Hum in 5E5A build

[Bluefinger]

Hi,

I am having troubles finding the reason for the humming in a 5E5A build. I am using a ground bus which is terminated at a power tranny bolt. The amps hums with both volume controls at 0. When I turn them both up to 4 it disappears and above four it comes back again. When I disconnect the inputs from V1 the hum is lowest at volume 0 and goes up the higher I turn the volumes. Disconnecting the input on the PI voming from the tone stack makes the hum disappear and so does disconnecting the tone pot inputs. Disconnecting the signal inputs on the volume pots doesn't change anything so I assume the hum is coming from somewhere around the vol pots, V2 and the surrounding components but the wiring looks good. Any ideas?

Voltages are ok. The plate voltage on V2 is a bit high but not dramatically.

any ideas?

Thanks in advance!

[loudthud]

Does the hum go away immediately when you put the amp in Standby mode or does it fade out over a period of a couple of seconds? Did you use a layout that you can post a link to?

See if you can make any sense out of this thread:

http://music-electronics-forum.com/t16328/

[Bluefinger]

Does the hum go away immediately when you put the amp in Standby mode or does it fade out over a period of a couple of seconds? Did you use a layout that you can post a link to?

See if you can make any sense out of this thread:

http://music-electronics-forum.com/t16328/

Thanks for the reply!
I used more or less the original layout: http://www.ampwares.com/schematics/pro_5e5a.pdf

The exceptions are that I am not using a brass plate. I run a ground wire from the upper right tranny bolt alongside the upper edge of the circuit board and down to the power tubes.

The 68k input resistors are attached directly to the jacks and then I run shielded wires straight down to V1.

The wire from the tone stack to the PI is a shielded one as well.

There is also a switch that lets me select between twi -fb values and no -fb at all.

The hum disapears within less than half a second I'd say when I switch to standby.

can a bad coupling cap cause hum???? I haven't tried replacing those yet ...

thanks!

BF

[Bluefinger]

I just read through the 5F4 topic you posted a link to. That was very interesting. I will redo my whole grounding scheme before I proceed troubleshootoing.

Is there a preferred location for the grounding point of the preamp section? Maybe a dedicated bolt at the right chassis wall?

One thing that is not quite clear to me is the grounding of jacks and pots. These components are grounded to the chassis by themself anyway. Would you suggest isolating them from the chassis to ensure that the ground connection is only going through the ground wire to the selected point or would adding the ground wire be just fine?

thanks a lot!
BF

*****
Edit: From rereading that thread again I seem to understand the pot issue. No isolation from the chassis is necessary because of the shielding but the lugs that go to ground should not be connected to the pot housings but only to the ground buss (or star ground point), right? The question still is, if I isolate off the input and output jacks and connect them to the same grounding point, where at the chassis should it actually be terminated? Or would it be ok, not to isolate off the input jacks and use the connection they make with the chassis as the grounding point for the "silent" ground?
*****

[loudthud]

The hum disapears within less than half a second I'd say when I switch to standby.

That's a symptom of hum getting into the signal path from the heater supply.
　
The layout shows one side of the heater supply grounded. Did you do it that way? You really need to use twisted pair wiring with a virtual center tap (a pair of 100 ohm resistors to ground).

can a bad coupling cap cause hum???? I haven't tried replacing those yet ....

Not likely.

Is there a preferred location for the grounding point of the preamp section? Maybe a dedicated bolt at the right chassis wall?

I suggest just using the input jacks. They are close enough together that you can use any of the ground lugs interchangeably.

Edit: From rereading that thread again I seem to understand the pot issue. No isolation from the chassis is necessary because of the shielding but the lugs that go to ground should not be connected to the pot housings but only to the ground buss (or star ground point), right?

That's usually best.

The question still is, if I isolate off the input and output jacks and connect them to the same grounding point, where at the chassis should it actually be terminated? Or would it be ok, not to isolate off the input jacks and use the connection they make with the chassis as the grounding point for the "silent" ground?

I sugggest you run a bare wire "ground buss" from one of the input jacks behind the pots to the Presence pot. Use this as the quiet ground. The output jacks may not need to be isolated. I would try it with grounded jacks first. Just connect the ground wire from the output transformer to the jacks and another wire to ground if needed. You can use transformer bolts for the other grounds.

[Bluefinger]

Thanks a lot for the detailed response!!!!

I did use a twisted pair of heater wires. The tranny I am using has a real center tap for the heater wires (green/yellow) so I am using this one. I guess this eliminates the need for the 100 ohms resistors.

all the best!

[Bluefinger]

Hi,

I just wanted to thank you. After studying your response and your posts in the 5F4 thread I think I finally understood the theory behind star grounding and how things are interacting. So I have redone the grounding in the amp and it's dead quiet now ... well, there is a little hiss but not half as loud as on my stock Fenders. I owe you a BIG ONE!!

all the best