Isolation transformer installed....hum

[Enzo]

Help me out with the diode.

[olddawg]

I don't see why you have to leave the original diode in place. If you do you better put the bridge in front of it. As far as him is concerned. If you are using an isolation transformer the unit is by definition isolated from earth ground unless you have a case or chassis disconnected from the circuit. Have you tried simply reversing the orientation of the 2 wire plug, like a ground reverse switch?

[pontiacpete]

If you want to add a bridge, here is how to do it. This gives you a couple of benefits. It makes the current in the iso transformer symmetrical so it is less noisy, runs cooler and avoids and danger of saturation due to the half wave rectifier. It also supplies the amp with 120Hz DC so you get less ripple. You MUST keep the original diode in place BTW.

I suggest that you move the 300 ohm resistor to be first in the high voltage side of the chain. This will reduce the hum voltage on the heaters and with any luck will reduce your overall hum level. If hum get's into the very first stage (12AU6) and since that is before the volume control that part of the total hum will change with the volume level.

[ATTACH=CONFIG]41224[/ATTACH]

Thanks for this Nickb. I have use this one in the past
http://music-electronics-forum.com/t34429/
(the heater is wrong in this diagram, it should connect to negative leg of the bridge)

[nickb]

I've redrawn it the way I would probably do it. I had a real hard time reading the schemo so please check carefully and I'll revise as necessary. The original diode is left in place to prevent the heaters running off the B+ rail. It would be too high a voltage, increase the hum and the rms current in the transformer.

Oh, what amp make & model is it? I seem to have missed that.

PontiacPete.pdf edit: see corrected version later in thread.

PS:The advantage of having the heaters after the rectifier is the Vpp is halved and that should help reduce the hum from the heater. 'Course. there's always more than one way to skin a cat

[pontiacpete]

Thanks again Nick, I wired this up and it has definitely removed alot of the hum.

This is a Kent model 5999
Wish I could get you a better picture of the schematic. I scanned using a phone app.

[nickb]

Thanks again Nick, I wired this up and it has definitely removed alot of the hum.

This is a Kent model 5999
Wish I could get you a better picture of the schematic. I scanned using a phone app.

Well, at least you now have practical experience of using "cut and paste" on your "desktop".

If you can simply look at the chassis and/or diagrams and check the component values, even if just the blurry ones, and let me know it will be helpful. Thanks.

[pontiacpete]

Well, at least you now have practical experience of using "cut and paste" on your "desktop".

Ha! More like cut and tape

Right now it is wired without the floating ground. The hum levels have been reduced. Do you think if I put the 100K/ .05uf back in, will this make it even quieter? I'm guessing it won't.

Here's a better scan, hope this helps.

[Enzo]

Well it sure won't be hard to find out.

[nickb]

That's much clearer

Corrected version (inc R11 below) :Kent Model 5999 with Safety Mods B.pdf

I doubt separating the local ground from the chassis /safety ground will have much effect on hum. The benefit of having them separated is prevention of hum loops if you plug anything into the input whose ground is connected to safety earth also.

[pontiacpete]

I forgot to mention the R11 is a 47ohm 3w fusable resistor.

BTW I did try it with the floating ground and no difference.

[pontiacpete]

This is what it sounds like now with the safety mod. The negative end of the bridge rectifier is attached to chassis and there is no floating ground. All grounds are attached to the chassis

Still looking to make it better. Or is it just a crummy circuit?

[nickb]

This is what it sounds like now with the safety mod. The negative end of the bridge rectifier is attached to chassis and there is no floating ground. All grounds are attached to the chassis

Still looking to make it better. Or is it just a crummy circuit?

Can you post a gut shot? That sounds very much like rectifier buzz often attributable to a poor arrangement of grounds esp around the power supply. You have to take care when wiring the power supply to avoid getting the current pulses from the rectifier that charge the first filter cap into the signal path. It's pretty hard to tell how objectionable it is from a clip. A scope across the speaker is a measurement. If it's not that then signal wires can capacitively couple to the power supply circuit and pick up buzz. Re-routing and screening is the order of the day in that case.

Chances are you won't find a magic bullet. Killing hum and buzz tends to be one of those soul destroying jobs that take a great deal of effort for little reward. You'll probably make a number of small incremental gains.

[pontiacpete]

Here's a video clip

https://www.youtube.com/watch?v=WvbktwAx_TY

[nickb]

A still photo or two would be much better than a video.

[olddawg]

Chances are you won't find a magic bullet. Killing hum and buzz tends to be one of those soul destroying jobs that take a great deal of effort for little reward. You'll probably make a number of small incremental gains.

And even if you get it done. It may be there on the next stage you play on. I had a hum in my dirty channel only the other night. Thought I may have had a pre tube going south. Got home. Perfectly fine. Same rig. Which brings to mind. Make sure you don't have ambient hum radiating into the unit. I would plug it in somewhere else just for the hell of it. If you have fluorescent lighting or a dimmer or a major appliance on that AC mains you could be chasing your tail.