Re: '75 Pro Reverb bias problems AA1009
Posted: Sun Jan 16, 2022 2:34 am
So, I went to bed early and finally managed to get a decent night's sleep. I think being over tired wasn't helping my brain analyze this properly.
I pulled everything off the power tube sockets, got rid of all the old solder and made sure I was starting fresh. Put in some proper grounds to the chassis from the cathode with a big iron (always satisfying). Tidied up the wiring a little, relocated the B+ supply wire to the correct spot, new grid resistors, new screen resistors (I only have 680ohm here for now so those will do, pretty sure that just results in a cooler bias) and sorted all the connections.
Went through as per øøøøøøø's recommendation with the meter checking for rouge grounds and continuity on all appropriate points. These all read as good so I powered up and took readings. Both grids are reading at a solid and steady -47mV, screens are getting a strong 450-ish volts.
It's on bench test for now, letting the tubes bed in and thoroughly warm up. Bias on the meter is currently sitting at about 26mA which is a little cool but not crazy low. The bias pot inside is just a bias balance pot so I've got those as level as possible.
I've plugged a guitar in for a sound check and it's making the correct noises, no weird hums yet. Will come back to it in an hour, take another round of readings then hopefully zip it back up!
A preemptive 'thank you' to all helpers here
I pulled everything off the power tube sockets, got rid of all the old solder and made sure I was starting fresh. Put in some proper grounds to the chassis from the cathode with a big iron (always satisfying). Tidied up the wiring a little, relocated the B+ supply wire to the correct spot, new grid resistors, new screen resistors (I only have 680ohm here for now so those will do, pretty sure that just results in a cooler bias) and sorted all the connections.
Went through as per øøøøøøø's recommendation with the meter checking for rouge grounds and continuity on all appropriate points. These all read as good so I powered up and took readings. Both grids are reading at a solid and steady -47mV, screens are getting a strong 450-ish volts.
It's on bench test for now, letting the tubes bed in and thoroughly warm up. Bias on the meter is currently sitting at about 26mA which is a little cool but not crazy low. The bias pot inside is just a bias balance pot so I've got those as level as possible.
I've plugged a guitar in for a sound check and it's making the correct noises, no weird hums yet. Will come back to it in an hour, take another round of readings then hopefully zip it back up!
A preemptive 'thank you' to all helpers here