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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 2711 times ]
6V6Redplates1.jpg
6V6Redplates1.jpg [ 245.4 KiB | Viewed 2711 times ]
6V6Redplate2.jpg
6V6Redplate2.jpg [ 253.25 KiB | Viewed 2709 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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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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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Mon 25, 2017 4:32 pm 
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time to put the tubes on a good tube checker. hopefully they are not shorted and still have good emissions


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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Thu 28, 2017 5:13 am 
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At this point, the main cause of redplating in vintage tube amps seems to be damaged or defective bias supplies more than any failure in the tubes themselves. Ancient selenium bias supply rectifiers decay or the bias supply filter caps get leaky until you just can't get enough bias adjustment, or the unadjustable types, just falls until you are pulling too much idle current. Every now and then you also see the ground connection from the socket to the chassis gets corroded and you lost the return path for the bias - particularly on kits, where the sockets are mounted with screws that have gotten loose over the decades.

The very first thing to check if you have redplating is that the bias is correct and steady. I always replace the tubes that have redplated whether or not you can get it to stop by correcting the bias fault, although I am not sure it is necessary.

Brett


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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Thu 28, 2017 5:37 am 
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i've restored a few amps that used the massive 7027A cokebottle tube lineup of quads and pairs.

in one instance, the plates on one pair must have been so red for so long that their edges (where they came together by the glass) were actually twisted and warped like an airplane propeller.

the "square" center of the plates was also dimpeled and warped in its own unique way. they had a unique color that ranged from their original black to various shades of grey to light grey.

they definitely saw some redplating in their lives :shock: .

oddly enough, they tested fine, worked fine, and drew cathode current within 5-8 mA of each other :? .

i felt sorry for those poor tubes :cry:.

steve

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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Thu 28, 2017 4:10 pm 
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Dutch Rabbit wrote:
...
in one instance, the plates on one pair must have been so red for so long that their edges (where they came together by the glass) were actually twisted and warped like an airplane propeller.
...

i felt sorry for those poor tubes :cry:.

steve


Steve, I have seen the edges of the plates in 7591 tubes very crispy and festered. But as you mentioned, they tested and worked just fine. That 7591 (talking vintage USA manufactured, not the foreign impostors) is a very robust tube for what it is in that little envelope.

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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Fri 29, 2017 3:01 am 
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yes, i have too on some stromberg 7591's, but not anything like i did on those poor cokebottle honking 7027A's.

not only were they warped, they looked like my grandma's old 1950's skillet with various colors of "heated metal".

steve

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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Fri 29, 2017 3:32 am 
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Don Cavey wrote:

Steve, I have seen the edges of the plates in 7591 tubes very crispy and festered. But as you mentioned, they tested and worked just fine. That 7591 (talking vintage USA manufactured, not the foreign impostors) is a very robust tube for what it is in that little envelope.


The 7591A was used right up the specifications in many old tube amps because you could get a lot of power in a pretty short cabinet. That was fine when the two manufacturers (Westinghouse for sure, and I think their hated rivals Sylvania) developed them for use the the SAGE radar system where, very literally, the fate of the human race could possibly ride. So they were built to last. Now, they are basically replacements for medium power vintage tube amps and they aren't *neatly* as robust.

Brett


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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Fri 29, 2017 3:24 pm 
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I think that I have several 7591 tubes in a Scott amplifier on a shelf somewhere. if I remember they are robust versions of 6L6


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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Dec Fri 29, 2017 4:41 pm 
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I know that this is a little off topic but...

Westinghouse designed the 7591. It also carried several brands on them but if you look carefully, you will probably find that they were either made by Westinghouse or Sylvania. RCA rebranded them as did many others. I have some GE branded ones but they are clearly Sylvania by the font and ink used.

Toshiba and Hitachi made them in Japan as well. They are truly remarkable for what they are.

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 Post subject: Re: Redplating 6L6 pictures
PostPosted: Jan Wed 17, 2018 7:48 am 
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Greetings to the Forum:

I don't have any photos, but I have seen TV horizontal output tubes with the glass pushed in due to localized overheating. Perhaps someone could post a photo if they have one?

Regards,

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