Tweet
Hello everyone. I was recently given a pretty much trashed Carvin x100b by the guy I fix amps for. He said if I could fix it I could keep it, and I really want to fix it because I love the working ones that I've played. I don't know what year it is, but it's a 6l6gc model with 470 ohm screen resistors and no bias pot. It is a "pull high lead" version before the "hot rod" mod was put it. I have a few questions about it since I'm not very familiar with its innards and haven't fixed an amp this damaged before. By the looks of it, one of the power tubes cooked when it was last used and left a bunch of black char all around that socket and burnt some solder traces. I've fixed up the burnt traces and replaced the failed screen resistor for that tube. So here's my questions:
1. I don't own a variac or any kind of adjustable power supply, so is there a safe way I can test and make sure all the proper voltages are showing up in the right places? Would it be safe to just plug in and turn it on without tubes and make sure all the tube pins show proper voltages?
2. I know that sometimes when a power tube blows, it can take the output transformer with it. There's no visible damage to the OT or any charred wires near it, and all the windings show continuity, but is there a way I can be 100% sure?
3. From what I've read, all I need to do to add a bias control is to replace R127 (see schematic link at bottom) with a 100k or 250k trim pot? Correct?
4. How do I bias this amp? Where do I take readings from? I've never owned or had to bias a push-pull tube amp before so this is new to me.
5. Assuming everything else checks out, I have some used tubes here I can test with (they still measure as "good" on my tube tester). My question arises because I only have 2 6l6gc's, and they are from different manufacturers (randall and bugera). If I get it up and running I will probably run it with 2 tubes in a 50watt mode to avoid using the charred (outer) power tube socket just to be safe either way. With the added bias control, is it safe to use 2 unmatched tubes so long as I bias them to behave the same? To do the 2-tube 50watt mode properly, I set the output impedance to half of the actual impedance correct? Ie: set it to 4 ohm if I'm using an 8 ohm cab, or is it the other way around? Also, how about the adjustable power selector? Is it safe to leave it set at 100watt or should it be set at 50/25 watt?
Please forgive me for asking so many questions, I just want to double check with some experts before I go doing random stuff and potentially blowing things up. Any help is greatly appreciated!
Here's a link to the schematic: http://www.carvinmuseum.com/pdf/amps..._schematic.pdf
1. I don't own a variac or any kind of adjustable power supply, so is there a safe way I can test and make sure all the proper voltages are showing up in the right places? Would it be safe to just plug in and turn it on without tubes and make sure all the tube pins show proper voltages?
2. I know that sometimes when a power tube blows, it can take the output transformer with it. There's no visible damage to the OT or any charred wires near it, and all the windings show continuity, but is there a way I can be 100% sure?
3. From what I've read, all I need to do to add a bias control is to replace R127 (see schematic link at bottom) with a 100k or 250k trim pot? Correct?
4. How do I bias this amp? Where do I take readings from? I've never owned or had to bias a push-pull tube amp before so this is new to me.
5. Assuming everything else checks out, I have some used tubes here I can test with (they still measure as "good" on my tube tester). My question arises because I only have 2 6l6gc's, and they are from different manufacturers (randall and bugera). If I get it up and running I will probably run it with 2 tubes in a 50watt mode to avoid using the charred (outer) power tube socket just to be safe either way. With the added bias control, is it safe to use 2 unmatched tubes so long as I bias them to behave the same? To do the 2-tube 50watt mode properly, I set the output impedance to half of the actual impedance correct? Ie: set it to 4 ohm if I'm using an 8 ohm cab, or is it the other way around? Also, how about the adjustable power selector? Is it safe to leave it set at 100watt or should it be set at 50/25 watt?
Please forgive me for asking so many questions, I just want to double check with some experts before I go doing random stuff and potentially blowing things up. Any help is greatly appreciated!
Here's a link to the schematic: http://www.carvinmuseum.com/pdf/amps..._schematic.pdf
The only identifier I could find on the board as to the revision is "30-X100-4B"
Comment