Ampeg B15 N hum

[rymac]

I have Ampeg B15. It has a 100ohm hum control pot but doesn't seam to do anything. The pot is connected to the 6.3volt filament wires. The center lug goes to a 47K res. and 10D6 diode. I'm wondering if I can not use the pot and connect a 100ohm res. to each filament lead and connect them to ground. The setup used on most amps.

[rymac]

[IMG][/IMG]

[schraedertr]

Filament supply bias

This was designed to let you offset the 6.3V filament power from ground to the value of the cathode voltage. Minimizing the voltage difference minimizes the hum. If your preamp tubes are modern 12AX7 tubes, you probably don't hear any hum.

Obviously, if you can't hear any difference, you could establish a ground with a pair of resistors and be fine. But, if there is no hum, why change it?

[rymac]

The amp uses 6SL7 tubes and does hum. I don't hear much difference when I turn the hum pot??

[Enzo]

Dewire the pot and check to see if it is open.

[rymac]

I dewired the pot and tested it with a meter. I get about 100 ohms across the two outside lugs.
I put the amp back together and here's what I've got now.
With nothing pugged in I turn the volume up over half way and bass and treble about half. If I turn the hum pot all the way one direction I get a really loud hum. If I turn it all the way the other direction it does get quieter but still has a hum/buzz. I've tried different power tubes and all the filter caps are new??

[Enzo]

SOunds like it is working then.

Not all hum comes from the heaters. SO when this pot is set for minimum, then the rest of your hum comes from elsewhere. Is it 60Hz or 120Hz hum?

[rymac]

how can I tel the difference between the 60Hz and 120Hz hum??
I have a new grounded power cord and I got rid of the ground switch.

[Enzo]

Touch the end of a guitar cord. That is 60Hz. 120Hz is the ripple hum in a power supply and is one octve higher. On a scope youi count the bumps.

[rymac]

I checked out my hum versus the tip of the guitar cord hum. It's the higher 120Hz hum for sure. I'm using a 33uF 630v Solen non electrolitic cap for the main filter. Here's a gut shot

[IMG][/IMG]

[rymac]

I'm looking for help on what to do next. My Ampeg has a 120Hz hum with nothing plugged in and gets louder as you turn up the volume. I have 6SL7 tubes.
Replace the main cap or something else??

[drewl]

I just fixed an older one this week with hum.
Check the input jacks, some aren't wired with the jack grounding when nothing is plugged in.
Also make sure the jacks have a good ground to the chassis.
Aside from the high voltage filter cap, check the output tubes cathode cap if this amp is cathode biased.