• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

300b Circuit Mod.

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I am building the SV-300B SE power-amp, it's circuit comes from Sun Audio (/jc morrison?). I want to listen rich/warm mid-range vocal song with comparatively enough detail. Now, I want to ask some questions:

1. Is there any parts value I should change to suit my need?

2. Use electrolytic capacitors or mkp cap for the cathode bypass, what is the sound different between them?

3. If I don't care about the power and tube life, but the sound quality. 320V, 340V, 360V plate voltage for the 300b is better?

4. Is there any improvement if I put one small choke for the 300b filament voltage regulation?

Many Many Thanks!



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Increasing the plate voltage, B+ changes the style of the sound. Higher the voltage, clearer and faster the sound it will get.

However, I am wondering how you could do it?

For better result, it would be nice if you could change the electrolytic capacitor in the power supply to oil or properlene type. You would be amazed how the sound changes.

Have fun.

When you increase the B+, you should keep the filament constant to 5V. The tube life will be shorten dramatically if the filament voltage exceeds by few percent.

It would be nice if you could have stablized DC supply for the filament by using Low-Drop-Out 5V regulator IC such as LT1085.

Garbage and Johnny, really thanks for your reply.


The message has been posted a year ago, I think it is hard to contact the owner, anyway. Thanks!


My power transformer has some taps such as 290V, 330V ac that I can select.

I would consider to use oil or MKP cap for PS (but it is big and expensive !!!) How about if I parallel a 15uF MKP with a 150uF electrolytic capacitor?

What is a sound quality different between using a choke and regulated IC for the 300b filament supply. Anyone knows?

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.