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Old 13th January 2007, 09:28 PM   #1
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Question valve wear symptoms?

Hello, I've always asked myself what are the symptoms of a valve that is wearing out.... thank you
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Old 14th January 2007, 01:29 AM   #2
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Default Re: valve wear symptoms?

Quote:
Originally posted by testarossa2k
Hello, I've always asked myself what are the symptoms of a valve that is wearing out.... thank you
Reduced gain, increasing distortion, and plate current that drops almost as soon as you power down. Good ones will keep emitting for awhile even as the cathodes cool off, and the plate current will tail off gradually.
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Old 14th January 2007, 01:30 AM   #3
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Default Re: valve wear symptoms?

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Originally posted by testarossa2k
Hello, I've always asked myself what are the symptoms of a valve that is wearing out.... thank you
smoke in the exhaust and lower horsepower
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Old 14th January 2007, 01:51 AM   #4
SY is offline SY  United States
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Shortness of breath. Badda-boom.

Lower transconductance and gain, higher plate resistance and noise, wandering off operating points, lower power (output tubes), drawing grid current, glowing red hot where it never glowed red hot before, and the ever-popular dark heater. Pick any of the above, they all happen.
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Old 14th January 2007, 01:46 PM   #5
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Quote:
smoke in the exhaust and lower horsepower
good this one

I have a valve preamplifier (vincent SA-31) and it seems to my ears that one channel plays lauder. Now I'm doing some test to see if it's a room effect, cable or what.
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Old 14th January 2007, 07:46 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Additionally the getter may start to lose its mirror finish, or dissapear altogether in a well used valve.
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Old 14th January 2007, 09:30 PM   #7
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In class A UL p-p the symptoms with high gm tubes (6550/KT88 series) usually end up destructive with a red hot anode, screen resistors destruct and usually meltdowns the cathode resistor (if fitted). A fuse in the Bmax line is essential.

With 5U4 directly heated rectifiers, thermal gradients caused by repeated switching on/off over time gradually flakes off the filament coating-> leading to loss of emission.

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Old 14th January 2007, 09:46 PM   #8
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I just got a Radio Craftsmen RC-2 mono amp off of Ebay, and I have an old Sylvania Type 620 tube tester.
The amp lights up, but doesn't pass any audio, just as the seller said.

It uses a push-pull 2 x 6V6GTA output with a 5Y3GT rectifier, and 1 each of a 6J5 metal tube, and a 6SN7GT for the pre-amp stages.
The parts under the chassis seem very simple, in fact less than 18 caps & resistors total, not including the metal can caps (one with 2 elements, one with 3, both Mallory). On one web forum site one guy said he is running his with the original caps, but I'm ordering new ones just in case.

I couldn't find any free schematics for this, so I had to order one for $5, it will take a few days or so.

All the tubes test good in all areas, but both 6V6's test very low on transconductance, it's in the "red" on my tube tester, though all the others were fine. The only major mod I noticed was an aftermarket unversal output xmfr, but it seems ok, no shorts or anything.
One of its leads goes back to pin 7 of the 6J5 (one side of the heater), since I don't have the schematic yet this sounds weird.

There are no controls, just an RCA input jack, and speaker screws for 4,8, & 16 ohms, and no switch, just plug it in to start it.

Do you think the 6V6's are probably shot, even if the caps are ok, and that this alone would cause no sound?
I'm ordering new tubes and will replace all the caps before trying it again.
One weird thing is that there seems to be 2 power resistors with markings new to me, one says RW-4-301K but tests at 300 ohms,
the other says RW-7-222K but tests at 22k ohms, I can't see the logic behind this. Also I should probably replace all the resistors, since I have nearly 100 1/2 watt ones, and some of the originals seem to have drifted more than 20 percent.

---------

My other question, does anyone have a great link to a DIY project or schematic or brand/name/model of any great amp using 2 6V6's in push-pull, with some audio control pots, etc for a real guitar amp.
I'm seeing at lot of circuits on places like Duncan's & MachMat, etc, but what's someone's real fave for sound simplicity, etc. (Maybe I should make a new thread for this last question, unless someone can point me to a great thread on this).
Don't need quite to power of 6L6's.
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Old 15th January 2007, 11:14 AM   #9
bembel is offline bembel  Europe
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Hi,


Quote:
One weird thing is that there seems to be 2 power resistors with markings new to me, one says RW-4-301K but tests at 300 ohms,

301K means 30 x 10^1 (last number being the multiplier) K is the precision, so 222K stands for 22 x 10^2= 2200 ohms.
I think that the prefix is for power rating of your resistrors (confirm with the size )

Hope this helps...
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Old 15th January 2007, 06:42 PM   #10
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K being 10% tolerance.
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