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Bummer... why is my B+ too high?!
Posted: Wed Mar 04, 2020 8:08 pm
by lifer
Man, I got a real bugger here. 100w Super PA.... I blew two, V1 tubes. So, i knew something had to be up.
From back to front....my B+ is too high. First gain stage preamps are about 100 volts higher than should be. Fuse holder reads about 525v.....but should probably be around 470v.....Power is an MCI unit....suppose to be "plexi voltages".
All my grounds are good. Power supply drop resistors are all new, metal films. Tubes are good.
Any ideas....I'm out of ideas.
Thanks!!
Re: Bummer... why is my B+ too high?!
Posted: Wed Mar 04, 2020 10:42 pm
by axeman
Did you rewire the amp at all?
Re: Bummer... why is my B+ too high?!
Posted: Thu Mar 05, 2020 7:47 am
by lifer
yessir.....it's a new build. it's one of those bugs that drive ya nuts! -lol I'm sure the PT is wired correctly. -and OT. Dang thing.....just can't find what I fkd up.
Re: Bummer... why is my B+ too high?!
Posted: Thu Mar 05, 2020 10:27 am
by axeman
Did you check those voltages with or without tubes? also upload a photo
Re: Bummer... why is my B+ too high?!
Posted: Thu Mar 05, 2020 12:03 pm
by gloeidraad
How high is the heater voltage?
Re: Bummer... why is my B+ too high?!
Posted: Thu Mar 05, 2020 12:38 pm
by axeman
should be around 3.3ACV
Re: Bummer... why is my B+ too high?!
Posted: Thu Mar 05, 2020 10:22 pm
by plexified
I would pull the tubes for safety and check the Power transformer secondary taps for another voltage. Usually in Plexis for example you have 525v or you have 460v rectified depending on the tap you use. Mains rectified is X 1.4 for your conversion.
So for example if you have 120v at the wall your PTX would be tapped @ 375 Volts X 1.4 = 525 volts rectified
@ 325 Volts X 1.4 = 460 volts rectified
This is what your looking for. I had a Super PA Plexi with this issue and Ken Fishers advice was this :
Orange = Common Red = 120 and I had 525volts
He said use Red as Common and wire Violet to 120 and that got me @ 460 and was purrrfect.
So not sure of your wiring diagaram, but I would look for the secondary voltage you want at the PTX secondary with nothing hooked up, and then factor in the Rectifier conversion of Voltage X 1.4 to get your end result. You should also have 6.3 Volts on the heaters measured across them ( red/blk ) and that should be constant regardless of the secondary tap used.
I hope this is the tweak you need, this thing should sound amazing , Good Luck .
Re: Bummer... why is my B+ too high?!
Posted: Fri Mar 06, 2020 8:12 am
by lifer
Tell ya what fellas.......i'll check all that...again. Thank you!! I'll upload some closeups, etc. after.
Fine tooth comb time!
PS- yeah, same voltages w/o tubes installed.
Re: Bummer... why is my B+ too high?!
Posted: Fri Mar 06, 2020 8:14 am
by lifer
plexified wrote: ↑Thu Mar 05, 2020 10:22 pm
I would pull the tubes for safety and check the Power transformer secondary taps for another voltage. Usually in Plexis for example you have 525v or you have 460v rectified depending on the tap you use. Mains rectified is X 1.4 for your conversion.
So for example if you have 120v at the wall your PTX would be tapped @ 375 Volts X 1.4 = 525 volts rectified
@ 325 Volts X 1.4 = 460 volts rectified
This is what your looking for. I had a Super PA Plexi with this issue and Ken Fishers advice was this :
Orange = Common Red = 120 and I had 525volts
He said use Red as Common and wire Violet to 120 and that got me @ 460 and was purrrfect.
So not sure of your wiring diagaram, but I would look for the secondary voltage you want at the PTX secondary with nothing hooked up, and then factor in the Rectifier conversion of Voltage X 1.4 to get your end result. You should also have 6.3 Volts on the heaters measured across them ( red/blk ) and that should be constant regardless of the secondary tap used.
I hope this is the tweak you need, this thing should sound amazing , Good Luck .
Oh yeah my bro......i LOVE the PA's. I had a '71 years ago. Loved it. Like a dummy, I sold it......always regretted it!!!
So, finally built this one from an original '67 chassis.