Metro 50w - can't bias any higher than 30-34ma

page361 · Post by **page361** » Sun Sep 11, 2011 4:05 pm

One of my ongoing projects...but can't set the bias any higher than 30ma on V5 and 34 on V4...any suggestions?

toner · Post by **toner** » Sun Sep 11, 2011 4:25 pm

You can either increase the first bias resistor before the diode or decrease the resistor going to the bias pot. If you have 47k to the pot, try 33k.

Also, put the hotter tube in V5. That side puts less stress on the tube.

page361 · Post by **page361** » Sun Sep 11, 2011 4:46 pm

Thanks -does it matter that the PT is only putting out around 305V DC at the bridge rectifier? that is really low...is my PT dying? It's around 430V when the stdby switch is turned on.

toner · Post by **toner** » Sun Sep 11, 2011 7:02 pm

...see SDM's reply below...

SDM · Post by **SDM** » Sun Sep 11, 2011 10:19 pm

Don't forget this is a Metro 50 watt here, the main filter cap comes after the standby switch in these amps. When in standby mode you thus have no smooth DC (ripple aside), but rather 120HZ peaks and dips that the meter is seeing instead. If meter reads this waveform as an RMS voltage, you'd expect to see approx .707 x the filtered DC voltage with amp in standby mode. .707 x 430V= 304V.

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Metro 50w - can't bias any higher than 30-34ma

Metro 50w - can't bias any higher than 30-34ma

Re: Metro 50w - can't bias any higher than 30-34ma

Re: Metro 50w - can't bias any higher than 30-34ma

Re: Metro 50w - can't bias any higher than 30-34ma

Re: Metro 50w - can't bias any higher than 30-34ma