A couple observations since my last post.
I've been scouting around for pics and schematics for this amp. The first thing that I discovered was that the black, uncovered back panel appears in other pics, so maybe it is original. Odd, but, OK.
There seem to be several versions of this amp, but I have not found one that has the same tube compliment as this one. The model number is 835, but other 835 seem to have a different preamp, and there are other designations (N and D variants come to mind), but they are also different. I'm beginning to wonder if any two 835s are the same? LOL. The Jensen alnico C-15-N also seems to be an odd choice. I'll have to check the date code on that....
I've decide to get to work. I figure I'll recap the power amp and install a 3-prong cord, and see where things stand once it is operational (I haven't actually plugged it in - there is no real point to doing so at this stage). The guts of the lower chassis looks like this.
The electrolytic caps for the power section have been replaced with two 40mfd Atoms. That's a reasonable solution, and I could just replace those. But there is this now-unused cap can, and that bugs me. I think I'll re-stuff it, as there seems to be nothing available with the correct voltage and temperature ratings. The can for the preamp I can easily replace with a current production 3x20mfd can at 450v/85C. Maybe I will do that, or maybe I will re-stuff it. Haven't decided.
I'll also replace the cathode resistor by-pass caps. I'll probably just use standard 50mfd/50v caps for those, unless I convince myself that I should try to more closely match the values present. I'll also replace the Astrons with something current (I used mojotone caps in my M15 with good results) and check the various resistor values.
The pot that I earlier assumed was a bias control looks to be a heater balance circuit. I guess that could have been predicted if I had just done some research.
That is one very cool amp! And it cleaned up so well.
I never take the time with the cosmetics - something that my wife would very much appreciate.
Getting the trem to work seems to be a frequent topic of conversation for vintage ampegs. On my M15, I had to change a resistor value just make it audible. And it really seems to care about what tube goes in that hole. Oh, and there is a cap that is missing from the online schematic. Without that it sounds like a helicopter. But It works now.
Oh, on my M15 I actually replaced the "death cap". The old one had leaked all over everything and shorted. Ampeg seems to have used a bunch of different values for that cap, and I think I stuck in a 0.1uf because I had one handy. Not sure what it's for. It doesn't do anything that would protect the power switch... It's not technically a death cap, as it doesn't lift the ground or anything. Doing without seems like a reasonable option.
Happily, there is no trem with the 835 that is on my bench now. But I'm sure there will be other interesting challenges that come up.
I've been scouting around for pics and schematics for this amp. The first thing that I discovered was that the black, uncovered back panel appears in other pics, so maybe it is original. Odd, but, OK.
There seem to be several versions of this amp, but I have not found one that has the same tube compliment as this one. The model number is 835, but other 835 seem to have a different preamp, and there are other designations (N and D variants come to mind), but they are also different. I'm beginning to wonder if any two 835s are the same? LOL. The Jensen alnico C-15-N also seems to be an odd choice. I'll have to check the date code on that....
I've decide to get to work. I figure I'll recap the power amp and install a 3-prong cord, and see where things stand once it is operational (I haven't actually plugged it in - there is no real point to doing so at this stage). The guts of the lower chassis looks like this.
The electrolytic caps for the power section have been replaced with two 40mfd Atoms. That's a reasonable solution, and I could just replace those. But there is this now-unused cap can, and that bugs me. I think I'll re-stuff it, as there seems to be nothing available with the correct voltage and temperature ratings. The can for the preamp I can easily replace with a current production 3x20mfd can at 450v/85C. Maybe I will do that, or maybe I will re-stuff it. Haven't decided.
I'll also replace the cathode resistor by-pass caps. I'll probably just use standard 50mfd/50v caps for those, unless I convince myself that I should try to more closely match the values present. I'll also replace the Astrons with something current (I used mojotone caps in my M15 with good results) and check the various resistor values.
The pot that I earlier assumed was a bias control looks to be a heater balance circuit. I guess that could have been predicted if I had just done some research.
(11-20-2017, 08:18 AM)vibrating string Wrote: So check out my posts at http://www.vintageampeg.com/forum/Thread...aramp-1956
That is one very cool amp! And it cleaned up so well.
I never take the time with the cosmetics - something that my wife would very much appreciate.Getting the trem to work seems to be a frequent topic of conversation for vintage ampegs. On my M15, I had to change a resistor value just make it audible. And it really seems to care about what tube goes in that hole. Oh, and there is a cap that is missing from the online schematic. Without that it sounds like a helicopter. But It works now.
Oh, on my M15 I actually replaced the "death cap". The old one had leaked all over everything and shorted. Ampeg seems to have used a bunch of different values for that cap, and I think I stuck in a 0.1uf because I had one handy. Not sure what it's for. It doesn't do anything that would protect the power switch... It's not technically a death cap, as it doesn't lift the ground or anything. Doing without seems like a reasonable option.
Happily, there is no trem with the 835 that is on my bench now. But I'm sure there will be other interesting challenges that come up.