• 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.

5AR4/GZ34 GT Britain

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I have an RCA labeled rectifier and am wondering if I'm going to be cooking it soon. I have it plugged into my 300B SET stereo amp. It is outputting 475VDC at pin 8 and has 175ma being drawn from it.

It was recommended to me on another thread and I originally thought it would be too small. After searching the web, I found that depending on what brand, the ratings changed.

Anyway, I searched my stuff and found that I had one. I wanted to hear what it sounded like and plugged it in. It sounds pretty good as compared to my 5U4GB, which is buzzing now.

Please let me know.

Joined 2004
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Should be ok, dyna ST-70's ran their mullard GZ34 at over 200mA and at voltages sometimes in excess of 500V without any untoward reliability issues. Just be sure to let it cool for a couple of minutes before turning the amplifier back on.. (Prevents arcing with partially warmed cathodes, something the GZ34 is notorious for - even old mullards and teles do it, and it is usually fatal to the tube.)

Check the voltage ratings of your supply filter caps, at these voltages they had better be rated at better than 500wvdc.

The higher rectified output voltage may have slightly changed the operating point of your output tubes, if cathode biased this is probably not going to be much of an issue.. If fixed bias the plate current may have increased quite significantly and should be checked.


Thanks for your responses. My first cap is 500VDC and my stage caps are 440VAC, so whatever that means for DC. But the voltages at the caps are below that anyway. I checked the voltages at all of my tubes and found that my driver is running at 410 vice 390 and my output tubes now get around 400 and are drawing 70ma. The sound of the amp has gotten more detailed, but I will bring my driver tube voltages down. I think it's a little high.

No kidding about the slow turn on rate. It is nice. At the rate this thing comes to full power, I can turn on my filament power and main power at the same time!

Yes, the tube's "guts" looks the same as your pic. But I don't remember if there was that much flashing on the top.
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