• 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.

B+ too high.

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Hello there,I'm building the Blueglow kt88 single ended amplifier. As I'm in the UK I'm using the 240volt transformer. Power supply wired up and using a variac I'm getting way too high(double) b+ voltage, heater voltages are correct. I understand the b+ will be high unloaded but what I'm seeing aint right!. In summary: 120 volts in and b+ is 608 and heaters half what they should be. 240 volts in and heaters are correct but b+ is twice what it should be. Anyone know what may be the cause?Regards LL
and b+ is 408 across rectifier input pins.

You are measuring "b+" at the plate pins of the rectifier? Meter is set on DC or AC?

800 or so volts AC, unloaded at the plate pins of the rectifier tube is about right for one of my KT88 amps (SSE). The transformer I use is 750 VCT.

The term B+ refers to the DC voltage that feeds the amplifier. It is usually measured at the B+ connection of one of the OPTs (usually a red wire). The rectifier tube must be in place to get any B+, and 500 volts or so in an amp without output tubes is not uncommon.
Something like this. The red line is B+ and the green is the pulsed AC on your rectifier.


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I also Built the KT88 Blueglow amplifier...
under no load the B+ is going to be off the charts...
its not important till the amplifier is under load...
using the 5AR4 rec tube the B+ across the main filter cap under load
should be right around 460V DC..

you should avoid running the B+ with no load... it will
be to high for the power supply cap.. 100-100 500V

if you need any help just message me.. ALSO.. it s a awesome amplifier !.



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