Are you sure your amp isn't oscillating (motorboating)? ;-)

No, I'm not sure, Sander. Can you suggest a simple test to determine
whether the rumble I am hearing only on my phono inputs is turntable
rumble or motorboating? Thanks!

said:


Are you sure your amp isn't oscillating (motorboating)? ;-)


No, I'm not sure, Sander. Can you suggest a simple test to determine
whether the rumble I am hearing only on my phono inputs is turntable
rumble or motorboating? Thanks!


Actually, I was joking about the motorboating, from the gist of your
post it was clear that motorboating can't be the source of your
problem.

A simple test to determine whether the amp or the turntable is at
fault: Disconnect the leads from the turntable, and put shorting cinch
plugs in the phono sockets. (Shorting cinch plugs: solder signal and
ground together in the plug to form a short, this won't hurt your
amp).

If the amp doesn't hum or rumble anymore, the TT is the problem.
If it still rumbles, the problem is in the RIAA amp, and could be
related to heater supply, a defective filter cap, drifted resistor- or
capacitor values, or a faulty tube.
If the latter, try to swap the preamp tubes for some that are known to
be OK.

If the problem is with the turntable, it could be mechanical
resonances from the motor, a worn out platter bearing, a loose ground
connection to the cartridge, or hum pickup from a mains lead.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005