OT "common" question

[reverendbk]

The amp I am working on is a Silvertone 1482, mostly modded to a 5E3. I have added a negative feedback loop. It is a wire from the (only tap) 8 ohm tap of the OT through a 68K series resistor to the cathode of the second gain stage, similar to a 5E4 or 5E7.

My problem is, the other OT secondary grounds directly to the chassis, and the speaker is soldered to it. I read on Randall Aiken's site that this can create interference, and indeed the amp hums loudly. Aiken says to connect the grounded side of the OT secondary to an isolated jack (sleeve), and connect the sleeve to "the point where the global negative feedback is implemented".

Does he mean to connect the "common" directly to the second-stage cathode (wouldn't this be + feedback?), or to the ground side of the cathode resistor with the other OT secondary tapped to the cathode?

I'm obviously new, and have limited time to screw around, and limited money to buy a new amp/ have a tech fix it.

[Enzo]

The NFB should affect overall gain and distortion, but not so much hum.

If you want to know if grounding the output transformer secondary - as most amps have done for decades and decades - is causing hum in your amp, then disconnect the secondary and connect it directly to a speaker with clip wires for test. That will be the same as an insulated jack.

When it comes to hum or noise or interference or any other malady, there is a big difference between CAN contribute and DOES contribute.

Personally I would think your hum came from other sources.

[tubeswell]

The amp I am working on is a Silvertone 1482, mostly modded to a 5E3. I have added a negative feedback loop. It is a wire from the (only tap) 8 ohm tap of the OT through a 68K series resistor to the cathode of the second gain stage, similar to a 5E4 or 5E7.

My problem is, the other OT secondary grounds directly to the chassis, and the speaker is soldered to it. I read on Randall Aiken's site that this can create interference, and indeed the amp hums loudly. Aiken says to connect the grounded side of the OT secondary to an isolated jack (sleeve), and connect the sleeve to "the point where the global negative feedback is implemented".

Does he mean to connect the "common" directly to the second-stage cathode (wouldn't this be + feedback?), or to the ground side of the cathode resistor with the other OT secondary tapped to the cathode?

I'm obviously new, and have limited time to screw around, and limited money to buy a new amp/ have a tech fix it.

Just to be clear, the NFB side of the OT secondary should be going to the speaker socket tip. The other side of the OT secondary is the ground return, and that can (and probably should) be wired to the speaker socket sleeve/ground lug.

Experiment with different value NFB resistors.

Your hum could be caused by other things, like inadequate lead dress, miswired components elsewhere, unwanted signal/power supply coupling, bad grounding. (Edit whoops I see I've just been overtaken by Enzo's post)

Can you upload a pic/photo of your build?

[reverendbk]

thanks for replies

Sorry I posted very tired. As soon as i can borrow somebody's digicam I can post pics here. I meant to say that adding the NFB loop did not add hum, but rather this is the first amp I've seen where the OT secondary connects directly to the chassis.
I did some more troubleshooting after reading your post and about "3/4" of the hum is volume dependent. The wire from input to first stage grid is ridiculous; tapping it is like tapping a hot microphone, so there we go i guess... premature post, but thanks for the replies, this forum has kept me from making some grave errors (for the amps, not me).