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In Reply to: RE: How to fire up a 20 OTL??? posted by djn on April 08, 2020 at 14:10:05
One thing I would do to make it easier to use would be to get rid of those Edison Price Music Posts- if you don't set them up right they can be a nightmare. I'd replace them with Cardas posts; to me they are a lot easier to use.
The Edison Price was designed to be able to clamp very hard on a banana post inserted into to it. If you plan to use regular speaker cable lugs though, the bananas still need to be in place, otherwise you can't tighten them down without damaging the post!
I would also examine the filter capacitors in the power supply- if any of the vent plugs near the cap terminals are not perfectly flat I would replace the capacitor(s). Usually this is a little red or gray plug a bit over 1/8" across.
I would also look for any resistors that appear discolored; if you find any replace them.
Finally, the output section should have some fairly large resistors on the tube sockets, 2 per socket. If there are not any there I would think about installing them; they would be on pins 3 and 6 and would be in series with the wires that otherwise connect to those pins. If they are there, I would measure all of them to make sure they are good. Further, the value should be at least 1 ohm (2 or 3 is better) and they should be at least 5 watts each (all the same value of course).
Follow Ups:
Hey Ralph, looks like the resistors are on pin 3 and 6. I think that is what I am seeing in my pix.
I'll check out the cap plugs tomorrow. I spent today organising my bench and building a dim bulb tester.
It looks from the photo as if those resistors on pins 3 and 6 of all 16 different power tube sockets are soldered direct to untethered wires, on the resistor leads opposite to the tube pins. So you have uninsulated connections floating in air, 32 of them in fact. If any were to touch the chassis, you might have a disaster. I suggest mounting each of those free leads on a stand-off to stabilize them in space, to which you could then re-solder the wires that go to them. I hope I am making myself clear. OR it could just be an optical illusion I'm seeing.
Hi Lew, I was seeing the same thing today. I was inspecting all the solder joints and some of them look awfully close. I like your idea as I couldn't come up with one.
Just realized that in many cases, if not most, you can just de-solder, install a piece of heat shrink to cover the exposed joint, then re-solder and use the heat gun. No need to install so many stand-offs.
Hi Lew. Working in the shop today I found a can of Plastic Dip and wondered if I could just paint the bare joints with this stuff. I called the company and they said once dry it can stand temperatures of 200+ degrees then I called a shrink tube company and they said the same for shrink tube.
I think I will just paint the bare joints with Plastic Dip. Thoughts ?
Good idea. Those connections are on the cathodes, which could have high voltage though. I would think that Plasti-Dip would have a high enough insulation rating (certainly better than the current situation), but I wonder that if you shortened the resistor leads also, if they'd be more mechanically sound.
Cory
Good point Lew.
because there are so many loose ends there (32, if I count right), but the peace of mind will be worth it.
Better than fixing a fried amp.
Look for bulging caps on the top side- that is the only way to tell they've been getting hot.
Check all the film caps for broken leads. Without terminal strips for mounting, they are all at risk!
Gotcha
Thanks Ralph. I just found a pair of Cardas posts this morning. Those other ones just look like trouble. I'll check all your other recommendations too. I tried to fire up my old Variac this morning but no output.
With no tubes in the unit, put a light bulb in series with the AC power and turn it on. The bulb will light up; if the caps are good it will go out in a few seconds, if not it will stay on or stay on dimly. But the light bulb itself will limit current so nothing gets damaged.
Yep. I plan on doing that Ralph.....I've worked on this Veriac for 10 years on and off with no luck. On a whim I checked the fuse again and found it tested good but then I noticed the ends were kind of dirty so I cleaned them off with 600 grit Emery paper and boom. It works. All that time and it was just bad contacts. Go figure.
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