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Metro 50w - can't bias any higher than 30-34ma
Posted: Sun Sep 11, 2011 4:05 pm
by page361
One of my ongoing projects...but can't set the bias any higher than 30ma on V5 and 34 on V4...any suggestions?
Re: Metro 50w - can't bias any higher than 30-34ma
Posted: Sun Sep 11, 2011 4:25 pm
by toner
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.
Re: Metro 50w - can't bias any higher than 30-34ma
Posted: Sun Sep 11, 2011 4:46 pm
by page361
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.
Re: Metro 50w - can't bias any higher than 30-34ma
Posted: Sun Sep 11, 2011 7:02 pm
by toner
...see SDM's reply below...
Re: Metro 50w - can't bias any higher than 30-34ma
Posted: Sun Sep 11, 2011 10:19 pm
by SDM
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.