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Preamp Deal of the Century

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slowhand
kgturner's avatar
kgturner
663 posts
 
Marius:

Which tube out of the bunch would be the likliest culprit? The Cab Dual uses two 101D, two 6SN7, two 6H30, two 5881, and a single 5AR4. I'd be fairly pricey to completely retube the Cabernet as the 101D's aren't cheap. I figure I can start with the 6SN7 first to see. Any other opinions?
amandarae
512 posts
 
Hello Kgturner,

I doubt it if the hum is cause by a tube. Do you have a scope? I ask because you can measure what freq your hum is. 60 hertz and it could be mechanical (tube or transformers vibrating) or filament supply voltage, 120 Hz. and it surely is coming from the rectified voltage of the B+. So which one is it and how much is the amplitude?

One thing to try is to have a shielded cable(shield connected at the preamp side only)from amp to preamp. Try it reverse also and see what happens.

regards,

Abe
kgturner's avatar
kgturner
663 posts
 
Abe:

I don't have a scope nor any access to one that I'm aware of. The hum sounds like a typical ground loop hum to me (60 Hz). I can try the shielded cable route, but I'd likely have to build it. What you're saying is connect the ground and shield at one end of the RCA and just the ground at the other end?
clipsal
80 posts
 
kgturner,

I also had a hum issue from one channel with my Cab Dual, but effectively reduce it by turning down the gain on the amps. A futher reduction in hum was obtained by re-routing the two umbilical cords on the Dual.
amandarae
512 posts
 
Abe:

I don't have a scope nor any access to one that I'm aware of. The hum sounds like a typical ground loop hum to me (60 Hz). I can try the shielded cable route, but I'd likely have to build it. What you're saying is connect the ground and shield at one end of the RCA and just the ground at the other end?

Yes, that is correct but lets change the terminologies so as not to get confuse.

Lets call what you call "ground" as negative phase. So,on one end, positive phase to + of RCA pin, negative phase connected to the shield together then to the negative of the RCA pin, on the other end negative phase and positive phase connections only on the RCA's.

regards,

Abe.
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