Alembic Guitars Club
Alembic products => Owning an Alembic => Troubleshooting => Topic started by: bumhucker on November 27, 2018, 08:39:08 PM
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hi-I'm the proud owner of a ca. 1974 F2B and a ca. 2000 F2B. I remember reading how the resistors at the outputs were changed later to include more w/different values. I have searched quite a bit and can't find why this was the case and to what effect. My 1974 actually has this update but the 2000 has just the 47k at the outputs. Also, my 2000 is WAAY higher output than the 1974. I am wondering if that is related to the output jack restore values or not (i'd guess maybe not...).
I am having a ground loop hum with both units when using them via the FX return on my GK MB500. It's not bad but not great either... A tech isolated the electrical ground from the chassis of the 2000 model with diodes but still no change. this happens if the units aren't racked up together and if they are. Any info would be awesome!
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Just an update for posterity—-
I put 12AU7 tubes in the 2000 F2B and the output is now at a very useable level with the same lovely tone.
The ground loop hum is less audible due to the gain decrease and actually was not even present on stage the other night. It may just be worse at my house-either way it’s tolerable for sure.
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The output resistors have always been 47k from the factory. And the original gain of both would be the same, when using 12AX7 tubes. I'm guessing that the additional resistors you find in the 1974 unit were selected to pad the output for use in an effects loop, or possibly to patch the two channels in series for overdrive sounds. Especially if the additional resistors were only in Channel A, to provide attenuation for patching the output of Channel A to the input of Channel B and using unmodified Channel B output to drive a power amplifier.
Ground loops are tough, but using short signal cables, kept close together (to minimize the area included in the loop) and also keeping the power cords of the two units close together bundled to be the same length and plugged into adjacent power outlets. Dressing the power leads to minimize the included area in the "loop" between the signal and power grounds will reduce the ability of a magnetic field to induce much hum into the loop.
Your use of lower-gain vacuum tube 12AU7 also makes sense for use in effects loop, without overdriving the effects return input.
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Thanks so much for the valuable info-I made those changes to cables etc. and it has helped. The 74 has become the ideal home studio pre and the 2000 is just killer in my live rig!