big bob Posted January 21, 2011 Share Posted January 21, 2011 So I've been cleaning up the sound city and I noticed it has a strange mod, which I have no Idea what it does.. Don't get me wrong, the amp sounds and works great.. just wondering what it does? because I have no amp experience please excuse my lack of technical jargon. I know the amp has been moded because of the alteration to the chassis. there is a large blue/green box that has been added to the works looks like this anyone have any Idea what it does? this is all I have ben able to find on line http://www.vintageamps.com/plexiboard/viewtopic.php?f=9&t=84673 looks like the same deal.. any thoughts are appreciated .. Link to comment Share on other sites More sharing options...
mars_hall Posted January 21, 2011 Share Posted January 21, 2011 Looks like a large oil-filled supply filter capacitor to me. Link to comment Share on other sites More sharing options...
bolero Posted January 21, 2011 Share Posted January 21, 2011 I'm no expert either but that makes sense, and is exactly what the guys on plexipalace concluded: a 600v oil filled cap you may want to post there too, someone could probably even tell you who installed it!! there is a large community of tech enthusiasts over there Link to comment Share on other sites More sharing options...
big bob Posted January 21, 2011 Author Share Posted January 21, 2011 Looks like a large oil-filled supply filter capacitor to me. Any Idea what is brings to the party. Um, what it does? Link to comment Share on other sites More sharing options...
big bob Posted January 21, 2011 Author Share Posted January 21, 2011 I found this as well, but pardon my ignorance as I don't read circuits hardly at all, could this be what was done Could be the unlabeled cap in the schematic but it is too hard to say from a distance Link to comment Share on other sites More sharing options...
kbp810 Posted January 21, 2011 Share Posted January 21, 2011 Any Idea what is brings to the party. Um, what it does? Many believe that the large oil in metal can capacitors can reduce unwanted oscillations, making them an ideal filtering cap. Link to comment Share on other sites More sharing options...
NoNameBand Posted January 21, 2011 Share Posted January 21, 2011 I think it is a GPS device from Bob's "Big Brother" to spy on Heritage guitar lovers. Probably, a Gibson tech put it in there. I'm just guessing. Link to comment Share on other sites More sharing options...
bolero Posted January 21, 2011 Share Posted January 21, 2011 filter capacitors regulate the power supply in an amp it probably doesn't have *much* effect on the overall sound, unlike the capacitors & resistor values in the tone circuit etc I doubt it was a "tone" mod, probably a utilitarian part to replace something, or maybe they even used them for a while when they built them although power supply mods can affect how an amp sags etc when it is run full tilt, and can affect the bass response etc and that's about all I know on that subject!! Link to comment Share on other sites More sharing options...
big bob Posted January 21, 2011 Author Share Posted January 21, 2011 WOW ran the beast for awhile this evening, All I can Say is WOW.. This thing sounds F-ing sweet It makes all those stomp boxes I hated sound Great... Oh this will be fun... Link to comment Share on other sites More sharing options...
mars_hall Posted January 21, 2011 Share Posted January 21, 2011 I found this as well, but pardon my ignorance as I don't read circuits hardly at all, could this be what was done Could be the unlabeled cap in the schematic but it is too hard to say from a distance I hesitate to say anything here as I am not a tube amplifier expert. However, I feel I know enough to understand what is going on and maybe pass some along some degree of understanding. The oil-filled cap could be the unlabeled one in the schematic. Notice the filament (heater) voltage is not referenced to ground but rather to some voltage in the range of 50v +/- 6.3v depending on the values of the resistors used to replace the pot. The 100K allows the filament voltage to come up a bit slower than instantaneous as the adjacent cap is charged up to its steady state voltage. These will reduce transients on the filament and premature failure. The filament voltage will more of less track changes in the plate voltage (350v) but with a slight delay. The delay is caused by the charging or discharging of the supply caps. Filter caps in a supply circuit are there to smooth out the AC ripple seen across the transformer and are placed after the pass diodes. During half the AC cycle, current is passing through the upper three diodes and on the other half through the lower three diodes. That voltage is continuously changing and the capacitors at that point serve to level it out though not perfectly as they charge and discharge. The imperfect filtration creates a noise (typically 60 Hz in the US) which is passed throughout the system. The 60 Hz and its harmonics become the noise floor. The noise floor can also impacted by drains on the supply where the current supply is temporarily required to supply more current than the supply can instantaneously handle. This causes the supply voltage to momentarily go low or "sag" and the change causes further stages of amplification to saturate or clip sooner than normal, since their supply voltage and power curve is momentarily lowered. Sag is not generally considered a factor in supplies that use diodes, as this one does, vs one with tube rectifiers due to the diodes reacting to change much faster. The larger the value of the supply cap, the lower the frequency of the ripple that is being filtered. Conversely, the higher the value of the cap, the more higher frequency ripple is being filtered out. You will normally see a combination of both in the power supply circuit as they react at different rates to changes in the input voltage. Hope this helps in some small way. Link to comment Share on other sites More sharing options...
kbp810 Posted January 22, 2011 Share Posted January 22, 2011 WOW ran the beast for awhile this evening, All I can Say is WOW.. This thing sounds F-ing sweet It makes all those stomp boxes I hated sound Great... Oh this will be fun... In the end that's all that really matters! Play it loud and play it proud! Link to comment Share on other sites More sharing options...
big bob Posted January 22, 2011 Author Share Posted January 22, 2011 I hesitate to say anything here as I am not a tube amplifier expert. However, I feel I know enough to understand what is going on and maybe pass some along some degree of understanding. The oil-filled cap could be the unlabeled one in the schematic. Notice the filament (heater) voltage is not referenced to ground but rather to some voltage in the range of 50v +/- 6.3v depending on the values of the resistors used to replace the pot. The 100K allows the filament voltage to come up a bit slower than instantaneous as the adjacent cap is charged up to its steady state voltage. These will reduce transients on the filament and premature failure. The filament voltage will more of less track changes in the plate voltage (350v) but with a slight delay. The delay is caused by the charging or discharging of the supply caps. Filter caps in a supply circuit are there to smooth out the AC ripple seen across the transformer and are placed after the pass diodes. During half the AC cycle, current is passing through the upper three diodes and on the other half through the lower three diodes. That voltage is continuously changing and the capacitors at that point serve to level it out though not perfectly as they charge and discharge. The imperfect filtration creates a noise (typically 60 Hz in the US) which is passed throughout the system. The 60 Hz and its harmonics become the noise floor. The noise floor can also impacted by drains on the supply where the current supply is temporarily required to supply more current than the supply can instantaneously handle. This causes the supply voltage to momentarily go low or "sag" and the change causes further stages of amplification to saturate or clip sooner than normal, since their supply voltage and power curve is momentarily lowered. Sag is not generally considered a factor in supplies that use diodes, as this one does, vs one with tube rectifiers due to the diodes reacting to change much faster. The larger the value of the supply cap, the lower the frequency of the ripple that is being filtered. Conversely, the higher the value of the cap, the more higher frequency ripple is being filtered out. You will normally see a combination of both in the power supply circuit as they react at different rates to changes in the input voltage. Hope this helps in some small way. Link to comment Share on other sites More sharing options...
big bob Posted January 22, 2011 Author Share Posted January 22, 2011 Stupid I-Phone.. Mark thanks for all your knowledge, I feel like a rube in that it took me so long to decipher all that you posted, and still knowing I probably only caught half. But I certainly wanted you to know that your incite is greatly appreciated. The amp sounds great as is so I don't foresee changing anything. If the big oil cap is to reduce noise I will say it does a great job.. I'm off to the lab tomorrow to start my plans for the cab. Oh boy once again, thanks for all the help. Lance Link to comment Share on other sites More sharing options...
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