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In the Forum: Melquiades Amplifier
In the Thread: Bias help
Post Subject: Bais and filament voltagesPosted by Romy the Cat on: 8/19/2013
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DA, looks like very nice  and lucid assembly. How do you like the sound of the thing?

The  output stage 200mA /187V. It is a bit too small but I will do. I do not know what the gap you have in your OPT. If it is limited to 200mA then stay there, you might crack the voltage to 200V if you need a little bit power from your 6C33C.

The  driver stage 205v (test point A) and -4.3v bais – that is perfect.

The DC drift at inputs – that shall not be. Where the drifts come from? The gas tube shall stabilize both positive and negative voltages. Are you sure that your preamps do not drive DC? Also, what the drift you are talking about? If you have the voltage dancing around -0.5 to +0.5 mA then I would consider it normal and you will not be able to stabilize it further. Try to hand-connect the single signal wire to your input jack. You shall not be able to hear any clicks or any effect indication that you are connecting or disconnecting. Also, I see a volume control at your input. Make sure that it is has impedance that in each position of the volume control the bias voltage does not change. You might pull the positive gas tube out of socket and shot the inputs. Then run the volume control up and down – your bias voltage shall be stable. Do not forget that positive bias that you apply to the input is virtually a shorting or juts a way to balance out the output impedance of your preamp and to kill DC what the amp is not connected.

The bias lines around 175v is too low and 5.5 mA current is very low. The gas tubes do not provide good regulation what they are too soft loaded. In my estimation you need to drive 20-30mA across the gas tubes in order then to be more stable in operation and surprisingly to sound better. If your transformer give too little voltage for bias supply then you need to adjust the volume of the R15 and R16 resistors or perhaps the R11 and R12 in order to have on the B and C point around 190V- 195V.

You do need to find a way to set the filament voltage higher. Do you think the guys who did your transformers fuck it up? If so you can take it with them. You do need 6.3V and I would make them to adjust it. How do you drive your amp? If you use something like Pure Power or PS audio regenerator then you might set it to 125 instead of 115V or something like this. This would be a not kosher fix and I would go for transformer makers instead.
It is look to me that they underpowered your transformers both bias and filament and perhabs the output plate transformer.

The Cat

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