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Old 23rd May 2011, 08:41 PM   #1
zxx222 is offline zxx222  Europe
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Hello,
finally I assembled amp on breadboard and it runs well – nothing exploded so far, music flows through my Fostex 126e BIB's effortlessly, no hum is audible. I want to thank everybody there for shared ideas and experience that allowed me to build it and make it running (this was my first amp build).
I used following iron – Hammond 374BX power transformer, Hammond 193J choke, Edcor GXSE 15-8-5K opt’s. Tubes- Electroharmonix el34 and 12At7, sovtek 5AR4 rectifier.
I would be thankful if anyone could comment about voltages I measure: B+ (at R4) 440V, cathode voltage of power tubes (R17/R27) is 30,6V; cathode voltage of 12AT7 is 2,4V.
If I calculated well, I run power tubes within specifications (55mA, 22,4W dissipation with cathode resistor of 560ohms). My concern is 12AT7 tube with measured cathode voltage of 2,4V. Is it ok to run it at 11mA? I made DN2540 cascode instead of ixcp10m45s and R13/23 is 180 ohms. Should I adjust it to lower current or is it ok?
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Old 24th May 2011, 02:28 AM   #2
rknize is offline rknize  United States
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George's design has the CCS set to 9mA, IIRC. That's leaning on the 'AT7 pretty hard already. The current-production JJ tube I had in there didn't last terribly long. I have a JAN one in there now and it seems OK (though I haven't pulled it out to check it yet). 11mA seems like a bit on the high side.

55mA is quite a bit for an EL34 as well, but I know some of the modern ones can handle it. Not sure about EH...I have some but never put them in the SSE.
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Old 24th May 2011, 07:47 AM   #3
zxx222 is offline zxx222  Europe
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How long your JJ lasted and what is normal service life for these tubes? These EH's are not expensive and will I use them for start up and burn-in. I had plans to use kt88's, but I think I will make some tube rolling to find out what I like most.
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Old 24th May 2011, 03:31 PM   #4
rknize is offline rknize  United States
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I had a batch of JJs go soft on me after only a few hundred hours on them. This was in two different amps. Some people have had similar experiences while others have had no trouble at all. They do seem to measure well in terms of linearity and are cheap. I had started a thread on the topic:

JJ ECC81/12AT7 Longevity

To be clear, it still worked in the amp. The CCS keeps raising the plate voltage as the tube wears to compensate for drops in emissions. Eventually it will run out of headroom, though. At some point the tube will distort from operating so far out of the intended region. I didn't notice any such issues when I pulled the tube, but I didn't do an intentional A/B test. The JAN tube in there now is actually showing signs of getter wear. I should probably recheck what the actual plate current is now.
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