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Old 17th January 2006, 04:16 PM   #1
Tubesin is offline Tubesin  Macao
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Default Schematic

Hello, can you advise me where to get a schematic for Audio Research amplifier D115? I need it to check the working voltages in the mentioned TP points because one of the power 6550A tubes in right channel just burned out after a few poping sound. Thank you
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Old 17th January 2006, 04:38 PM   #2
anatech is offline anatech  Canada
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Hi Tubesin,
Just checked, sorry I don't have it.

One tube went bang? Probably a tube fault or I'd expect more to go. AR likes to regulate the screens, so check that. One reg does both channels I think. You may have lost a resistor or two, so check them. There is always the bias supply to look at also.

What brand of outputs were you using?

-Chris
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Old 18th January 2006, 12:45 AM   #3
Tubesin is offline Tubesin  Macao
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Hello Anatech, the tubes are 6550A Billington Gold. When I replaced the banged one, the poping still happens, so, as you suggested, I have to check the screen supply. It is operating in pentode mode because the output primary have only b+ and 2 plates HV. Thank you
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Old 18th January 2006, 01:29 PM   #4
anatech is offline anatech  Canada
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I wonder if you are losing the negative grid bias. Might be a loose socket. These can be hard to find.

-Chris
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Old 18th January 2006, 06:50 PM   #5
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Tubesin

I don't have the schematic either but this may help.
Since this is a stereo amp, use the working, good? left channel as a reference. Sorry I can't help with the schematic but I will try and search for it on the net.

Wayne
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Old 19th January 2006, 01:03 AM   #6
Tubesin is offline Tubesin  Macao
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Up to now, I discovered one side of theparalell tubes cathode to ground resistors of 1 ohm? are open. Still looking and studying further the circuits, thank you
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Old 19th January 2006, 01:12 AM   #7
anatech is offline anatech  Canada
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Hi Tubesin,
Now there is a sure sign of excessive current. A blown resistor may be arcing and you really have a blanket problem affecting all those tubes.

The guys at AR are pretty decent. Try sending them an email requesting a schematic. Let them know what you have found.

-Chris
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Old 19th January 2006, 02:04 PM   #8
Tubesin is offline Tubesin  Macao
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Yes, I'll do it. I suspect the power tubes are not strong enough, or, wearing out is the cause. I'll replace the resistor and a fresh set of tubes to see. Since all others resistors to the tubes look according to the values on the respectives colors, or, maybe, some over loaded and maybe weak or noisy or arcing when power on. Yeah, its very nervous. Also, there is a tube regulated supply in the middle of the chassis, it supplies to the voltage and driving tubes of both channels it looks. Does the big power supply capacitors harmed with the failing tubes? I attach a pic, the resistor at the same level of V15 and V13 are open to ground.
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Old 19th January 2006, 02:30 PM   #9
anatech is offline anatech  Canada
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Hi Tubesin,
That regulated supply is most likely for all the screen grids, both channels. The caps will be fine, age effects excluded.

I strongly recommend that you obtain a schematic from the manufacturer. I do have other schematics in electronic format if it will help you.

Do not just install a new set of tubes and "spark" it up. If you yank the outputs you can run the amp up to about 90 VAC. Watch for smoke (you shouldn't get any, but we are dealing with a failure). Now you can take measurements to check grid bias voltage and screen voltage. You can bring the AC voltage higher to make the plate voltage agree with normal values, the heater voltage will be slightly low.

-Chris
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Old 19th January 2006, 03:20 PM   #10
Tubesin is offline Tubesin  Macao
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Hi Chris,
The screen regulation circuit board is in between opt and power transformer. For screen regulation, I think does not need to use KT88 as a pass-tube. I'm still thinking how to reactivate the amp and the use of a variac is a must. But I don't know if the relay in the lower part wl activate or not if I inject 100volt to the primary(the unit is 220V). I attach a photo of the underside for your appreciation
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