harpburn
89 posts
Feb 06, 2014
7:30 AM
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Hello harp players out there,
This is a question concerning an amp of mine - a 70s carlsbro 50. It's a bit of a feedback nightmare, but I have discovered, hidden behind some metal plating, preamp tubes. I want to change v1 to a 12AU7, but I am wondering if I need to rebias the amp if I do so? There seems to be some sort of dial on the back of the amp, which goes from 3.75 to 15 (?) and another going from 110v to 240v. Currently, there are only two tubes in the preamp section, despite the fact that there are four slots.
Your advice is much appreciated, guys
Last Edited by harpburn on Feb 06, 2014 7:30 AM
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rainman
127 posts
Feb 06, 2014
8:43 AM
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The 110v 220v is a voltage selector for your AC source. US would be 110v. The 3.75-15 is output impedance selector in ohms. It would match your speaker impedance load. If you were to ad another speaker to the load you could adjust that to match the total impedance. neither would be helpful to eliminate feedback but they should be property set. I would think you could change tubes without re-biasing.
Last Edited by rainman on Feb 06, 2014 8:50 AM
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Greg Heumann
2587 posts
Feb 06, 2014
9:44 AM
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You don't need to rebias when you change preamp tubes. You need to check bias when you change power tubes.
How many preamp tubes are there and what are they?
If 2-3 12AX7's then change the SECOND and THIRD to 12AU, not the 1st. Usually that gives best results. If there's only one preamp tube then dropping to a 12AU is probably too big a step.
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chromaticblues
1542 posts
Feb 06, 2014
12:18 PM
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@Harpburn
Bribg it to someone that knows what they are doing.
You'll save money in the long run!
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TetonJohn
188 posts
Feb 06, 2014
2:41 PM
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I don't know this amp, but it is a bit odd that there are 4 sockets and only 2 tubes. Someone who knows this amp or who can find a schematic should chime in.
For, example, does it have a solid state power section or are two of slots for power tubes? Maybe you already know all this, in which case, never mind...
Last Edited by TetonJohn on Feb 06, 2014 2:42 PM
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