Tube Amp - Grinding Buzz at Startup

[dawsonaudio]

I just got done building an Ampeg SB-12 bass amp and am getting a lot of buzzing/hum as the amp warms up.  When the amp is turned on, the tubes light up and after about 10 seconds, a grinding buzz/hum sounds starts to fade in.  It increases in volume, but I have to turn the amp off to stop it before it gets too loud.  And the noise stops immediately after I turn in off.  My first thought was one of my new filter caps went south.  I took them out and they all show good capacitance.  Maybe it's a ground loop? 

I should say that the buzzing/hum happens with the power tubes and rectifier in...the 12ax7's are still out of the amp.

Is there a particular way of deciding which ground connections connect together back at the main power supply ground? I've normally brought things back to a master ground off the negative electrolytic capacitor/s. I am just not particularly sure which grounds should bus together back to the master ground point. Here's the schematic I'm working with? I currently have the main power ground connected to the chassis when it comes in the box. The three 22uf electrolytic capacitors are grounded together as the star ground point.  I have the first 12ax7 (V5) ground section resistors and volume potentiometer going back to the star ground.  And then the treble and bass ground resistor is grounded with the second 12ax7 (V4) and goes back to the star ground point.  Each of the sections is on a different ground wire back to the ground points.  The input jacks aren't sent back to ground as they are self-grounded to the chassis.  Do they need to be sent to the star ground as well?

Thanks for any help here.
Nate

 

[Dreams]

The field coil should be used as a choke, not between CT and ground. I'll bet if you bypassed it and used a permanent magnet speaker instead the noise would go away.

 

[Dreams]

If your schematic matches the build, that is.

 

[ruffrecords]

Two possibilities  I can think of. First you new filter caps are not properly  grounded - hard to tell without knowing what you have used. Second, with NO tubes in, check the grid bias voltage is correct.

Cheers

Ian

 

[CJ]

is the amp for bass or guitar?

echo above post, field coil replaces choke in pwr supply,

here is a link to a Gibson amp with field coil>

http://schematicheaven.net/gibsonamps/br9.pdf

maybe it does not care if it is in the ground leg, certainly safer from a voltage standpoint,

do you have a center tapped heater supply?

if not, stick a couple of 100 ohm  1/2 wat resistors from each leg to ground to create a virtual CT,

put a couple of cathode resistors in there so you can check tube current for matching, mis match will cause a bit of hum,

use 0.5 ohm to 1 ohm, usually they come in a 5 watt package for transistor emitter resistor, measure millivolts and use ohms law for current,

40 mv across 0.5 ohm = 20 ma, 30 mv across 1 ohm = 30 ma.  etc,