B+ too high.

[Lordlidder]

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

 

[john65b]

Transformer primaries hooked up correctly?

 

[Lordlidder]

Definitely. Gone over everything multiple times! Correctly wired. 120 volts in and b+ is 408 across rectifier input pins.

 

[Alan4411]

Give us a clue as to how you have the rectification configured.
Valve rectifier, bridge or what ever. And what transformer are you using?

 

[pblix]

What brand transformer are you using? I had the exact opposite once. Turned out the transformer was wound for the wrong line voltage.

 

[Tubelab_com]

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.

 

[Lordlidder]

Using edcor xpwr 144 760v/640v(360-320-0-320-360)@200ma. and power supply is a valve rectifier with capacitor,choke,capacitor configuration. Blueglow website has schematic,sorry I don't know how to add it here. Hope that helps some.

 

[disco]

Something like this. The red line is B+ and the green is the pulsed AC on your rectifier.

 

[Lordlidder]

Holy crap!! Isn't it great when testing you employ the correct scale and you get the required result! Sorry to waste your time. Put meter on dc and all's ok ,thanks tubelab. Oh God I can't believe such a schoolboy error!!

 

[pblix]

What about the heater voltage? I thought you said that was 1/2 what it should be? Or was that at 120VAC on the primary?

 

[Lordlidder]

Yes that was at half power on the primary....120 volts. So all sorted out now! Thanks all.

 

[dandee]

KT88

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 !.

Dan..