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Help needed: OTL3 Front End Bias not reaching 85V

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Posted on January 29, 2015 at 14:38:42
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 One of my NYAL OTL3 monoblocks sounded like it was clipping, so checking it out replaced a 6LF6 as it had a slight red glow when operating. After that, running through the setup found the front end bias is having a hard time going over 60V or much less than the 85V target. Did a quick tube check and they're all fine. Before I take it apart, has anyone seen this issue? Something simple, I hope...

 

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When was the last time filter caps were changed in the bias supply? nt, posted on January 29, 2015 at 14:46:25
Ralph
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RE: When was the last time filter caps were changed in the bias supply? nt, posted on January 29, 2015 at 15:02:37
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 Thank you for the fast response!

About ten years ago George Kaye went through the amp and I had the caps in the signal path replaced with DynamiCaps along with checking all of the power supply caps (some were replaced). Anything else to look at when I take it apart?

Regards,

 

RE: Help needed: OTL3 Front End Bias not reaching 85V, posted on January 29, 2015 at 19:43:25
Lew
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Location: Bethesda, Maryland
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 Check the Dynamicaps for shorts where the leads exit the body of the capacitor. They are fragile. The output tube "glowing" might have meant that the coupler cap to that tube is at least intermittent. This would alter bias (reduce or eliminate it) on that tube which would result in thermal runaway, and could have caused the glow. This is a long shot, I admit.

 

Yes- resistors that have gotten hot. nt, posted on January 30, 2015 at 08:12:44
Ralph
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RE: Yes- resistors that have gotten hot. nt, posted on January 30, 2015 at 09:42:53
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 Thank you, I'll check that too (I've replaced a few tube bias resistors over the years, so I know some have gone bad).

Best regards,

 

RE: Help needed: OTL3 Front End Bias not reaching 85V, posted on January 30, 2015 at 19:48:53
rhinohifi714
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Posts: 174
Location: oklahoma
Joined: June 13, 2009
 I would try swapping out the 6EJ7 from the working amp to the other and see If that gives you the 85v, if not you know it's something on the board.
good luck

 

RE: Help needed: OTL3 Front End Bias not reaching 85V, posted on January 31, 2015 at 09:16:43
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Joined: March 21, 2014
 Thank you!

Tried swapping out the 6EJ7, but no luck. Tested all of the tubes on a tester and they test fine too. Think I found a resistor that has significantly drifted and ordered a replacement to try (right by the front-end bias test point, so that would fit with what I'm seeing). The caps appear to be fine and I'm a bit surprised given the amps age...guess driving ESL's are good for it. My OTL3 doesn't really match the schematics I have that well, so I need to try it to make certain that's the issue. I'll post what I find.

Thanks again!

 

RE: Help needed: OTL3 Front End Bias not reaching 85V, posted on February 6, 2015 at 09:37:00
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 R37 wasn't it ... swell :(

 

RE: Help needed: OTL3 Front End Bias not reaching 85V, posted on February 6, 2015 at 17:57:56
Banat
 -Check for bad solder joints on all tubes sockets pins on underside of PCB board ,with special attention for Magnoval tube sockets pins dedicated for output power tubes and voltage regulator tubes , ( hope that original tube sockets are ceramic high quality types ) .
-Also check few small electrolytic capacitors for correct values which are placed on the top of PCB , they want to dry up when work in hot surrounding as is by any OTL amp.
-Check all resistors which are with mega-ohm values , and all others resistors with values above 220K , after long period of use those resistors sometime want to change(raise)the value .
-And yes , before doing anything from above make sure that NO electricity is present inside of that OTL unit , with special attention on PSU electrolytic capacitors , since they want to keep the potentially shock hazard electrical charge for several days .

Best Regards

 

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