OTL Asylum

RE: HELP: DC at the input of an old MA1's.

63.224.21.219

In Reply to: RE: HELP: DC at the input of an old MA1's. posted by Lew on December 14, 2007 at 08:01:08

During warmup the 6DJ8 can have some peculiar behaviors. When the amp was built, nearly everybody had preamps and so there were no worries. After living with the 6DJ8 for a while it was obvious that things were going to be better if we went to a different tube, so we went to the 12AT7 sometime in early 1990.

The voltage you are seeing is grid current from the 6DJ8 and is very low current. There are several easy things you can do to sort it out.

1) install the 100K resistors to ground, replacing the 0.5M resistors.

2) install a pair of 4.75K resistors in series with the input, from pin 2 of the XLR to one of the 6DJ8 grids and another from pin 3 of the XLR to the other grid of the tube.

The combination of the above will curtail the voltage enough that even a slight load on the input will attenuate the spurious voltage a lot. For example if the output impedance of your CD player is around 600 ohms, the spurious voltage will only be about 0.9V.

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RE: HELP: DC at the input of an old MA1's. - MaceEar 06:48:13 12/21/07 (0)

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