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 Post subject: Redplating 6L6 pictures
PostPosted: Nov Thu 09, 2017 1:14 am 
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Not a question, but since we hear of the infamous red plate sometimes, I thot I'd post these pics.

The old Traynor PA amp was being mustered into service for a heavy metal band my nephew was affiliated with. I recapped it and added a 3-prong power cord, etc. On the bench, tubes facing up, the amp seemed to be fine and I was about ready to let it out the door. But when back on its feet, tubes hanging down, I powered it up and saw both power tubes practically on fire.
I turned it on again briefly to catch these pictures, no flash so they're blurry. There were no issues with bias or other, and a new set of tubes took care of the red plate issue.


Attachments:
6V6Redplates.jpg
6V6Redplates.jpg [ 254.04 KiB | Viewed 696 times ]
6V6Redplates1.jpg
6V6Redplates1.jpg [ 245.4 KiB | Viewed 696 times ]
6V6Redplate2.jpg
6V6Redplate2.jpg [ 253.25 KiB | Viewed 694 times ]

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Last edited by westcoastjohn on Nov Sun 12, 2017 7:22 am, edited 1 time in total.
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 Post subject: Re: Redplating 6V6 pictures
PostPosted: Nov Thu 09, 2017 1:36 am 
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Yikes! That second photo is a good photo of a classic example of red plating.


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 Post subject: Re: Redplating 6V6 pictures
PostPosted: Nov Fri 10, 2017 8:43 pm 
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What caused the red platting? Just bad tubes?

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 Post subject: Re: Redplating 6V6 pictures
PostPosted: Nov Fri 10, 2017 11:54 pm 
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Those tubes look like EL-34s, not 6V6s. Have you checked the g1 bias voltage, and the caps that couple the g1s to the preceeding stage?

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 Post subject: Re: Redplating 6V6 pictures
PostPosted: Nov Sat 11, 2017 12:01 am 
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TPAairman wrote:
What caused the red platting? Just bad tubes?

Could be bad tubes. I just had to side line pending further testing 3 6BQ5s that red plated in an amp I restored. They tested OK in the checker. Two new JJ brand tubes operate fine. Also incorrect bias to the signal grids. Too positive usually equals red plates. :cry:

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 Post subject: Re: Redplating 6V6 pictures
PostPosted: Nov Sat 11, 2017 12:50 am 
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Location: pensacola fl
Check both plate current and plate voltage. You can open the opt primary connection to B+ and insert your dvm set to current then divide by 2 to get an idea of current or if cathode resistors are there measure voltage across them use ohms law to get current. Remember plate voltage times plate current equals plate dissipation with no signal in. You need head room as well because with signal plate current can rise. I am aware that the cathode resistor method is not as accurate as cathode current also has screen grid current as part of the reading. Look up your output tubes in a tube manual to get the max the tube can handle. Over load it at your own risk of shortened tube life and other possible damage as well.


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 Post subject: Re: Redplating 6V6 pictures
PostPosted: Nov Sat 11, 2017 2:32 pm 
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stromberg6, I'm with you, the tubes look more like EL34's or possibly earlier Russian 6L6's and not 6V6's as the tubes look way too large to me. Harry


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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Nov Sun 12, 2017 7:16 am 
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The tubes were bad, but only when hanging down. Something got loose inside, so I think maybe the amp was treated badly? Kicked off the stage? :x

They are 6L6's, correct. It was a while ago, and I was confusing this one with another amp. Anyway, I just found the pics and thot I'd put them up.

They say that Mr. Traynor tested his amps by dropping them from the roof of his workshop.
Well maybe he did, maybe on a dare or to win a bet, but surely not with the tubes installed. :lol:

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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Nov Mon 20, 2017 8:46 pm 
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once the tubes have "cherried out" like that they are usually ruined.
Most common cause for this is running it with leaky coupling caps or the bias has gone way off, or both.
Mark Oppat


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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Nov Tue 21, 2017 3:36 am 
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Right, I think that might have been it, weakened by past overheating.
I probably replaced the coupling caps along with the electrolytics, but that info is lost in my memory archives. :lol:

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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Nov Wed 22, 2017 1:59 pm 
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oldradioparts wrote:
once the tubes have "cherried out" like that they are usually ruined.
Most common cause for this is running it with leaky coupling caps or the bias has gone way off, or both.
Mark Oppat


Depends how long the "red plating" has been going on. They can take such abuse for a short duration, but getting plates that hot for a long period is death sentence for the tubes.

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