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How to fire up a 20 OTL???

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Posted on April 8, 2020 at 14:10:05
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003



Hi All. I bought this Bruce Rozenblit OTL built from the Glass Audio article. The guy who built it added 2 tubes per side. It hasn't been turned on for 12 years. Is there anything special I should do except use my cheapo test speakers first.

 

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Variac, posted on April 8, 2020 at 21:00:12
garymuffley
Dealer

Posts: 360
Location: Maryland
Joined: March 29, 2006
If you have access to a Variac, that is the best way to bring up an amp or preamp that has been in storage in a slow and controlled fashion.

 

You might want to do a visual examination before you plug it in., posted on April 9, 2020 at 12:46:37
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
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).

 

RE: You might want to do a visual examination before you plug it in., posted on April 9, 2020 at 12:51:39
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
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.

 

RE: Variac, posted on April 9, 2020 at 12:52:50
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
Cant seem to make mine work. No output. Going to work on it now.

 

You could use the old light bulb trick., posted on April 9, 2020 at 13:27:10
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
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.

 

RE: You could use the old light bulb trick., posted on April 9, 2020 at 18:08:08
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
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.

 

Check the fuse, posted on April 10, 2020 at 17:07:41
garymuffley
Dealer

Posts: 360
Location: Maryland
Joined: March 29, 2006
Mine blew a fuse and, naturally, I checked that last.

 

RE: You might want to do a visual examination before you plug it in., posted on April 11, 2020 at 20:31:38
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003



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.

 

RE: How to fire up a 20 OTL???, posted on April 12, 2020 at 18:38:44
LtMandella
Audiophile

Posts: 1232
Location: CA
Joined: May 27, 2019
his forum?

If art interprets our dreams, the computer executes them in the guise of programs!

 

That kind of filter cap does not have a pressure relief plug., posted on April 13, 2020 at 10:25:23
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
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!

 

RE: That kind of filter cap does not have a pressure relief plug., posted on April 13, 2020 at 10:48:56
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
Gotcha

 

In addition to what Ralph said., posted on April 13, 2020 at 20:04:54
Lew
Audiophile

Posts: 10455
Location: Bethesda, Maryland
Joined: December 11, 2000
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.

 

RE: In addition to what Ralph said., posted on April 13, 2020 at 20:08:59
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
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.

 

It'll be a bit of a pain in the butt,..., posted on April 13, 2020 at 20:35:33
Lew
Audiophile

Posts: 10455
Location: Bethesda, Maryland
Joined: December 11, 2000
because there are so many loose ends there (32, if I count right), but the peace of mind will be worth it.

 

Heat shrink, posted on April 13, 2020 at 21:07:23
Lew
Audiophile

Posts: 10455
Location: Bethesda, Maryland
Joined: December 11, 2000
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.

 

RE: Heat shrink, posted on April 13, 2020 at 21:08:54
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
Good point Lew.

 

RE: It'll be a bit of a pain in the butt,..., posted on April 13, 2020 at 21:11:34
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
Better than fixing a fried amp.

 

RE: Heat shrink, posted on April 14, 2020 at 12:15:12
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003



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 ?

 

RE: Heat shrink, posted on April 15, 2020 at 14:56:29
Cory M.
Audiophile

Posts: 1172
Location: Midwest
Joined: April 10, 2005
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


 

Fired up the amp with variac and dim bulb tester...nothing, posted on April 19, 2020 at 13:01:15
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003



The bulb started coming up at around 30v and just kept getting brighter as I increased the voltage and stayed bright. No tube filaments lit up. So that is a dead short somewhere right?.

 

Got to start from scratch , posted on April 19, 2020 at 21:01:09
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
Geez, I don't think this amp was ever in service. I've been tracing all night and I am seeing cross eyed. no grounds anywhere even where it is called for. Some of the wiring is as backwards or non existent all together. I'll keep at it but the more I see the more I think I'll snip all the wires and start for zero.

 

RE: Fired up the amp with variac and dim bulb tester...nothing, posted on April 20, 2020 at 05:10:08
Cory M.
Audiophile

Posts: 1172
Location: Midwest
Joined: April 10, 2005
Yep, sounds like a dead short if that's all that's happening. The tubes' filaments probably wouldn't be visible 'til around 70 volts. Did you bring it up to full power? Excuse the question, but the incandescent bulb is wired in series with the AC output socket, right? (The duplex receptacle has me guessing).


Cory


 

RE: Fired up the amp with variac and dim bulb tester...nothing, posted on April 20, 2020 at 06:07:14
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003



Yes, in series. Here is the schematic I used.

 

The tubes can't be in the amp when you do the light bulb thing., posted on April 20, 2020 at 09:14:15
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
The load will light up the bulb otherwise!

 

Whoever did this was a bit sloppy, posted on April 20, 2020 at 09:15:41
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
Get some terminal strips and do a neat job.

 

RE: Whoever did this was a bit sloppy, posted on April 20, 2020 at 11:01:14
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003



Hi Ralph. Actually there is a terminal strip at every tube but I think he added the resistors to pin 3 and 6 as an afterthought and didn't use the strips.

 

BACK ON THE SHELF SHE GOES, posted on April 21, 2020 at 09:33:03
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
Ok, this thing is going back on the shelf. The more I looked the more issues I found. I ordered Bruce's book about OTL design and I'll pour over that and set a plan. Until then I dug my half finished GM70 SET out and will finish it. At least I KNOW what I have to do to finish that one

 

RE: BACK ON THE SHELF SHE GOES, posted on April 22, 2020 at 21:21:17
airheadair
Audiophile

Posts: 378
Location: California
Joined: October 18, 2010
Wow this is disappointing. Both the Futterman rebuild and the Transcendent rebuild failed.
I was very interesting in hearing how each turned out. My OTL3's are still going strong, thanksto Roger Modjeski. But he is gone now, and if I have trouble again, I fear I am doomed. I doubt that I will every find anything else that can handle my Stax speakers as well.

 

RE: BACK ON THE SHELF SHE GOES, posted on April 22, 2020 at 21:24:35
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
I have not given up on this amp. I just need to do some research and studying before I attach it again.

 

RE: BACK ON THE SHELF SHE GOES, posted on April 23, 2020 at 13:21:27
Lew
Audiophile

Posts: 10455
Location: Bethesda, Maryland
Joined: December 11, 2000
Are all of the NYAL guys gone from the scene?
Not to mention Atma-sphere amplifiers and Ralph.

EDIT. Bruce Rozenblitt is still active, is he not?

 

Nah- should be easy enought to sort out nt, posted on April 28, 2020 at 12:58:31
Ralph
Manufacturer

Posts: 3960
Location: Minnesota
Joined: April 24, 2002
-

 

RE: Nah- should be easy enought to sort out nt, posted on April 29, 2020 at 08:07:16
djn
Audiophile

Posts: 1955
Location: michigan
Joined: January 15, 2003
It should be Ralph. I just recieved the original glass audio magazine with this amp build and waiting for Bruce's book on OTLs. So while I wait to get smarter about OLTs I'm finishing my GM70 SET amp.

 

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