12AY7 needs help

Dear Dr. Jon Wong,

I got a problem need your help. I just build up a tube pre-amp (using 12AY7). However, I found that the noise is too big and I can even hear some sound when I turn on/off the light. So, I strongly suspect that the power supply is not good enough.

Currently, I am using a tube rectifier (6X4) + capacitor + choke + capacitor in the power supply stage. I am thinking to change to the LM317 solution. I have seen in your articles that you have built very good power supply using LM317T. So, I will appreciate if you can provide me a complete circuit or tell me where I can find one.

By the way, the Vcc for my pre-amp is about 300VDC, and my transformer has +/-230VAC and +/-250VAC output. Thanks!

best regards,

Cheng

Hello Cheng!

Sorry for the late reply. From the description of your power supply, I suppose the problems are due to the following:

1. You did not follow a start grounding scheme. All the grounding should be taken from one point only, usually the zero volt point of the filter capacitor after the rectifier.

2. The transformer has questionable quality, you can try to solder two 0.33uf 600V myler type capacitor across the 230V terminals of the transformer.

3. The RCA terminal should be isolated from the chasis. You can try using twisted wire to run from the RCA terminal to the PCB board. Make sure the ground do not form a big loop.

4. Filament heating should be DC and regulated.

A tube preamp with your power supply should be giving zero volt AC, ie, very quiet. Also, the output should be zero volts AC, ie, also very quiet. My pre-amp produces nothing even turning the volume all the way up. Use your DVM to trace the AC source.

Failing all, please watch out for my article on HOW TO BUILD A PRE-AMP.

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