Metropoulos Forum

I'm noticing a number of folks using a second bias trimmer on amps built with the Metro power transformer with low voltage switching. I built an amp using this transformer some months ago.

I wasn't really sure what did what with the three position standby switch, but looking into it, I gather the 33k resistor mounted on the switch is a bias resistor for low voltage mode? Is in necessary to adjust the bias in low voltage mode for proper operation?

I only adjusted bias in standard voltage mode, myself. The amp has run fine for months of daily use. Am I doing any kind of damage here?

Thanks for your time.

i would adjust it in high mode and check it in low mode to make sure all is kewl. get out your meter and test it for yourself.

Yeah the 33K (22K-33K as denoted in pic) on the standby switch effects the negative bias for the low voltage setting ONLY. 33K could be fine or perhaps too much, meaning the negative bias voltage is too low (at low voltage setting option), and tubes are running too hot. So do check as Mark suggests, and set the actual normal board bias pot for the high voltage option, then leave that alone. When you flip to the low voltage option, tweak the 33K value (or swap in a pot) until you bias up as desired there too. If 33K proves to leave the tubes too hot in the low voltage mode, lower it's value (which you can do by adding parallel resistor/s). A pot is really is ideal there instead as then you can dial in both bias settings for both voltage options very precisely.

Appreciate the info, gents. Thanks so much. I remember checking the bias when I built the amp in both modes, but I didn't realize that resistor impacted the bias and sort of assumed it was fine in either position. I'll double check it later tonight and see where I stand.

Thanks again.