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Old 30th November 2008, 04:10 PM   #21
llwhtt is offline llwhtt  United States
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Burnedfingers,

Is the top list of voltages when it is working or not working, same for the bottom list, working or not working. Also pin 3 and 8 are tied together so they should read the same no matter what. Check and make sure you counted the pins correctly. Also what are the voltages on the upper tube of the cascode, there should be clue there too.

Craig
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Old 30th November 2008, 04:19 PM   #22
llwhtt is offline llwhtt  United States
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Burnedfingers,

I just figured out your voltage lists, I was looking at it wrong. Andy used a tube for each side of the cascode, I was thinking a tube for the upper and a tube for the lower, clear as mud now. Anyway the side with 363VDC is way too high for a 6DJ8, so I would have say that tube is NOT conducting, is the heater lit up? The professional soldering OK on the tube socket? Swap tubes around and see if the voltages follow.

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Old 30th November 2008, 05:56 PM   #23
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Thanks for the response Craig

Heater connections are ok. Swapped tubes and voltages stayed within a few volts of what I posted. I pulled one of the LM334-z's and nothing changed so I re-installed it and took out the other LM334-Z and blew the zener diode so I am assuming that one of the LM334-Z's wasn't working. But then I could wrong
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Old 30th November 2008, 06:57 PM   #24
llwhtt is offline llwhtt  United States
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Is there a voltage on the schematic for pin8 ? Your photo is cut off where the line is leading to it. With the cross-coupling you are getting voltages that normally wouldn't be there if the tube was not conducting, hard to tell from here. For example, there would be no voltage on pin 6 if the that tube was not conducting, so the voltage is coming from the cross-coupling. You know it's going to be something real stupid, it always is. Keep at it. Oh, can you tighten the pins in the socket, that's always a forgotten problem.

Craig
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Old 30th November 2008, 09:01 PM   #25
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The voltage on pin 8 on the input side was 2.46 and 2.41 on the other side.

Plate voltage is roughly 100 low on the input side and 100 high on the other side. Your right its probably something really stupid but I'll be darned if I see it right now. I'm about frustrated enough
to pull it out and throw it in the garbage and put in something that works.
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Old 3rd December 2008, 11:36 PM   #26
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Ok, plate voltages are more in the ball park now. Recent tests now show that the current sources are to blame for its inability to produce a nice clean sine wave. The signal is good on the input side of the tube and gets nasty looking on the feedback side.

So the question now is can I replace these CCS's with a different CCS board.
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Old 4th December 2008, 01:15 AM   #27
SY is offline SY  United States
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Yes. I didn't get a chance to draw up a schematic today, so I'll wave my hands:

If you use the cascode bipolar circuit, use a couple of reasonably fast NPNs, like 2N5210 or '2222 or the like. Connect the bottom of the CCS to the negative rail, then use something like a 200k resistor to feed the LED references from the B+. The zeners can be clipped out, along with the resistors that set the original CCS chips' currents.

I don't see any logic in using two parallel CCS.
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Old 4th December 2008, 01:27 AM   #28
llwhtt is offline llwhtt  United States
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Possible reason for the parallel CCSs is the max. current for a 334 is only 10ma. By my calcs using the voltages on the posted schematic 420/225 VDC I get 6.5ma per side for total of 13ma., too much for a single 334. Yes-no.

Craig
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Old 4th December 2008, 02:32 AM   #29
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SY,

If you could post it I would appreciate it as I cannot find my CCS board and schematic information. I have 100 or so 2N5210 transistors on hand so they aren't a problem.

llwhtt,

Thanks for the reply. I believe the material I have states 5.8mA (170-175) voltage drop so the 6.5mA would be right in the ball park. If I had some LM334-Z's on hand I would try another pair per side but I do have some nice little CCS boards courtesy of SY that I could try.
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Old 4th December 2008, 02:41 AM   #30
llwhtt is offline llwhtt  United States
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There are so many voltages listed for the same node it 's hard to tell what number goes with what number. I got to thinking maybe one is for ST70/MKIV and the other set for the MKIII, don't know. Anyway at 5.8ma times 2 we're still over the limit of a single 334. Looks like you're on the right track though.

Craig
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