Bringing a B-52 ATX100 back to life!
Posted: Sun Nov 13, 2022 12:23 pm
A fully functional B-52 ATX100 head is selling for $300. If you like Fender clean, Mesa dirt and need an amp head that can handle the bedroom to a small theater no problem, then I recommend this amp (with a required mod listed below).
It is mostly a 3 channel Mesa Recto-verb 100w, with reverb on al channels, an effects loop and only the diode rectifier. It doesn't have the simul-class or some of the other scaling/tone features, but it nails the high-headroom Fender clean in channel 1 and Mesa high gain in channels 2 & 3. All foot switchable with effects loop.
This year I started repairing for the local Music Go Round, who recently took this B-52 in trade which was an awful deal for them. It was missing all 10 tubes (about $281 replacement for basic tubes at Sweetwater) and the back panel with fan (which isn't needed). They smartly decided to take it at a loss and 'trash it.' Instead of making me dumpster dive, they kindly gave it to me.
Luckily, I had all the required tubes in my 'used, but great' tube stock (worth about $170 in repair value to me). While I was diagnosing it on my bench, I found four of the filter caps were not functioning and the amp had an unbearable hum. Looking closer, I realized this was under-designed B+ power supply circuit (the second I've encountered this year). Basically, the amp was designed for 115v house power, which would provide 475v B+ when the tubes were installed and biased correctly. Most homes today exceed 115v and here in Missouri, 123v is common which results in >510v B+. The filter caps are rated at an absolute max of 500v (250v + 250v). Not enough overvoltage to blow them up, but enough to wear them to the point of intolerable hum in a few months of play.
MY REDESIGN of the FILTER CAP MOD: Replace the (2) 220uF-250v radial and the (2) 100uF-250V Chinese electrolytic caps, with (2) 220uF-350v and (2) 100uF-350v Rubycon (22k hours lifespan) electrolytic caps. Mouser part numbers: 232-350BXW220MEFR18X and 232-350BXW100MEFR18X. This mod adds about 90v of over-voltage protection and should last 20 years. Cost me <$25 delivered.
I am going to play this one a bit and then sell it off. PM me if you are interested.
It is mostly a 3 channel Mesa Recto-verb 100w, with reverb on al channels, an effects loop and only the diode rectifier. It doesn't have the simul-class or some of the other scaling/tone features, but it nails the high-headroom Fender clean in channel 1 and Mesa high gain in channels 2 & 3. All foot switchable with effects loop.
This year I started repairing for the local Music Go Round, who recently took this B-52 in trade which was an awful deal for them. It was missing all 10 tubes (about $281 replacement for basic tubes at Sweetwater) and the back panel with fan (which isn't needed). They smartly decided to take it at a loss and 'trash it.' Instead of making me dumpster dive, they kindly gave it to me.
Luckily, I had all the required tubes in my 'used, but great' tube stock (worth about $170 in repair value to me). While I was diagnosing it on my bench, I found four of the filter caps were not functioning and the amp had an unbearable hum. Looking closer, I realized this was under-designed B+ power supply circuit (the second I've encountered this year). Basically, the amp was designed for 115v house power, which would provide 475v B+ when the tubes were installed and biased correctly. Most homes today exceed 115v and here in Missouri, 123v is common which results in >510v B+. The filter caps are rated at an absolute max of 500v (250v + 250v). Not enough overvoltage to blow them up, but enough to wear them to the point of intolerable hum in a few months of play.
MY REDESIGN of the FILTER CAP MOD: Replace the (2) 220uF-250v radial and the (2) 100uF-250V Chinese electrolytic caps, with (2) 220uF-350v and (2) 100uF-350v Rubycon (22k hours lifespan) electrolytic caps. Mouser part numbers: 232-350BXW220MEFR18X and 232-350BXW100MEFR18X. This mod adds about 90v of over-voltage protection and should last 20 years. Cost me <$25 delivered.
I am going to play this one a bit and then sell it off. PM me if you are interested.