Wednesday, December 28, 2011

DeVry. I just spent a few minutes of study trying to craft a neat time-line fashion historical interconnect between this amplifier and the DeVry technical institute. I wound up digging up ties to Lee de Forest and Bell & Howell. They are all connected in some manner. I'll leave it at that. It's just too convoluted a maze to touch upon here, and I'd like to flow some solder sometime today. Lee de Forest is an interesting character steeped in early electronics design lawsuits, let me kick this off with a quote:
"So I repeat that while theoretically and technically television may be feasible, yet commercially and financially I consider it an impossibility; a development of which we need not waste little time in dreaming." – 1926

Had they listened, what would we be doing today?

To sum it up: Hot chassis film projector amplifier.

The enclosure really speaks to my growing fascination with modular synthesis. Observe that the selenium wafer stack is bypassed with a silicon rectifier. The silicon rectifier exhibits a much lower internal resistance of roughly 0.6 volt forward drop than the selenium wafer stack, which exhibits roughly one volt drop per wafer - almost ten times as much resistance in its conductive cycle. Therefore, during the conduction cycle the electricity will follow the path of least resistance and bypass the selenium stack. However, each selenium wafer only withstands roughly 20-25 PIV, which is why they are stacked to provide a working voltage to begin with. By not removing the selenium stack from the circuit, its 100-125 volt PIV rating still stands during the non-conductive phase of the AC cycle, which, may I remind you, is non-isolated 120 volt mains.

In short, bypassing in this manner is a terrible idea.

Engraved by hand on the chassis at the tube sockets are the following designations from left to right: 12AU6, 12AX7, 35C5 and between the two sockets at the right 35C5. The right-most socket is wired differently, and during a few moments in which I had blocked the memory of the solid state rectifier from my mind, I considered this an error until I did a little more hunting.

Discovering pictures of a similar DeVry projector at auction produced the layout legend above.

Another note. I won't tear this one up, sort of speaks for itself.

Gut shot! Finally some guts. One of these weeks or months I'll dispel with the commentary and just cut to the chase and post innards. I owe it to the internet.

Thordarson transformers. This company has been going since 1895, keep the dream alive. I personally feel that the creed from 1937 far outweighs their current motto, steeped as it is in the way of things...

For all the shit I've heaped on selenium, the rectifier stacks sure are pretty.

I'll post a proper retrofit eventually. I realize my content is flighty, best to just roll with it.


Gilamp said...

Hello, I have same amp, can they be converted into guitar amp?

crochambeau said...

Hi Gilamp!

I think anything, with enough effort, can be converted into a guitar amplifier. Whether or not it's worth it is another question.

To be honest, I have not messed with this unit further, but a few points of concern jump out at me:

Needs an isolation transformer (or a proper power transformer and different tube complement) to be safe to operate.

The three 35C5 power tubes need to be figured out, I'd guess the odd one out is a VR, but I don't think I've looked at a schematic.

That selenium stack is a ticking time-bomb.

Honestly, I wouldn't be surprised if the amount of work needed to make this a passable guitar amplifier exceeds the amount of work it would take to strip the chassis and build something from scratch, but that may just be caution/pessimism speaking. If you do chase a guitar amp with it, I'd love to see the result!

Gilamp said...

Thank you for your respond I appreciated it.Your info was great , yes I will be caution. You might be right to start from scratch, cabinet case is in good shape maybe redesign. Thanks for all your help looking forward to maybe seeing more of your projects. I have many others.