Very unbalanced plate voltage and bias @ power tubes - 50-watt Plexi Kit
Posted: Mon Oct 30, 2023 4:30 pm
Hey guys,
Been awhile since I posted here, as I haven't had any issues at all with the kit I put together a long time ago. It's a "mostly" MetroAmp 50-watt Plexi kit in a Weber chassis. I kinda went about things the long way... anyhow it was working fine for a really long time, and then someone knocked it off my speaker cabinet at a rehearsal and it took a hard hit onto a concrete floor.
This cracked one of the power tubes, and I'm guessing it affected _something_ else, too. Ever since falling, it doesn't sound right - it breaks up way too early - like on 1 or 2 with the guitar plugged into any single input. the II inputs breakup later than the I inputs, but they still breakup way before they should.
I was just going through it and I don't see any obviously broken components or connections. After trying a bunch of different pre-amp tubes w/o much change to the early breakup, I decided to try another pair of power tubes. When I checked the bias on the tubes, one of them reads about half of the other one.
I'm reading the bias at the resistor mounted between pins 1&8 and ground. So if V5 measures -33ma, V4 reads -16. I put the Mullard Reissues back in that have been in there since I had to replace my Winged C tubes, and the readings were different, but followed the same pattern -- -24ma on V5, -11 on V4. I cranked it up to -33 on V5 and then V4 went up to -15.
So I guess the question is --- where do I start looking to find what's wrong with this crazy thing?
There is a master volume in this amp. I don't recall if it's a LAR/MAR or a PPIMV. It's whatever one was available first in the Metro store. All that wiring still seems solid and appears to be working fine, but I could be totally wrong about that, too.
Thanks for any help you guys can offer!
Ryan
Been awhile since I posted here, as I haven't had any issues at all with the kit I put together a long time ago. It's a "mostly" MetroAmp 50-watt Plexi kit in a Weber chassis. I kinda went about things the long way... anyhow it was working fine for a really long time, and then someone knocked it off my speaker cabinet at a rehearsal and it took a hard hit onto a concrete floor.
This cracked one of the power tubes, and I'm guessing it affected _something_ else, too. Ever since falling, it doesn't sound right - it breaks up way too early - like on 1 or 2 with the guitar plugged into any single input. the II inputs breakup later than the I inputs, but they still breakup way before they should.
I was just going through it and I don't see any obviously broken components or connections. After trying a bunch of different pre-amp tubes w/o much change to the early breakup, I decided to try another pair of power tubes. When I checked the bias on the tubes, one of them reads about half of the other one.
I'm reading the bias at the resistor mounted between pins 1&8 and ground. So if V5 measures -33ma, V4 reads -16. I put the Mullard Reissues back in that have been in there since I had to replace my Winged C tubes, and the readings were different, but followed the same pattern -- -24ma on V5, -11 on V4. I cranked it up to -33 on V5 and then V4 went up to -15.
So I guess the question is --- where do I start looking to find what's wrong with this crazy thing?
There is a master volume in this amp. I don't recall if it's a LAR/MAR or a PPIMV. It's whatever one was available first in the Metro store. All that wiring still seems solid and appears to be working fine, but I could be totally wrong about that, too.
Thanks for any help you guys can offer!
Ryan