Rickresource Rickenbacker Forum

Well, I for one am excited and want a review when you get music out of that thing!

well, good then bad news. I got it all back together, fired it up (I did the warm up) and it worked. Had a good bass output, sounded good. Set the bias using the europrobe to the specified 38 milliamps. It gave off some smells, but no "burning electric" smells, more heating up smells. At one point after 5 minutes or so I did hear a "boiling off" sound for about 5 seconds, definitely from one of the transformers or the capacitors that are inside a cover on the tube side of the frame, but I didn't see any smoke or smell burning. So I decide I would leave it on for an hour as a burn in and see what happened. After 10 minutes or so I went upstairs for 5 minutes and when I came back down she had blown the fuse (doesn't that always happen like that ). A quick look over I noticed the .002 cap off pin 5 to ground of the 6L6 had come off ground (this is the one I fixed). I soldered it back in, warmed up, powered up and within 3 seconds the fuse blows. I pull all the tubes thinking maybe I fried the 6L6 when the ground came off, powered up and same thing, fuse blows after 3 seconds. She gives off a low "hmmm" (a component makes it, not through the speaker) when I flip full power then the fuse goes. I opened the cover that covers 6 very large Power Supply Electrolytic capacitors on the tube side, it seem a bit damp and maybe slight oil film. I am guessing one of these caps may be bad. Is there a way to check these without power on, just using an ohm reading? Should not they be "open" when no current is applied, so if I get a reading to ground thru one with no power- is it bad? I want to try un-powered checks first as the fuses are $2 a pop at my local hardware store. Any trouble shooting tips to isolate problems are appreciated. I am actually having fun with this amp.

It seems that probably both capacitors have failed. These are the 70µF 350V capacitors.
Probably one had an internal short cut causing the second one to be overloaded (too high voltage).
If these capacitors are out of order the fuse will blow even with the standby switch off. Change both, there are such capacitors available at 10 or 20$ each. Please check the 220kOhms resistors, too. They should have no internal short cut and still measure about 200-240kohms
After 40 years many electrolytic capacitors do fail.
One hint: leave the small capacitors from pin 5 (6L6s) in to avoid hf-instability. At least until your amp works fine again.
38mA of idle current are all right for the 6L6 tubes. There seems to be no transformer failure.
You can check the 6 rectifier diodes, too, although I think they are o.k.
BUT PLEASE BE CAREFUL WITH THE HIGH VOLTAGE OF 500V ON THESE CAPACITORS.
Always leave the amp untouched for half an hour after switing off power.
You better give this repair job to a professional
Wolfgang

Hi Wolfgang, The fuse blows after I power on the standby switch (full power), it's fine with only the power button on (warm up). That puts it past the standby switch. I think one of the 20MFD/500v capacitors went bad. The one below the 12AT7 (on the schematic) oozed out the side, and the ohm meter shows no resistance thru it. I ordered all the electrolytic caps from http://www.tubesandmore.com/ and will replace them one at a time and see if that does it. I checked the 220k resistors around the 70MFD/350v circuit- they're within spec. That was the only cap that showed no resistance thru it, and I believe the "boiling off" sound I heard was this cap going bad.

this is good news. Just replace the bad 20µF capacitor and your amp is fine again.

Wolfgang

You should just replace all of the electrolytic caps in the amp. They have a finite lifespan (usually 10-15 years) and with non-use, they dry up quicker. If one leaked it's paste electrolyte out, the rest will soon follow. (That stuff is toxic btw and will eat into the metal so try to clean it all up) Replace every electrolytic cap in the amp, including the power supply caps under the can, the bias supply caps on the small board inside, and the cathode caps on the main board. Electrolytic caps are polarized, so they will have a + symbol on one end, or arrows to show which end is the negative end. They have to be hooked up the right way or they will explode. The bias supply caps go in backwards, with the positive end of the cap going to ground, and all the rest go with the negative end to ground. You should also replace the power supply dropping resistors with some modern metal oxide resistors. You can bet those original carbon comps have drifted, and modern metal oxie won't burst into flames like those CC's do. You can get some metal oxide at a local electronics store like Fry's Electronics or the like. NTE brand are what they usually sell and these are fine. They're just the blue 2 watt jobbers. Don't try to find any at the Rat Shack though...you won't. You need 2 watt resistors for these spots...not the 1/2 watt or 1 watt the Rat Shack sells. The power supply dropping resistors are the ones up under the can. Go with blackface values for a better sound.

After you've replaced all of the electrolytic caps, then with the tubes out, turn the amp on and see what happens. It should run without blowing the fuse. If it still blows the fuse, then disconnect all of the seconday leads from the power transformer and see if it still blows a fuse. (Make sure they are not touching anything when disconnected) If it does, the power transformer is bad. If it doesn't blow a fuse, then put the tubes in and see what you have. BTW, you will get a much better sounding amp if you modify it to the AA864 circuit, or even the AB165 circuit.

Keep us up to date!

Greg

Quick update- received the caps today. I replaced the bad 20MFD/500VDC cap that is in the pan. Powered up and all is well. Back to normal operation. I will replace the other pan caps and the 2w pan resistors, and the rectifier cap this weekend. Eventual goal is to restore it to an AB165. I'll keep you posted on my progress.

Thanks for the update Chris - I've been watching for one but realized you had parts on order so didn't post asking. I'm having fun!

Good stuff. Blackface that thing for sure!

A full day of amp head rehab today. I replaced all the can caps, and all the resistors under the can. I replaced the rectifier cap. I replaced all the chocolate drop caps with orange drop caps. I rewired the bias balance pot to a common bias adjustment pot. Set bias to 37 ma. (As a side note, I checked the bias adjustment of both 6L6 while still bias balance- one tube could go 10-50 ma, the other would only go 24-26 ma while swinging the pot. They both are now in synch at 37 ma with the bias line mod) Sounds good at this point. All the pots are quiet. The amp has a very slight hiss to it that you notice if you are not playing. Definitely is affected by the volume and treble pot (brighter/duller hiss). I checked all the tube pin voltages. The only thing not close, but still within the +/- 20% are pins 1 and 6 of the 12AT7. Should be +340VDC, they are 299 and 296. The lead in is at +440VDC, so maybe the 47K resistors are wearing? This is the line that the 20MFD cap in the can shorted- maybe that affected it? Should I bother with replacing the 47k resistors? Also, can you tell me what the voltages of pins 2 and 7 of the 12AT7 should be from the schematic?
http://www.schematicheaven.com/fenderam ... _ac568.pdf

Played around with it this morning- the sound is warm and buttery. There is a rattle in the head I have to track down, something is loose. When I play I hear this sound like the vibration of a blown light bulb- the filament on glass when you shake a blown bulb. Not sure if it's the tubes, or maybe the pan caps vibrating against each other. I can reproduce it with the power off and if I vibrate the cab a bit- brings out the same sound. Will go around and tighten everything up and add some foam to the cap can. Anyone try using a dab of silicon caulk on the caps to keep them tight? Should there be any ambient tube "rattle" that is normal?

OK, I isolated the noise to one of the 6L6 tubes- I believe I have "tube rattle". I can hear it if i tap on it with a pencil. When playing it is quite noticeable and I think it is also amplified. Solutions? I have seen these tube amp dampers on Ebay

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... %26fvi%3D1

Don't know if that will work, or if new tubes are required. They are brand new tubes, and I was careful on installing them (only grabbed the base, never the glass). Any feedback from previous experience with this is appreciated.

by the way, here is the finished project

You'll find if you go all the way in changing that over to an AB165, that those 47k resistors will get replaced with different value ones in addition to quite a few other changes. The rattling tube is just a microphonic tube. There isn't anything you can do about it except to replace it with one that doesn't rattle. I wouldn't worry about the voltages being off. Since the wall voltage is different today, all bets are off. As long as none of the voltages are drastically out of whack and it sounds fine, then you are good to go. Those chocolate drop caps as you called them are notoriously known as brown turd caps and they were lousy caps with high ESR and bad tone from the get go. Anything in their place will sound and perform better. The amp looks nice!

Greg

Hi guys, sorry to hijack your thread, but i have a 1978 Bassman 135 Silverface and it is sounding a bit noisey.............. which i know is usually the point .
I have replaced all of the pre amp and power amp tubes with Electro Harmonix valves and it sounds really nice, very tight, punchy sound. But all the while there is a lot of back ground noise, kind of loud hissing/ white noise.
I have been reading your thread about replacing the cap's, my question is which caps to change. Is it just the ones located under the can on the transformer side or other ones also. I live in England and all the parts i need are very expensive and really hard to get hold of. I need to move to the USA .