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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: RE: Scott 299B Output tubes glowing Red posted by onekid on May 6, 2016 at 18:31:14:
Since you've changed the coupling caps and you can get .99 vdc at the test point for each output pair, I'd think that the bias rectifier is OK right now but for how long if you didn't replace it? But that's another issue.
I'd be looking at the DC balance setting but that's hard with the way the 299B is currently wired with one 18 ohm resistor in the cathode lines of the output pair.
The overall bias voltage per output pair is adjusted with the bias pot and -15 volts on pin 1-2 is an approx voltage but the actual bias voltage needed to have each tube draw the right amount of current depends on the tubes and unless the tubes in each channel are well matched, the bias voltage will be different for each tube in the pair.
The DC balance pot in each channel shifts the bias voltage down on one tube and increases the bias voltage on the other tube in the pair so that hopefully they will each draw about the same current.
The .99 volts as measured from the test point is the voltage across the 18 ohm resistor in each output pair cathode line which seems right when converted to current with that value resistor BUT if one tube is doing all the work, it may glow red because it's over conducting.
You have the right voltage figure but you don't know how well both tubes are sharing the work to get it.
You can try the hum test to see if you can hear hum on each channel with the volume all the way down.
If that works for zero hum then see if an output still glows red but make sure you still have around .99 to 1 VDC VDC at the test points and not more.
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Follow Ups
- RE: Scott 299B Output tubes glowing Red - DaveV 20:31:15 05/06/16 (2)
- I did it right and rebuilt the 299B with individual - gkargreen 17:42:08 05/10/16 (0)
- RE: Scott 299B Output tubes glowing Red - onekid 09:32:37 05/07/16 (0)
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