05-08-2018, 07:54 AM
Hi All -
My R-12-R Reverbrocket (circa 1961) has been on the bench for a couple weeks. I last worked on it about 15 years ago, and it's been "in storage" for much of the last 10. It had some random problems in addition to the obvious need to replace electrolytics. I was originally planning to to a complete makeover with Zoso blue caps, but upon adding up the costs, I decided to go go about it piecemeal. Anyway, a few observations, just to see what other people are finding.
1. Filter caps. The last time I did this I was able to a find 4x20uf can at rated at 85 degrees. Those seems to be unavailable now. The last time I checked, the caps that I was buying from Mouser to restuff cans were out of stock, so I decided the best I could do was replace the can with a 30-30-40-40. Hardly the same thing. After some research, I decided that the 5Y3GT rectifier would likely survive if I used the 30s as the first two filter stages. So far, I haven't see any issues. YMMV. If it matters, I'm using an RCA in the rectifier spot. I have a few of those, but if one goes bad, I will revisit this decision.
2. All of the Mallory 150 coupling caps that I put in last time were leaking. That surprised me a bit, as these things are supposed to be of decent quality and are supposed to be "self-healing". Maybe I should have just let them "heal".
I replaced them.
3. I've been using a lot of Mojotone caps. Of of my big disappointments was with the current crop of 50/50 electrolytics. The one that I installed to bypass the power tube cathode resistor failed with a short. The amp was sounding like crap, and the temolo was a disaster. I spent a of time trouble shooting the trem until I discovered that the overall voltage of the amp was running low because of the excess power tube current. Happily, no harm done. But disappointing. I also have some extra 60Hz (not 120Hz) hum from the preamp. Based on my experiments pulling tubes it seems to be originating with the PI, so I'm suspecting that the bypass cap there has also failed in some way. I will report later when some Atoms get delivered. Probably need to get the scope out to trace further.
4. I did replace the 0.02uf coupling caps with blue Zosos. Those are really nice caps. Good leads. I like them, and they are not *that* expensive.
Unrelated to caps: this amp has always had an issue with receiving a local radio station. It currently is playing very nice classical music, which is not what I remember from before. Probably the station changed formats in the last 10 years. Anyone else have an idea where I might look? It's not dependent on the volume or dimension controls, so it's somewhere between V2 and the power tubes. I would have thought that 500pf caps around the PI would be bleeding RF out of the signal, but I've replaced those with no effect. I sort of feel like the signal is originating on the chassis and coming in on the grounded lead of the power cord. I suppose it could also be coming in through the NFB loop.
Anyway, it's been pretty quiet here lately, so I thought I would start a thread.
My R-12-R Reverbrocket (circa 1961) has been on the bench for a couple weeks. I last worked on it about 15 years ago, and it's been "in storage" for much of the last 10. It had some random problems in addition to the obvious need to replace electrolytics. I was originally planning to to a complete makeover with Zoso blue caps, but upon adding up the costs, I decided to go go about it piecemeal. Anyway, a few observations, just to see what other people are finding.
1. Filter caps. The last time I did this I was able to a find 4x20uf can at rated at 85 degrees. Those seems to be unavailable now. The last time I checked, the caps that I was buying from Mouser to restuff cans were out of stock, so I decided the best I could do was replace the can with a 30-30-40-40. Hardly the same thing. After some research, I decided that the 5Y3GT rectifier would likely survive if I used the 30s as the first two filter stages. So far, I haven't see any issues. YMMV. If it matters, I'm using an RCA in the rectifier spot. I have a few of those, but if one goes bad, I will revisit this decision.
2. All of the Mallory 150 coupling caps that I put in last time were leaking. That surprised me a bit, as these things are supposed to be of decent quality and are supposed to be "self-healing". Maybe I should have just let them "heal".
I replaced them.3. I've been using a lot of Mojotone caps. Of of my big disappointments was with the current crop of 50/50 electrolytics. The one that I installed to bypass the power tube cathode resistor failed with a short. The amp was sounding like crap, and the temolo was a disaster. I spent a of time trouble shooting the trem until I discovered that the overall voltage of the amp was running low because of the excess power tube current. Happily, no harm done. But disappointing. I also have some extra 60Hz (not 120Hz) hum from the preamp. Based on my experiments pulling tubes it seems to be originating with the PI, so I'm suspecting that the bypass cap there has also failed in some way. I will report later when some Atoms get delivered. Probably need to get the scope out to trace further.
4. I did replace the 0.02uf coupling caps with blue Zosos. Those are really nice caps. Good leads. I like them, and they are not *that* expensive.
Unrelated to caps: this amp has always had an issue with receiving a local radio station. It currently is playing very nice classical music, which is not what I remember from before. Probably the station changed formats in the last 10 years.
Anyone else have an idea where I might look? It's not dependent on the volume or dimension controls, so it's somewhere between V2 and the power tubes. I would have thought that 500pf caps around the PI would be bleeding RF out of the signal, but I've replaced those with no effect. I sort of feel like the signal is originating on the chassis and coming in on the grounded lead of the power cord. I suppose it could also be coming in through the NFB loop.Anyway, it's been pretty quiet here lately, so I thought I would start a thread.