That'll do it; where does it go?zulu said:http://parts.digikey.com/1/parts/761339-resistor-18k-ohm-5w-carb-comp-of183je.html
That'll do it; where does it go?zulu said:http://parts.digikey.com/1/parts/761339-resistor-18k-ohm-5w-carb-comp-of183je.html
; where does it go?
Sorry ... it really wasn't a trick question ... :lol: I tried looking around on that schematic but I'd like to put off another glasses prescription ... that schematic is a blur. :evil: Did you pick the value up off the schematic or by reading the color code?zulu said:You'd think I would know the answer to that :lol:; where does it go?
I'm forgetting the amp doesn't match the schematic anyway ... :? there's no guarantee it's there in the first place!zulu said:From the color code. I will find it in the amp and/or on the schematic, (which seems clear as a bell to me :wink: ) In case it's in a spot where I can sub a 15k or 20k.
adorshki said:You Guys are STILL at it? What's it been like a week now without sleep? This is actually some kind of religious initiation like when the shamans make the poor wannabe eat something guaranteed to be really scary, right?!?! :lol: :lol:
Cap, you're a saint! :wink:
Hi Zac ... didn't see these pics ... I guess I got distracted listening to the Stones ... We don't see that stuff because it isn't there ... Identification of the secondaries leaving the transformer:Zac said:... the two brown wires ...I don't see anything like that, but I don't see like you see.juan said:should be a diode and then a small filter cap which I don't think I've seen so far
Is that the belt or the suspenders? I'll guess belt. no, suspenders.This amp is strictly cathode-biased.
That thing that looks like a piece of candy is a cap; don't know what it's doing and I don't think it's on the schematic
kind of a crazy amp to be learning on, although next is an Ampeg B-25, it's so simple compared to this.There are your two heater supplies running around the amp lighting up your tubes --http://www.youtube.com/watch?v=PAbMazCRNso ... crossing each other in the green circle. Just going by the pics, it looks like the tan/white supply just runs along the back edge and feeds only the reverb preamp tube. I have no explanation what they're doing other than to say that the transformer, with its extra bias tap, was designed to support fixed bias but somebody decided it would be a cathode-biased amp and used that bias supply as an auxiliary filament supply ... take a little load of the primary filament supply. ???
The Boyz at Stonemarmot say: "A better way is to change one of the caps inside the amp. I would recommend changing C17. This way you only need to change one cap to affect the response on both channels. If you change C17 from 0.1 microfarads to 0.0047 microfarads, you will limit the low frequency response of this stage to about 32 Hz and the whole amp to about 33 Hz. This will still comfortably pass the lowest note on a standard four string bass. This same change will also work on the Ampeg B-22-X and B-42-X." Hi Zac: well ... sure seems reasonable ... dat be de cap ... an interstage coupling cap that collects the signal from the first / pentode stage of the 7199 and connects it to the input of the second / triode stage while blocking the DC ... we don't want to listen to amplified DC. If you decide to make the switch, you want 600V rating on the replacement; if you don't find a .0047 in your favorite flavor, .005 will do:zulu said:An interesting blog on the B25 if you're curious, according to this guy the circuit has a flaw of allowing frequencies as low as 9hz, easily 'rectified' with a cap swap.
Hi Zac: you're mixing Guilds and Gibsons here ... in guitar amps, the only time an electrolytic cap is connected + to ground is in a bias supply circuit ... which you don't have. If you have an 130ohm resistor from one 6GW8 pin 7 to ground and your 220uf/100v electrolytic cap is connected + end towards the tube and - end to ground, you have a cathode resistor bypass cap and it's installed correctly. If you substitute, put the - end to ground and the + end on either pin 7; the two pin 7s must be jumpered ... wire running from one pin 7 to the other ... this is the case now, no?zulu said:Hey John, you mentioned it its common to see cathode bypass caps on the bias circuit with the positive connected to ground. Do you think The 220uf 100v electrolytic is installed backwards, and when I swap it, how should the polarity be oriented?
Hi Zac: you're mixing Guilds and Gibsons here ... in guitar amps, the only time an electrolytic cap is connected + to ground is in a bias supply circuit ... which you don't have. If you have an 130ohm resistor from one 6GW8 pin 7 to ground and your 220uf/100v electrolytic cap is connected + end towards the tube and - end to ground, you have a cathode resistor bypass cap and it's installed correctly.
capnjuan said:Hi Zac: just thinking out loud here but you've refreshed all the bypass cathode caps and I understand you have some new power tubes inbound. You might want to hold off on reducing that cathode bypass cap from 220uf/100v down to 25uf/25 or 50v. I can understand wanting to do as much maintenance as possible while the amp's on the bench but if you switch out the bypass cathode cap ... with your new tubes ... with your refreshed preamp cathode bypass caps, you're not going to get any sense of what any of these might have done individually to the amp's tone.
I mean, you can put the amp back together and see if you got any uptick from the cathode bypass caps you've already replaced. You can shut down and replace the power tubes without yanking the chassis, and fire back up and see what if any uptick you got from the tubes. I'm saying if you're ok with the amp's tone with that existing cap in it and since it's reasonably new and for the sake of appreciating what you've done so far, you might want to hold off ... you can still do it any time you want ... just a thought.