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Blackface Bassman filter caps

5/26/2000 3:15 PM
Rick Doebele
Blackface Bassman filter caps
I just got a '67 bassman head and it has a quiet fluctuating static. Will replacing the filter caps help this? At some point the caps were replaced, the new caps date from 1979 - 94. There are six capacitors and they are 16, 40, 20, 40, 50 and 100mfd, does anyone know if these are the correct values? I can't seem to find a replacement listed anywhere for the 50mfd, can I use a 47? Any suggestions and advice for this beginner greatly appreciated. Thanks  
Rick D.
5/26/2000 5:20 PM
Ted Pimm

Rick, those values appear high compared to either the AA371 or AB165 values. If yours is a 67 it is probably one of these. Check the schematic and you'll see it calls for a pair of 'stacked' 70uf's as the main supply filter and interstage values of 20uF. The schematic calls for an 8uf as the last decoupling cap but mine was 16 or 20uF if I recall correctly.  
I doubt the sound you are describing is related to the filter caps. Problems with these will usually create a 60Hz HUM not a higher-frequency static sound. Could be thermal noise, poor layout (heater induced noise into grids).  
Be safe if you're new to this and read the FAQ.  
5/26/2000 7:09 PM
Rick Doebele

Hi Ted,  
You are right according to the schematic it shows two 70uf's... three 20uf's and an 8 (if I am reading this thing correctly) which is why I was confused to find what I did under the hood. If the sound is not related to the filters I'll just leave them.  
Thank you. Have a good Memorial Day Weekend.  
Rick D.
5/27/2000 6:12 AM

The noise you are hearing is probably either from bad preamp tubes or noisey 100K plate load resistors.  
Try changing the tubes first and then go after the plate load resistors and use new 100K 5% carbon film or 1% metal film instead of 10% carbon comp.  
5/28/2000 12:22 PM
Carl Z

In addition to the suggestions already listed here, it sounds like somebody did a real hack job when recapping that amp. I would strongly suggest replacing them with the correct values. A 20 yr old cap here is about done for anyway.  
Bassmans were a little weak in the filtration department so I would suggest replacing the whole lot of them with 20uF Spragues and the input caps with a pair of 100uF/350v caps. I think if you did this and replaced the plate resistors as per bruces suggestion you'll have an amp that's running mighty quiet and stays together a little better in the bottom end without getting too stiff.  
Carl Z  
P.S. Don't use Illinois caps. They're total shit!
5/31/2000 6:45 PM
Rick Erickson

"P.S. Don't use Illinois caps. They're total shit!"
Eloquently stated but not very informative. What exactly don't you like about Illinois Caps?  
6/1/2000 1:22 AM
Peter S

I'm probalby speaking out of turn here(as usual), but I hate to see a question go unanswered. I think what Carl was trying to say was that the quality of the Illinois caps is not very good compared to other brands. They are cheaper than the Spragues, but the old get what you pay for....probably applies here. I have found that after a few years, these caps tend to get leaky. For just a bit more money, the Sprague caps are rock solid and last a very long time.  
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