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Old 29th March 2012, 09:37 AM   #1
roccale is offline roccale  Italy
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Default Strange couple comportament 6L6GC RCA black plates.

I have a problem with my final, the SANSUI AU-111.
I recently put in place by doing a bit of overhauling electricity, luckily it was replaced a few things, keeping up the original sound character.
The amplifier is constituted by:
Stage with 5 pre ECC83/12AX7 MATSUSHITA (the so-called Mullard Japanese) I have to say if the fight very well with smooth plates and telefunken ECC83 Mullard cv4004, so that when I'm using this right ... I'm amazed ...! !
as the final stage is a pushpull of 6L6GC PLAQUE A 485V and negative grid-44v (about 36/38mA) driven by a pair of 12bh7a RCA BLACK PLATE and a Mullard ECC85.
I used until recently, a quartet of 6L6GC made in usa but is not present in the make, say good-valve, never had problems, and respected (and still comply if I mount) all operating parameters.
Lately, I've got a quartet, even though a sestina 2 nn yet been received, the RCA BLACK PLATE 6L6GC, which by all accounts is the best ever made 6L6GC.
By mounting these tubes you realize immediately the validity of the same, an 'other music on all fronts, depth, soundstage, detail, placement of instruments, in short, excellent, little to say ...
There is one though.
In two of these tubes during operation a corner of the plaque becomes red, not red like the filament of course, not so red glow issue but obviously it shows.
I thought at first to measure how many came on foot 8 mA, and I discovered that with the trimmer to a minimum (each has its 6L6 trimmer for adjusting the negative bias in short) with a negative of about-48V, the tube sucks 60mA, and 'other 56th ...
The other 2 are a-44v with 35/36mA ...
I thought it was an issue of the final so it was easy + change the position of the tubes, so I did.
I put one of incrimniate 2 per channel, and the result is the same ... same ...
I changed more position and the behavior is the same.
The conclusion is that the blame for the phenomenon of tubes ...
The amplifier is electrically in order.
So I asked myself, but perhaps sarannpo exhausted ...
Not at all, the two offending measure, respectively, 2500 and 2400 mocrohmos, with a pass fail to 1300 tubes ... so very strong, the other two (those that do not turn red) stood on the 2100-2200 microhmos ...
Why does this happen??
Obviously I'm not using those two pipes for fear of any consequence on the finish, especially the output transformers that are precious HASHIMOTO 60 years, as the 'unit.
Added to this is to say that there is no effect on the sound even with the red pipes (repeat not think they are hot but the center of a bass note of a flush valve 2).
Obviously, with those two tubes there is a considerable dissipation of heat come from 6L6 and with the other valves not exist at that level, and the power transformer heated at least a 20% to +.
Can anyone help me understand this phenomenon and confirms to me if they can be affected by using it with the 2 valves in question ...
I'm waiting for the other 2, but it sounds really divinely comes too ... want to hear ...

I had to fall back on good old sunaudio sv300be who took the powder, but there is no history, it destroys the sansui ...
Thanks all.
Marco
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Old 29th March 2012, 10:47 AM   #2
DF96 is offline DF96  England
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Some paragraphs splits would help in reading.

You may have gassy valves. This creates grid current which upsets the bias and causes excessive anode current, hence the glowing anodes. A gassy valve may measure OK for current and gain.
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Old 29th March 2012, 11:01 AM   #3
roccale is offline roccale  Italy
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Many thanks.
In your opinion is dangerous use this tubes on my amplifier in this condition?
Marco
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Old 29th March 2012, 11:17 AM   #4
DF96 is offline DF96  England
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Two dangers: excessive heat will shorten the life of nearby components, especially electrolytic capacitors; the gassy valves may go into thermal runaway which could damage the output or mains transformers through overheating.

I would stop using them.
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Old 29th March 2012, 11:21 AM   #5
roccale is offline roccale  Italy
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Perfect.
In fact I'm using other tubes.
Really thank you very much.
M
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Old 29th March 2012, 12:34 PM   #6
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I have no experience in 6L6 tubes, so I may be wrong, but searching for a datasheet, it appears that 485 anode voltage is a bit high,

http://www.drtube.com/datasheets/6l6gc-sed1996.pdf

Besides, the actual 6L6s are "normal" and the original are a bit over dimensioned?
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Old 29th March 2012, 01:06 PM   #7
roccale is offline roccale  Italy
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This voltage is provided in this amplifier, please see the diagram.
Thank you.
Marco
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Old 29th March 2012, 07:10 PM   #8
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Is the bias rectifier a modern (silicon) device? An older (selenium) rectifier ages poorly and should be replaced. Also check (actually, just replace) C81 to give yourself as much bias voltage as you can get.

All good fortune,
Chris
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Old 29th March 2012, 07:27 PM   #9
roccale is offline roccale  Italy
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Hello, can you show me on the diagram which the bias rectifier mean?
Many thanks.
Marco





Quote:
Originally Posted by Chris Hornbeck View Post
Is the bias rectifier a modern (silicon) device? An older (selenium) rectifier ages poorly and should be replaced. Also check (actually, just replace) C81 to give yourself as much bias voltage as you can get.

All good fortune,
Chris
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Old 29th March 2012, 10:38 PM   #10
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On the drawing, the top set of windings on the right side of the power transformer is the tap for the bias. Note that the voltages are negative. The diode in that circuit is the rectifier in question.
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