| Author | Message | ||
     Tim Connolly |
Hi, I have a 212-65 which has been getting a loud background hiss as of late.It is a hiss and not a hum so maybe it is in the preamp section? The amp has the solid state preamp section. I have removed the big can electrolytics and tested them. They seem ok but I'm not really sure. One is 116mf and the other is 127mf. I did the 9 volt test where you charge them up with a 9 volt battery and see how long they take to discharge. Both of them discharge slowly and steadily losing only about a half a volt in one minute. I have no problem changing them out as just a matter of routine maintenence but as you know they're pretty hard to find. I plan to use Deoxit on everything,tighten up the tube sockets,change the tube socket caps and resistors and do the IF shielding per the tech bulletin. Is there anything else specific I should look to change? Ic's,diodes,op amps, while I have it open. How about the coupling caps? I have a little experience rebuilding some tube radios and a couple of stereo tube amps. I respect the high voltages with the hand in the pocket etc.wooden probes and the like.Tube guitar amps are fairly new to me. I have a couple of tube testers that are mutual conductance capable to help match the output tubes. I also use an isolation transformer for line power and have a variac to power up new components. Any help would be appreciated. | ||
| mike kaus |
Well, that tried and sometimes untrue method of charge them up and watch them discharge is only valid if you had 9 volts in use. Other than that, it only tells you they can safely store 9 volts. Throw them away and replace them. You could test them with a regular capacitor tester, show good and blow apart next week. The elctrolytic dries out and when they go, they just poof sometimes. Those values sound a little spooky- I don't see them anywhere on my 65 watt schematics but that doesn't mean anything-these are for the 12ax7 PI driver amp. If they are in the main filter section, you can use 150mF at whatever voltage you have on them and they will be fine. I think mine show 450 v but I don't have my glasses on. Remember, always use the rated voltage or higher and you should be OK. You should replace ALL the elctrolytics, not just the filter section. You should have some on the main preamp board as well as <the rectifier board>. The bias supply cap(if you have a 12ax7) is VERY inportant because if you lose bias, you lose tubes and other things that are expensive. Tubes SHOULD be bought matched from a reputable dealer- I don't really suggest fleabay for tubes unless you know who you're buying from. Also, for MM's I'd stay away from anything that say Sovtek or it's name alikes. If it says Svetlana, it's not really the old Svet's. You want tubes that are branded with a syrillic C or SAY Winged C. Or the JJ tubes are also very good if you have the El34 style amps. If you go JJ EL34, there is a variant that they call the E34L's that hold up real well in MM's. These are what I use but hey, we all hear things dif> I wouldn't change anything else until you retension sockets, replace the tubes and bias properly. You might be surprised. Mike. |