JohnnyG Posted December 3, 2004 Report Share Posted December 3, 2004 looks likeyou're not the only one having to deal with funkeh caps now. i got the old PA tube amp i bought off BigD today and im currentlly tracing it out. has 2 big metal can caps in it. one is a straight 40uF 450V. the other is a 40uF 450V and a 25uF 25V cap in one can. christ knows where im gonna find any of those if i need to replace it. thedoctor: how can you tell that the cap is bad? not dismissing your opinion, just interested as to what leads you to that conclusion. interesting about reforming capacitors as well. im assuming thats a result of them sitting around unused for so long Quote Link to comment Share on other sites More sharing options...
thedoctor Posted December 3, 2004 Report Share Posted December 3, 2004 Yeah, twenty years is definately the death knell for paper/foil caps. If you want to test old caps to see if they are worth reforming, use a standard capacitance meter or an ANALOG voltmeter. Digital meters got no place here. Look for leakage at the X10k scale. The real test is to hook them to a power supply capable of hitting their max voltage rating and slowly charge them. If they are still in one piece at full voltage, put em in. By the way, don't forget that a good one will hold power for weels! Ruin your whole damn day. Don't discharge a cap by shorting it. Discharge with a 250 ohm resistor for about 10 seconds. justradios.com has lots of high-voltage caps for sale but limited metal can stuff. You got the scematic so just leave the can in place and buy three modern electrolytics of equal or higher rating and hide them. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted December 3, 2004 Report Share Posted December 3, 2004 OOPS! Forgot to address two things. It is in-capacitated. I got two reasons to say that cap is bad. A bad output tube is the major culprit for all the damage he reported and the cap took everything that resistor didn't. Also, look at that disgustingly clear,detailed picture of the bottom of that cap. That is burnt seperator paste coming out the unused tab slot. Age has its benefits only if you remember your mistakes. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted December 3, 2004 Report Share Posted December 3, 2004 That is burnt seperator paste coming out the unused tab slot. I also did notice that stuff oozing out. I wasn't sure if it was indicative of anything, though. I have no experience with the old style caps like that... Quote Link to comment Share on other sites More sharing options...
JohnnyG Posted December 3, 2004 Report Share Posted December 3, 2004 ah ok. i can see what you mean now. many thanks for that, definatelly something to remember. i assumed that even if a power tube went and shorted to ground then the cap would be ok because its not actually in the direct line of the short (if u get what i mean) you learn something new every day Quote Link to comment Share on other sites More sharing options...
thedoctor Posted December 3, 2004 Report Share Posted December 3, 2004 In this case, you learn something old everyday. JohnnyG, I use an old Bogen PA amp as a converted guitar amp. It has some strange pentode power tubes in it and every cap was bad when I got it. WE CAN FIX YOURS! Give me as much info as you can find on it and I will get you a schematic and help you test/modify. Nothing as cool as one of those old spaceheaters sitting behind you! Quote Link to comment Share on other sites More sharing options...
JohnnyG Posted December 3, 2004 Report Share Posted December 3, 2004 lol. atm im about half way through drawing out a schem of thething. everything is fine so far except for 2 things. one is that while i havnt got there yet i suspect that the power amp topology is gonna confuzzle me. second is that there are these old school capacitors inthere lol. they're plastic. square (some are smaller oblong ones) and they're flat. they have the value codes on them in the form of 6 little coloured dots. but i have no idea how to read them. the middle dot on one side is silver on all of them so i assume thats tolarence. any other help would be welcome lol with regards to the layout this thing has a 6j7 pentode at the input (i was really stumped for awhile untill i realises that the wire off the top of the tube went to the jack lol. couldnt figure out where the signal went) then theres the volume control. then into a 6j5 single triode from there it has some kooky tone controls that really confused me for a bit. the tone pot is some strange blend pot. blends between two simple RC networks at each end and thentheres another RC conected to a 4th lug. i worked out using my DMM that in the middle its a direct connection to that and then as you move towards either end. from there it goes into a 6sl7 dual triode (havnt worked out all the connections yet) and after that there are 3 6v6 pentodes. which is what im not sure about. i know enough about amps to know all about single ended and push pull topologys etc etc but what are 3 tubes for? i dont think the dual triode is being used as a phase splitter (will soon find out) and i didnt think you could use a triode as a phase splitter. Quote Link to comment Share on other sites More sharing options...
lovekraft Posted December 3, 2004 Report Share Posted December 3, 2004 Hey, doc, have you had any luck replacing those old canned multi-caps with the new JJ replacements? The specs look good, even if the prices aren't great, and I'm tired of having to replace a can with 3 or 4 discrete caps, and getting dirty looks from customers who think I've "modded" their power supply. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted December 3, 2004 Report Share Posted December 3, 2004 JohnnyG, you're breakin my heart. Those old cap color codes were all I knew fer years. You read them clockwise from the top/left. The silver dot should be bottom/center. It's been years but here goes: black-0 brown-1 red-2 orange-3 yellow-4 green 5 blue-6 violet-7 gray-8 white-9 Starting from top/left is: TYPE-ignore top/center-1st digit top/right-2nd digit bottom/right-multiplier bottom/center- tolerance class-ignore As for JJ replc. caps, I don't know them by that name. Sounds like repackaged mil. spec stuff to me. Let me know where to see some. Three 6v6s? single-push dual-pull? That's a tube joke.lol Quote Link to comment Share on other sites More sharing options...
unclej Posted December 3, 2004 Author Report Share Posted December 3, 2004 boy i sure do like it when you guys start talkin' all that technical stuff. Quote Link to comment Share on other sites More sharing options...
JohnnyG Posted December 3, 2004 Report Share Posted December 3, 2004 aaahhh ok. i figured the colours were the same as resistor codes i just didnt have a clue how to read them. as for the power amp section its actually fairlly normal....well i say that. its push pull using 2 of the 6v6 tubes. the third tube isnt even connected into the signal chain which has really confused me. it seems to be connected up in some wierd way to a mono type input. its got the heater connections but then everything else is wierd. the cathode is connected straight to ground and then the other pins all seem to be connected to something that im guessing is a large variable inductor i have zero idea what the hell it could be, short of guessing at some sort of tube burn in section to store a spare power tube ready in case one goes??!!?? id really love to know. other than that the tone controls are a little wierd and it seems that if the dual triode is used as phase splitter it isnt a design ive seen. im gonna check it through all again just to make sure i got it all right. so just to make sure with the cap thing. i have a cap here that is White Red red brown silver red so that would be 220 nF ye? Quote Link to comment Share on other sites More sharing options...
thedoctor Posted December 3, 2004 Report Share Posted December 3, 2004 (edited) Your numbers are correct. That is one old booger if it uses a 6SL6 or 7 as a splitter. Course, you got tubes with external HV, also. Old. Good iron, I'll bet. Full speed ahead. Lovecraft, if you can fit em in, pry the can apart at the bottom and yank the socket out. Drill and scrape that lovely goo out and fit the new ones inside with a good heatshrink on the leads. I haven't had any heat problems with that but my amps seldom get over "6" anymore. At least thats what my wife says. NO lol. Edited December 3, 2004 by thedoctor Quote Link to comment Share on other sites More sharing options...
JohnnyG Posted December 3, 2004 Report Share Posted December 3, 2004 i have no ideahow oldit is. Bigd told me he had hardlly any info on it lol. im fairlly sure that the 6sl7 is a phase splitter but i cant see how. the cathode of one is joined to the B+ supply directly at the rectifier through a 220k resistor. the cathode of the other tube is joined to the plate of the 6v6 its driving via a 560k resistor. the cathodes arent joined together which is how im used to seeing phase splitters. in truth this isnt really a major problem since i can just rewire it to be a "normal" phase splitter when i rejig the tone controls. the only real problem is that im fairlly sure that the power tranny is for 120v usa and im not sure if it has two primary windings so i can turn it to UK voltage. its a really big power tranny but it does have secondaries for B+, Heaters and then the tube rectifier heaters. i may have to take it out the amp and take the cover off the top to have a look there are one or two other little oddities like the spare 6v6 power tube that doesnt seem to do anything but it'll be alot easier to ask questions as soon as ive got the schem up on the computer. Quote Link to comment Share on other sites More sharing options...
lovekraft Posted December 4, 2004 Report Share Posted December 4, 2004 ...As for JJ replc. caps, I don't know them by that name. Sounds like repackaged mil. spec stuff to me. Let me know where to see some... JJ Electronic From the folks who brought us JJ replacement tubes - did I mention that they're not cheap? Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted December 4, 2004 Report Share Posted December 4, 2004 I am partial to the Sprague "Atoms" myself. BTW, I have some J&J tubes in one of my amps. I think they're nice tubes. Quote Link to comment Share on other sites More sharing options...
thedoctor Posted December 4, 2004 Report Share Posted December 4, 2004 Lovecraft, those look fab! Did I just say "fab"? Watched the movie BirdCage last night. Robin Williams as a gay guy. Those prices are very good, in my experience. The asians make good components. Ugly, though. Just guessing, those are UNIChem or Matsu caps. Good stuff. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted December 4, 2004 Report Share Posted December 4, 2004 I just looked at that J&J link. That's good to know that there is a source for those can type caps. My Seymour Duncan Convertible has two of those type of caps that will probably have to be replaced one day... Quote Link to comment Share on other sites More sharing options...
unclej Posted December 4, 2004 Author Report Share Posted December 4, 2004 if you'll google "can capacitor" you'll find several sources for them. i did that the other day but couldn't find the exact match to the one i needed. i just googled "mallory type fp" and found several more. Quote Link to comment Share on other sites More sharing options...
Slabbefusk Posted December 4, 2004 Report Share Posted December 4, 2004 Aww I'd love to tinker with a tube amp, if I only had the skills . This is not meant to be a thread hijack, but I want to ask a question: If I had a preamp connected to my effects loop Return, so that it would be Tube Preamp -> Tube Poweramp (in this case the slave amps). If I then used an A/B Box to switch between the preamp, where I would have my high gain tone, and the ordinary input on the slaveamp, where I would have my clean tone. Could I damage my output tubes if I slammed them with my high gain preamp just like that? The slave amp in question is a Musicman RD-100 with 6L6GC tubes in the power amp and SS in the preamp. Quote Link to comment Share on other sites More sharing options...
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