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Old 24th June 2011, 03:52 AM   #21
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Originally Posted by tbie View Post
Even with output tubes B+ is hanging around 440V
Then lower C4 value to get around 370 for B+ when output tube bias is set around 65mA.
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Old 28th June 2011, 07:17 PM   #22
tbie is offline tbie  United States
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Sorry everyone, been busy these past few days. I tested everythign after resoldering the output sockets, changing MOSFETs on both channels, and 10M45s on Left. I started making measurements.
with rectifier and 5842 tubes in,

B+ = 440, I could reduce it to 354 by turning bias pot on left, no change noticed when R bias pot is turned in either direction.
grid voltage
L = -104
R = -85
plate voltage at coupling cap
L = 145V
R = 0, no measurement what so ever.

Is the coupling cap to blame on R for the troubles. I plan to change coupling caps in next couple of days and check.
Is there anything I need to check? I did check the pots and they do measure appropriate resistance.
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Old 28th June 2011, 08:43 PM   #23
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Swap the tubes and see if the problem follows.
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Old 29th June 2011, 12:02 AM   #24
tbie is offline tbie  United States
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No, swapping tubes doesn't seem to change anything. R still has problem.
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Old 30th June 2011, 12:21 PM   #25
tbie is offline tbie  United States
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Time for retesting, actually did this yesterday afternoon but couldn't post.
without tubes,
B+ = 0.00 I did not reduce voltage from 500 on the meter.
B- = -316
filament voltage
R = 4.99
L = 4.99
Board setup for 300B
With rectifier tube
B+ = 460
B- = -276
Output grid voltage
R = -85-92 (varying on its own)
L = -99V
With input tubes(5842)
B+ = 460
B- = -264
Output grid voltage about the same as above
Plate voltage
L = 46V no change in voltage manipulating cathode pot
R = 0.00 with cheap meter
with auto digital meter (ideal) varying voltage from 112-357 and climbing without touching cathode pot.
Stopped testing at this point. Coupling caps changed to Jantzen standard Z 0.22uf 400v, these are the caps that I couold get my hands fastest while waiting for russian PiO caps.
Resistors in signal path are all working well, pots are good, caps in signal path are looking goood. No bulging, no smell, no corrosion around.
Could the input tubes be at fault? Could they produce this oscillating voltage? Why can't I change voltage on left? I would appreciate any answers or suggestions?
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Old 30th June 2011, 04:38 PM   #26
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Can you post a close up picture or two of your board?
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Old 30th June 2011, 05:08 PM   #27
rknize is offline rknize  United States
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It almost sounds like the grid is floating. Can you measure the bias at the gate of the FET? Do the two side match there? Maybe the bias pot is bad.
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Old 16th December 2011, 03:02 PM   #28
jrobson is offline jrobson  South Africa
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Hi tbie

Did you ever get your problem solved?
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