Hi All,
I've been a big Tubelab lurker for years, and I've thought about jumping into one of the projects. Recently, I nabbed a big DIY 845 SE two chassis amp, and I'm trying to figure out what to do with it. The driver circuitry is sub-optimal (6sn7 to the grid of a 5687 to a parallel 6sn7 driving the 845).
I've got two HUGE OPTs of unknown original (6"x6"x4", potted in wax, very heavy) and a big power supply (separate HV, filament, and bias transformers, a mechanical timer, 3B28s for the HV with a big 8H choke and giant oilers, 5AR4 for the bias, SS for the filament--no separate B+ for the driver circuit). I haven't gotten the PS fire up to see what sort of B+ it'll give me, but it looks like it'll work. I measure the OPTs at 13.5k, but I question whether I'm right.
If I build this into something else, I'm not concerned about maximum power or sonics. I just want it to work, not be too ridiculously complicated, and to light up pretty like 845s do. I'm torn between trying to use some of the PS components I have and doing monoblocks with onboard power supplies (I don't love umbilicals, let alone at this voltage). I'm fine doing quite a bit of point to point or breadboard, but time is the issue.
Could I follow in the path of the 845 project and build based on a TSE-II board? How about a SSE? Or maybe pairs if I do monoblocks ....
A reasonable person would just build a TSE-II and call it a day. But when I sit here staring at those monster OPTs, it seems like they need a job.
Paul
I've been a big Tubelab lurker for years, and I've thought about jumping into one of the projects. Recently, I nabbed a big DIY 845 SE two chassis amp, and I'm trying to figure out what to do with it. The driver circuitry is sub-optimal (6sn7 to the grid of a 5687 to a parallel 6sn7 driving the 845).
I've got two HUGE OPTs of unknown original (6"x6"x4", potted in wax, very heavy) and a big power supply (separate HV, filament, and bias transformers, a mechanical timer, 3B28s for the HV with a big 8H choke and giant oilers, 5AR4 for the bias, SS for the filament--no separate B+ for the driver circuit). I haven't gotten the PS fire up to see what sort of B+ it'll give me, but it looks like it'll work. I measure the OPTs at 13.5k, but I question whether I'm right.
If I build this into something else, I'm not concerned about maximum power or sonics. I just want it to work, not be too ridiculously complicated, and to light up pretty like 845s do. I'm torn between trying to use some of the PS components I have and doing monoblocks with onboard power supplies (I don't love umbilicals, let alone at this voltage). I'm fine doing quite a bit of point to point or breadboard, but time is the issue.
Could I follow in the path of the 845 project and build based on a TSE-II board? How about a SSE? Or maybe pairs if I do monoblocks ....
A reasonable person would just build a TSE-II and call it a day. But when I sit here staring at those monster OPTs, it seems like they need a job.
Paul
Why not bring that amp up and running as is so as to see what works and what doesn’t. See what kind of B+ you have and how much drive you’re getting from the front end. A properly driven 5687 followed by a 6SN7 cathode follower might not be a bad driver stage. Have you drawn it up?
You should be able to accurately measure the OTs without uncertainty. What’s the issue there?
No doubt you’ve seen George’s 845 powered by the TSE 45.
Sounds like a fun project. Good luck.
You should be able to accurately measure the OTs without uncertainty. What’s the issue there?
No doubt you’ve seen George’s 845 powered by the TSE 45.
Sounds like a fun project. Good luck.
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I got the B+ and filament to fire up. Looks like just under 900V on the B+ with no load. While it was working the other day, it looks like I lost a resistor in the bias supply. More on the way. I've got some bleeder resistors coming too because the builder evidently thought some marker reminding anyone under the skirt "No HV Bleeder" is enough safety for 900V. After the parts arrive I can stick some tubes in it.
The issue with the OPT, at least to my limited understanding, is I don't know what the intended secondary impedance was. If I assume 8 ohms, seems like about 13.5k for the primary, which is awfully high, especially for a "low voltage" 845 amp. If I assume 6 ohms, though, its more like 10k, which would make more sense. But I have no idea what these OPTs are or where they came from. Given the size, they have to have been intended for 211/845 amps.
And yeah, I saw George's TSE 45. Which makes me wonder about the TSE-II. Or whether the simpler, cheaper SSE would be a possibility. But as you can tell, while I've built some tube amps, I am no tube design expert.
Paul
The issue with the OPT, at least to my limited understanding, is I don't know what the intended secondary impedance was. If I assume 8 ohms, seems like about 13.5k for the primary, which is awfully high, especially for a "low voltage" 845 amp. If I assume 6 ohms, though, its more like 10k, which would make more sense. But I have no idea what these OPTs are or where they came from. Given the size, they have to have been intended for 211/845 amps.
And yeah, I saw George's TSE 45. Which makes me wonder about the TSE-II. Or whether the simpler, cheaper SSE would be a possibility. But as you can tell, while I've built some tube amps, I am no tube design expert.
Paul
My 845 amp has been idle for nearly 10 years. It put too much heat (about 500 watts) into my small room to use for more than an hour at a time in south Florida. Now that I live in a climate where below zero temps in the winter are common it could be useful, but exposed 450 and 1100 volt wiring and young curious grandkids are not compatible with each other, so it remains on the shelf.
I have thought about rebuilding it in a much nicer and safer form factor, but it's pretty low on the long project list. If I was to rebuild it now the driver board would definitely be a TSE-II. The power supply would be completely new, probably with two 500 or so volt supplies stacked on top of each other, using solid state rectifiers.
I don't see why it wouldn't work, but I have never actually tried it. It just seems like a big fat DHT like the 211 or 845 should be driven by a smaller DHT like a 45 or 2A3, so that's what I used. My 845 amp was actually built before the SSE existed, so that might have influenced my decision a bit too.
Most speakers have a pretty wide impedance variation over frequency. Chances are what you have will work fine if they are good, and can stand the B+ voltage.
My OPT's actually came from a guy who made a living ripping people off. I got these early on when he made some decent sounding OPT's but upon opening one up I see that they were made with paper and package shipping tape. How long they will work at 1100 volts remains a mystery.