First Build, 5E3, strange problem, help please

[cfidave]

I am new to the forum, but have already learned a great deal from many of you. I have completed my first 5E3 build and have run into a strange problem.
The amp is virtually silent with no guitar plugged in. Almost no hum, and a little white noise with volume & tone at 12. So far so good. Plugging in a guitar, any guitar, and I get a loud buzz that increases with volume, and more so as the tone knob is increased. Much more noticable on the two #1 inputs. Touching the guitar strings, quiets it a little, but does not eliminate the buzz. It is not the guitar, cable, or any of the tubes, as all have been swapped, and the buzz persists. I have used the Hoffman buss bar grounding scheme, and checked grounds which seem solid on the DVM. I am wondering if either or both of the tone caps ( 500pf & 0047uf) being bad could cause this. I have no way of checking those caps. All resistors read the correct value. Seems like a grounding issue, possibly at the input jacks, but they check out O.K. Any help would be appreciated. Thanks much.

[tubeswell]

Got any pics of your build you can post here? (Helps with trouble-shooting). If not, maybe some of these things might be useful:

1) You could try a split-ground (run separate wires from all all pre-amp ground return points - including the pre-amp filter ground - to one of the input socket ground lugs. I find keeping the PI grounds with the pre-amp grounds in this scheme is quieter than grounding it with the output stage grounds. Then run the all the output stage ground returns - including the 'plate & screen node' filter grounds - and your PT 2ndary CTs, via separate wires to another ground point on one of your PT chassis bolts (with a solder tag terminal).

2) How are your heaters wired? Do you have a heater winding Centre Tap, or have you used 100R-to-ground from each 6.3VAC lead, or have you done it the way Fender shows on most of their schematics? This can make a difference in hum

3) In your amp, are the wires neatly organised, or are they all over the place like spaghetti? If it is the latter, you could have unwanted coupling from hotter wires going into more sensitive parts of the circuit, which can induce hum. Where possible keep all wires up against the chassis (this 'eats up' the EMF radiation around the wires). Keep wires physically seperate, and keep supply wires away from signal wires by about 1" min if possible - where it is necessary to cross other wires, keep them at right angles to each other. Keep AC pairs of wires (from the PT secondary windings) twisted around each other - this cancels out the EMF around those pairs.

4) For sensitive signal wires (such as input wires going to grid of first stage), have you tried using shielded cable (with the shield grounded at one end only - to avoid stray capacitance induced hum)?

[txstrat]

Have you already tried to pull the tubes one after another to figure where in the circuit the hum consists.
If you pull a tube and the hum disappears the reason for the hum is before that tube (or the tube itself, but you swapped them already).

Hope this helps

Matt

[cfidave]

Thanks for the quick reply guys. Here are some pics, of my build. As far as grounding goes. All power supply filter grounds, the bias supply ground, both center taps, ( including heater winding Centre Tap) and the green AC wire are grounded at the PT studs. All other grounds are to the buss bar on the back of the pots. I have trouble, at this point knowing & identifying what you mean by " pre-amp filter ground" and " PI ( phase inverter?)ground. I not sure if my grounding is what you are talking about. BTY, I used Mercury Magnetics transformers, if that helps.

As far as pulling the tubes goes: humming goes away with removal of either
V1 or V2. With V1 & V2 installed, humming returns and is not affected by removal of power tubes. I think that tells me it is in the preamp stage. BTY, the sound of this guy is amazing, I just need to track down this problem.

[hasserl]

In my experience you cannot beat the brass grounding plate for Fender style amps. It is a brass plate that mounts between the input jacks/pots and the chassis, all grounds off the board are fed to it. My first 5E3 I tried to incorporate a star ground, and I just couldn't get it to be as quiet as it should be. Went back to the brass grounding plate and all my noise issues went away. I've used it on all other Fender style builds since then and I've never had ground noise issues. My suggestion is that you get one of the brass grounding plates for these amps and install it, chances are very good your noise issues will go away.

[cfidave]

Thanks, I may need to try the brass grounding plate. I keep checking evrything I can think of, with no luck. I am going to try shielded cable between the pin 2 of the AX7 & the volume pot, as a last resort.

[michaelj]

Although it may sound "way" simplistic, you may want to try tube shields on your preamp tubes. This has made a significant difference for me both in my 5e3 build and in a el84 single stage pre / PP el84 pair build.

Mike J

[cfidave]

Tried the shields, no real change, but thanks for the suggestion. Grounding the heater center tap to pin 8 of the 6L6 has helped some. Since the amp is hum free with no input, even with controls at 12, I think I am picking up noise from the input ( guitar), that is being amplified. I am going to try and run some shielded cable ( gonna be a bear), and see if that helps.

[Chuck H]

You might also try isolating the input jacks from the chassis.

Also, Have double checked and are sure that the switch on either input jack is not lifting the ground with a cable plugged in?

Chuck

[dai h.]

my guess (since the layout looks stock and the workmanship looks clean) judging from the above is that the main problem is the grounding scheme. One of the things mentioned for example, about tying all the filter cap grounds together is wrong (preamp filter shouldn't be tied together with early filters, B+ winding, and filament ground). Probably (since the build is clean) there won't be too much to do to correct the situation and get the amp working as it should be.

[cfidave]

Chuck, not sure what you mean by lifting the ground. The number one input jacks are connected to the number two input jacks by tip of the #1 switch going to the switch of the number two jack, via the 1 meg resistor. That 1 meg resistor is connected to all 3 lugs of the number 1 jack.

[michaelj]

Sorry the shields didn't help.

After looking at your pics a bit closer, you might want to try "chopsticking" the leads on your pre and PI some. Seems like there are a fair number of parallel runs there, I have had better luck keeping the runs short and making sure that nothing [esp cathodes] runs parallel or too close to screens and plates.

I agree with the other guys re grounding runs. I don't use the brass plate myself but have installed "stars". Center taps and first filter to one PT bolt. AC ground to another. All the other grounds go to a separate "star".

Single coils [especially P90] still hum a bit but not enough to spoil the fun.

I also shielded my "back panel" with copper foil which helped even the single coil / P90 stuff.

Good luck..... It is a real clean looking build so I know you will sus it out.

MikeJ

[tubeswell]

I have trouble, at this point knowing & identifying what you mean by " pre-amp filter ground" and " PI ( phase inverter?)ground.

'Pre-amp filter cap ground' is the ground side of the decoupling/filter cap for the pre-amp supply (the filter cap nearest the pre-amp plates). This should be grounded with your pre-amp grounds for minimal hum - and not linked to your other filter cap grounds

PI = Phase inverter/splitter - the ground for this is the cathode resistor/bypass cap ground (and any grid bias resistor ground). I find its better to ground these with the pre-amp grounds as well.

[cfidave]

Hey thanks guys. I ran shielded cable from the first volume pot to pin 2 of V2.
grounded just on the volume pot. No real change.

The whole grounding thing has got me confused. I grounded it the way you see it on advice from Hoffmans web site,

http://www.el34world.com/charts/grounds.htm

Apparently, it is not the way to go. Changing it is no real problem and worth a try. I also noticed that when I plug a speaker cable in, the tip of the plug sees signal as well as ground. I am using the Mercury Magnetics output transformer that has no speaker ground wire, just the yellow wire that goes to the tip of the jack. Anyone know if it is normal to see ground at the tip of the plug. Thanks much for the help!

[txstrat]

I usually run a rather strong wire over the pots bodies as a replacement for the brass ground plate. I put ALL preamp grounds (including the filter caps in the preamp section) and the PI ground (including filter cap) to this wire.
All other grounds (power tubes, 100 ohms resitors for the heaters etc.) I put to a star ground at one of the PT bolts. That does the job in my amps. To be honest, I forgot about that in my recent 6G3 build (put all grounds to the star ground) and it hummed, until I changed the preamp as described.

Hope this helps

Matt