hummm issues

[Leif H]

I figured I'd move to this forum to get some help.

I've built a 5E3, I used Fender's / Webers layout from parts sourced here and there. Cap's from weber, as are the pot's.
Resistors are from my local electronics supplier.
Tubes are NOS, or used from a Filmosound amp I had started to mod into a 5E3, but I decided I wanted two channels.
Chassis and eyelet board are from ebay.

The layout went well, soldering it was a breeze.

I powered it up, and had some hum, actualy lots.
I swapped tubes, to no avail. the hum seems to disappear when I pull the PI, but not the pre amp tube.

I moved the grounds from the brass plate, to a star ground as follows;
Bias, 1st & filter caps have a shared ground
2nd & 3rd filter caps shared ground.
Ground from tone, Volume and jacks
separate ground from the bypass caps for V1 & 2.
Separate ground from output jacks.
PT and mains ground all tied to the chassis.

I've still got hum. In fact, it seems that the tone seems to really make it worse when turned up.

I tried removing the tone stack, which just made it worse.

I put a piece of co-ax from the Volumes to the PI tube, and that did nothing.

Thoughts?

[MWJB]

Tone & vol pots want a buss wire soldered to the backs of the pot bodies, this terminates at Normal/Mic 1 input jack ground terminal. Also terminating here should be V1 cathode resistor & bypass cap, V2 cathoder resistor & bypass cap, preamp filter cap.

Ground everything elso to a PT bolt, using one wire for each component.

Your heater wires look a bit wild from this angle, keep these away from grid wires to the preamp tubes.

How are the PT & mains grounds tied to the chassis, with a nut, bolt & solder tag, or did you solder to the chassis?

Can we see the tube socket wiring?

[MWJB]

Shielded wires to preamp tube grids should be grounded at one end only, heatshrink the other end to avoid shorts.

[Leif H]

Originally Posted by MWJB

Tone & vol pots want a buss wire soldered to the backs of the pot bodies, this terminates at Normal/Mic 1 input jack ground terminal. Also terminating here should be V1 cathode resistor & bypass cap, V2 cathoder resistor & bypass cap, preamp filter cap.

Ground everything elso to a PT bolt, using one wire for each component.

Your heater wires look a bit wild from this angle, keep these away from grid wires to the preamp tubes.

How are the PT & mains grounds tied to the chassis, with a nut, bolt & solder tag, or did you solder to the chassis?

Can we see the tube socket wiring?

To clarify, should I take the cathode resistor & bypass cap to the Jack ground terminal of normal input 1? and the other end of that ground buss running to the star ground.
I've bolted the grounds to the chassis, and soldered the wires into ring terminals bolted to the chassis.

The heater wires are run up and around the lip in front of the output jacks, then I tried to get them up and over the other wiring to the input tubes.
I'll get a picture of the tube wiring later...

[MWJB]

"To clarify, should I take the cathode resistor & bypass cap to the Jack ground terminal of normal input 1? and the other end of that ground buss running to the star ground." Normally the other end of the grounding buss is not connected to anything, try it like that first, still got an issue then try it tied to the star ground.

"I've bolted the grounds to the chassis, and soldered the wires into ring terminals bolted to the chassis." Good.

[MWJB]

Does your PT have a 6.3VAC heater centre tap wire (usually green/yell), or did you use 2x100ohm resistors as a virtual ground?

[Leif H]

Originally Posted by MWJB

Does your PT have a 6.3VAC heater centre tap wire (usually green/yell), or did you use 2x100ohm resistors as a virtual ground?

No I didn't do the virtual ground, and I don't have a centre tap...

where is the bst place to put this? terminal strip mtd. on chassis?

[loudthud]

Are those the 100 ohm resistors for the virtual center tap on the 6V6 sockets?

OOOPs took too long looking at the picture.

[MWJB]

Couple of options, assuming that heaters are wired as a "twisted pair" - e.g. wire from pin 2 of one 6V6 goes to pin 2 of the next & then on to ONE side of the preamp tubes (either 4 & 5 tied together, or pin 9), the other heater wire goes to pin 7 of the 6V6s & the OTHER side of the preamp tubes...

Mount them on the pilot light, each resistor goes to a terminal. Free ends may be grounded to the pilot ligt bracket that screws to the chassis (remove bracket from the chassis & use a big hot iron) or free ends can be tied together & run a ground wire from the junction to the PT bolt ground.

Or, mount them on the tube socket, one from 6V6 pin 7, the other from pin 2, free ends tie to pin 8 (cathode resistor & cap). This provides a dc bias to the heaters & can reduce hum.

[Leif H]

Here's the tube's wiring, the heater's wiring is getting messier as I prod...

[Leif H]

Originally Posted by loudthud

Are those the 100 ohm resistors for the virtual center tap on the 6V6 sockets?

OOOPs took too long looking at the picture.

I swear that chassis has that effect on me too. I've been looking too long at this too... I'm dreaming about it, seriously.

I play so much better in my dreams!

[MWJB]

Yellow speaker wire should go straight accross the chassis floor, not loop around tube sockets. You have yellow & green wires from the OT...seems unusual, what does the legend say as regards OT taps?

Ther appears to be a jumper wire accross the main speaker jack...please confirm?

If you do the heater wires last it's best to wire them "up & over" the tube sockets, rather than flat against the chassis floor (which you would do if wiring the heaters first).

[MWJB]

No, they are 1500ohm grid stoppers.

[Leif H]

Originally Posted by MWJB

Yellow speaker wire should go straight accross the chassis floor, not loop around tube sockets. You have yellow & green wires from the OT...seems unusual, what does the legend say as regards OT taps?

Ther appears to be a jumper wire accross the main speaker jack...please confirm?

If you do the heater wires last it's best to wire them "up & over" the tube sockets, rather than flat against the chassis floor (which you would do if wiring the heaters first).

They yellow wire on the speaker output is the ground, I was out of white.
the second yellow you think you are seeing is just a reflection. (I'm sorry I'm not much of a photographer).

The output wires are greeen / black and the jacks are wired according to the layout (webber, fender).

I'm going to re wire the heaters tonight as they are now a mess. And keep them spun tighter and above the tube sockets as suggested.

[MWJB]

If the output wires are green & black, you don't need the yellow wire. The jacks ground through the chassis connection. You are most likely causing a ground loop hum.

I am confused by your output jack wiring, it doesn't match the Weber layout & the Fender layout has no detail in this respect (you would be very lucky to build a properly functioning amp based purely on Fender schems & layouts, they are service documents, not detailed "how to" instructions).

According to the Weber layout, with the upright tip contact of the jack socket farthest from you (as they appear in your photo), the terminal to the LH side of the main speaker jack (Tip) gets the green wire from the OT, this wire is then continued to the ext jack tip terminal. The terminal farthest to the RHS (sleeve) gets the black wire & a jumper to the middle terminal (switch) on the main speaker jack. DOUBLE CHECK this, the sleeve/ground terminal is easy to spot because you can normally physically see the terminal connecting to the centre ring of the jack.

[Leif H]

Originally Posted by MWJB

If the output wires are green & black, you don't need the yellow wire. The jacks ground through the chassis connection. You are most likely causing a ground loop hum.

I am confused by your output jack wiring, it doesn't match the Weber layout & the Fender layout has no detail in this respect (you would be very lucky to build a properly functioning amp based purely on Fender schems & layouts, they are service documents, not detailed "how to" instructions).

According to the Weber layout, with the upright tip contact of the jack socket farthest from you (as they appear in your photo), the terminal to the LH side of the main speaker jack (Tip) gets the green wire from the OT, this wire is then continued to the ext jack tip terminal. The terminal farthest to the RHS (sleeve) gets the black wire & a jumper to the middle terminal (switch) on the main speaker jack. DOUBLE CHECK this, the sleeve/ground terminal is easy to spot because you can normally physically see the terminal connecting to the centre ring of the jack.

originally, I did not have the speaker jacks grounded, only though the chassis, I thought this was point, that I can remove.

The Output transformer green wire goes to the right jack, and jumps to the left (green jumper) the black wire goes to the left jack, and jumps to the right (bare wire) the left jack is switched to ground.

I possibly have a tripple ground loop there then... I'll go back and try as you have it explained above.

Now that we are on the grounding the jacks, should the input jacks just be grounded through the chassis not the ground lug on the sleeve?

I may try that as well... unless that's a no no.

[MWJB]

"... I'll go back and try as you have it explained above." Look again at the Weber layout..I stress again, identify the grounded (sleeve) terminal first, just in cas your jack sockets don't match the standard pattern.

From what I can establish at the moment your jacks only work because you have the green wire going to the switch (usually the centre terminal).

The input jacks also ground through the chassis, no additional ground connection is required, other than the tie point for you buss wire from the pots & preamp cathodes.

[Leif H]

Originally Posted by MWJB

"... I'll go back and try as you have it explained above." Look again at the Weber layout..I stress again, identify the grounded (sleeve) terminal first, just in cas your jack sockets don't match the standard pattern.

From what I can establish at the moment your jacks only work because you have the green wire going to the switch (usually the centre terminal).

The input jacks also ground through the chassis, no additional ground connection is required, other than the tie point for you buss wire from the pots & preamp cathodes.

My jacks are different than Weber's, and IIRC both jacks may not be the same, the pin out could be different, it's what I had on hand from years gone by and what was left of the FilmOsound.

Thanks, I think I may have several ground loops going on here, which would explain the hum... hopefully tonignt, I have practice volume above the hum.

[Leif H]

I checked the output jacks wiring and removed the ground jumper. I still had hum.

I removed the ground wire that I had going from each input jack to leave it grounding from the sleeve to the chassis, wired the volume pot's ground to the ground terminal of input jack one.

Still humming along.

I guess the next step is putting V1 and V2 cathode resistors / bypass cap grounds to the input 1 ground terminal and tidying up the heater wiring and finally putting the virtual centre tap on the heater wires.

What wattage should I use for the 100R resistors, 1/4 1/2 or 1 watt resistors?

[MWJB]

The 2x100ohm virtual centre tap should have been your first job...without it you will always have hum. 1/2W resistors.

Don't waste time by revising the grounding scheme "piecemeal", do the whole job as previously described.

[txstrat]

Originally Posted by Leif H

I moved the grounds from the brass plate, to a star ground as follows;
Bias, 1st & filter caps have a shared ground
Thoughts?

I'd separate the bias cap ground from the main filter caps ground, since this can induce hum directly into the grids of the power tubes.
+ take care that the heater wires of the preamp tubes go to the same lugs. Pin 9 to pin 9 and pins 4/5 to pins 4/5.

[MWJB]

Preamp filter cap wants to be grounded with preamp cathodes @ input jack, not with main & screen supply caps.

[Leif H]

By filter cap, I assume the bypass caps on the cathode resistors?

and bringing these wires up to the input's wont that cause noise? do I keep them tight to the chassis?

I'm rummaging all round the net looking for images of wired chassis, and having a hard time finding how these wires are routed.

My first order is to get the virtual center tap as you said earlier, and tidy the heater wires.

Lets hope that does it.

[Leif H]

Originally Posted by MWJB

The 2x100ohm virtual centre tap should have been your first job...without it you will always have hum. 1/2W resistors.

Don't waste time by revising the grounding scheme "piecemeal", do the whole job as previously described.

Last night when I got home I didn't have any 100R resistors... I swore I grabbed some, but there were four 470R for the screens on the output tubes. I guess I left them on the counter and picked up the wrong ones.

[txstrat]

Originally Posted by Leif H

By filter cap, I assume the bypass caps on the cathode resistors?

Filter caps filter the ripple out of the B+ voltage.
Bypass caps bypass some current alongside the cathode resistor.
Coupling caps decouple the DC of a plate from the next grid while they let the signal (AC) through.

Originally Posted by Leif H

and bringing these wires up to the input's wont that cause noise? do I keep them tight to the chassis?

I usually have my B+ wires floating above the circuit components.
Ground wires can be close to the chassis. Works for me.

[Leif H]

Thanks guys!

It's rockin' now!

It's as quiet as it should be.

Just like to thank you all for helping me with this, It's much appreciated!!

Now... what to do next?

[thehoj]

Originally Posted by Leif H

Thanks guys!

It's rockin' now!

It's as quiet as it should be.

Just like to thank you all for helping me with this, It's much appreciated!!

Now... what to do next?

So what ended up being the biggest reduction in hum?

Was it grounding the preamp cathodes with their respective filter cap? Should the bypass cap for that cathode also be grounded at that same point?

[MWJB]

"Should the bypass cap for that cathode also be grounded at that same point?" cathode resistor & bypass cap can share a ground wire, but it's best to have the cathode resistor & cap of any following stages with their own wire to the preamp ground point. In-phase cathodes, of the same tube (e.g. if you gave each half of V1 its own cap & resistor in a 5E3/5F6A type circuit) can share a wire too.

[Leif H]

I think the largest culprit was not having the center tap or virtual center tap on the heater wires.

[thehoj]

Originally Posted by Leif H

I think the largest culprit was not having the center tap or virtual center tap on the heater wires.

Maybe I missed what state this was in before hand. Was it not grounded at all?

[Leif H]

Originally Posted by thehoj

Maybe I missed what state this was in before hand. Was it not grounded at all?

Yep, no center tap, I missed that the Weber schematic had it, and I don't have a center tap on my hammond xfmr.

I'm now educated, what is it they say, hard lessons learned, aren't easily forgotten?