Problems With Simple Gounded Cathode Gain Stage - diyAudio
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Old 23rd January 2006, 02:33 PM   #1
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Default Problems With Simple Gounded Cathode Gain Stage

I completed my 3rd modification of a Sony 685 CD/DVD/SACD player by placing a 12AU7 ground cathode gain stage to take the analogue signal just before the OP amps (front right and left only).

Click the image to open in full size.

A few problem developed that I have not had with my other exact mods to this player.

BUZZZZZ. Lots of audible buzz, too much to be listenable.

Loss of gain, a .5 volt square wave input nets a .3 volt output.
See screen shot, this is with the intput signal before the 1.5 K resistor, the lower trace is the output.
Click the image to open in full size.

This is the trace with the signal inputed after the 1.5 K resistor.

Click the image to open in full size.

The 3.3uF caps seem to be guilty as handling them increases the buzz.

Additionally the stage only biases in the .4 volt range on one side and .2 volts on the other. Changing tubes does not make a difference.

TubCad calculates that the bias should be -3.42 volts, a gain of 22.7 db, my B+ is 270.

POWER SUPPLY FILTERS
Click the image to open in full size.

TUBE STAGE
Click the image to open in full size.

Any suggestions for trouble shooting would be appreciated.

dr_sleep
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Old 23rd January 2006, 03:56 PM   #2
Jax is offline Jax  Sweden
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It seems to be the heater wiring.

12AU7 wants 12.6V between pins 4 and 5 or 6.3V between 4 and 5 strapped together and pin 9.

The cathodes should light with an orange glow when properly heated.
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Old 23rd January 2006, 05:50 PM   #3
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The design is to starve the heater filaments but I will connect them in the usual way for 6 volt operation to see if this will effect the BUZZZZ..

Thanks,

dr_sleep
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Old 23rd January 2006, 05:55 PM   #4
SY is offline SY  United States
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May I assume that the power supply grounding is as casual as the signal circuitry you're showing? If so, we may have found the source of the buzz...
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Old 23rd January 2006, 08:31 PM   #5
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Well, I am concerned about the grounding scheme as the outputs and the gain section are grounded to the chassis while an additional separate ground is formed with the power supply, a ground taken from the cathode resistors, grid leak resistors and output ground. Perhaps using insulated outputs, not grounded to the chassis would cure the problem.

I still am confused about the bias which seems low for the operating conditions. All the resistor values check out and it is not tube related.

My plan will be to insulate the output from the chassis by changing the RCA's and not grounding the cathode resistors to the chassis also but just the main star ground.

Any other bright ideas?

Thanks,
a "Casual" dr._sleep
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