Weird reading on GSG 6SL7

2nd harmonics for the masses

Weird reading on GSG 6SL7

Postby lth1 » Mon Dec 31, 2007 2:44 pm

Finally received the Weber SS 6AX5 (only took them 47 days). Started to do a comparision between the WAX5 and the 6AX5-GT rectified GSG with 6B4G installed, 6A3 installed, and the SS 300B version. With all 5 variations I am getting strange reading on the 6SL7 driver. Tried 3 different 6SL7's and all had similiar readings.

PIN
1 0V
2 57.6 - 68.3VDC
3 .112 - .118 VDC
4 0
5 143VDC
6 1.71VDC
7 6.12VAC w/51V bias
8 6.12VAC w/51V bias

What am I doing wrong, or what is causing the low readings on the first triode section? The readings were idential on the two triode sections on the original voltage test, shorted input, no signal.
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Postby TerrySmith » Mon Dec 31, 2007 3:50 pm

Check your 270k plate resistors, 3k cathode resistors. If ok, you may have a weak 6SL7.
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Postby lth1 » Mon Dec 31, 2007 6:26 pm

Terry,

I had checked the cathode resistors, and just now the plate resistors, and all the other resistors on the board, all check out OK. I have used 3 different 6SL7's, all tested better than new on a hickok 800.

Thank you for the responce.

Lee Hankins
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Postby TomMcNally » Mon Dec 31, 2007 7:06 pm

Lee -

I'd do this ... with all power off and capacitors discharged:

measure across all resistors ... comparing from side to
side on the 6SL7's sections. Then measure from each
point to ground and compare ... the actual readings to
ground are not important - what you are looking for
is "what's different"

It's either a wrong value or bad or solder joint
problem with R7 ... maybe backward or bad cathode
capacitor, or wrong value or bad R5.

You might also pull one end of the coupling caps loose
and take some readings if you don't get anywhere.

That's the nice thing about a stereo amp - you have a
good channel to compare to.

You'll find it ! Let us know what it was.

... tom
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Postby lth1 » Mon Dec 31, 2007 11:23 pm

Thank You Tom,

That did the trick, the GSG magic is back! It was the cathode cap, completely blown; I believe that it was blown back when I had problems with the SS rectification.

I had already check all of the resistors, but did not think about measuring from point to ground and comparing the two sides.

Listening to SS GSG with TJ meshplate 300B and 6SL7GT "tall boy" tubes, absolutely stunning.

Happy New Year to All
Lee Hankins
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Postby TomMcNally » Mon Dec 31, 2007 11:27 pm

Hey good to hear Lee !

Happy New Year and enjoy the amp !

... tom
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Postby Shannon Parks » Tue Jan 01, 2008 5:10 am

New Year. Check.
Help a tube brother fix his amp. Check.

You guys are the best - here's wishing everyone a super deluxe 2008!
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