little demons this thing may have...

Please note that this is not the schematic for this amp, but the schematic for what this amp started as. Though as I look at it right now... I remember changing the 220uf filter caps out for 80uf when I was modding the amp. I did this because: 1. The AB763 schematic uses 80uf (or similar) 2. I went from 4 output tubes in UL mode to 2 Output Tubes. I wonder if I am under-filtering the power section for the needs of the voltage doubling PT. Should I put the 220uf's back? (Crap. I think I threw the originals away. Oh well, they are 30+ years old. I can get new ones.)

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  #35  
Old 03-07-2012, 10:09 PM
guiltless guiltless is offline
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A little update. I redid the grounding scheme to what is on Hoffman's site. The hum is almost completely gone. And when I pull V3 (ab763 normal tube) there is absolutely zero buzz, even with every control dimed and the NFB switched out.

Both the ab763 and the Marshall side still hiss some when everything is dimed. But compared to the volume, it's negligible. I still plan on hunting down the buzz in the normal channel. It may simply be due to layout as that channel is right next to the in and out holes for the dog house.

Now it is time to hunt down guitar buzz that goes away when I touch the strings and gets louder when I touch the LP style pickup selector ('68 RI Tele Deluxe)
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  #36  
Old 03-07-2012, 10:48 PM
trobbins trobbins is offline
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If the hiss reduces when you short an input tube grid to 0V, then you may be at a level where you get some advantage from using lower noise resistors around the input stage (eg. 2W metal film).
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  #37  
Old 03-07-2012, 11:36 PM
guiltless guiltless is offline
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Quote:
Originally Posted by trobbins View Post
If the hiss reduces when you short an input tube grid to 0V, then you may be at a level where you get some advantage from using lower noise resistors around the input stage (eg. 2W metal film).
Thanks for the tip! I'll try that tomorrow!
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  #38  
Old 03-08-2012, 05:06 PM
guiltless guiltless is offline
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Any thoughts on going back from the 80uf electrolytics to 220uf? Does the coltage doubling setup need the extra capacity?
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  #39  
Old 03-09-2012, 11:05 AM
guiltless guiltless is offline
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Well, the Marshall 100w Superlead uses a pair of 100uf's for 100watts so I should be good with a pair of 25w KT66's or 30w 6L6's running the 80uf's.
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  #40  
Old 03-09-2012, 08:47 PM
trobbins trobbins is offline
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If hum isn't your problem, then changing the main filter cap can affect the sustain and character of the amp, so imho its better to stay close to original unless you actually want to change the 'flavour'.
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  #41  
Old 03-09-2012, 09:12 PM
guiltless guiltless is offline
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Quote:
Originally Posted by trobbins View Post
If hum isn't your problem, then changing the main filter cap can affect the sustain and character of the amp, so imho its better to stay close to original unless you actually want to change the 'flavour'.
Agreed. That is why I am going to stick with 80uf. The original "flavour" of the Bassman 135 was big, stiff cleans. I would like my amp to more closely emulate BF-Era for the normal, and Plexi for the Bass channel. The amp has more than enough power supply and OT capacity as it is; especially since I am running a tube rec sometimes and only 2 output tubes. I think bumping back up to 220uf would make the amp too stiff, especially since the 100w Superlead gets by with a pair of 100uf's!

Now the preamp on the Superlead runs a much stiffer power supply for the preamp part of the rail than I am, but again, that is to taste. I don't need the extra ripple rejection there as it is pretty quiet, at least as far as hum goes.
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Last edited by guiltless; 03-11-2012 at 11:46 PM.
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