Hum Question

[Bombacaototal]

Have been scratching my head over this issue with the amp/build attached which has been featured on a previous post about the reverb hum.

Haven’t had much time for amp projects lately unfortunately, lot’s going on elsewhere...but cannot wait to have more time and start again

Anyway I had a few minutes today and took the guitar and amp out and noticed the following:
1) whenever I turn the amp on with the guitar plugged in, for the first time I turn the volume knob of the guitar there are loud crackles and if I flip the pickup selector switch there is a loud pop. These seems to settle fairly quickly and after that no more issues. But if I turn the standby, as soon as I bring the amp back on, the above repeats
2) if I am not touching any metal part of the guitar, like the strings, bridge, switches, pot or pickups there is a continuous hum/buzz. As soon as I touch any of the above parts of guitar the hum/buzz is gone
3) without touching the guitar (as per item 2) but with the guitar properly plugged in, there is the hum/buzz described above, but if I touch any switch of the amp (ie bright, deep, mains, standby) or any jack (ie input, fx loop, Footswitch) the hum/buzz is gone. Same happens if I touch the handle screws. But if I touch the mounting screws nothing happens

I had not noticed these issues before but cannot be sure they were not there

I’ve used AN Wonderland chassis and grounding is as per wonderland. I’d say the main difference is a single input jack with the FET switchable by a toggle and hence the different input jack type, which is isolated by washers and grounded appropriately

I have checked all star grounding points and made sure they are as tight as possible

This amp is very quiet and I do not have any ground loop issues. The tests above were done with my trusty frnder custom shop strat and vintage gibson L5, with equal results

The descriptions above were with the amp inside the headshell

There is clearly an issue with the grounding but I have no clue where to start troubleshooting this one.

Any advice?

[xtian]

Plug in a guitar cable, but no guitar. Measure DC from tip to sleeve of free end of cable, with amp's Volume control up (but you can turn Master down so you don't have to listen to the loud buzz this will create).

[erwin_ve]

Make sure your amp is plugged in a earthed wall outlet.
That's what I would look for first.

[professormudd]

nevermind

[Bombacaototal]

Make sure your amp is plugged in a earthed wall outlet.
That's what I would look for first.

Thanks Erwin, interesting point, I think all UK plugs are earthed, but I am using a power conditioner which is powering an Eurovolt (european brown box) which controls the incoming voltage before hitting the amp.I wonder if the Eurovolt may not be earthed? I noticed the same thing on another amp, whereby I had some small hum until I touched the chassis, but once that amp went into the headshell the issue was gone (probably due to the mounting screws having physical contact with the chassis and headshell). I will try bypassing the Eurovolt and also check the IEC cables

Thanks Xtian, will do it and report back

[Richard1001]

2 and 3 are completely normal. It is no coinsidence that the bridge of your guitar (all guitars) is connected to a ground wire so the strings are grounded. You can see this on you strat, the ground wire is soldered to the claw holding the tremolo spings. On a Gibson there is a wire trough the body. So, no worries.

Problem 1 could be a little grid current flowing on your first triode of gate (if you use the fet input) You could try a different tube.

[Bombacaototal]

Reporting back:

Xtian, with red on tip and black on sleeve and with the meter measuring Voltage I get 1.9 VDC at start up, which slowly goes down to 0.001VDC (maybe within 20 seconds or less?)

Erwin, you nailed 2 and 3. It seems that either my IEC was faulty or that I was not pushing it in hard enough probably causing a floating ground. I replaced the IEC by spares and made sure they were plugged all the way, and the weird grounding issue is gone, phew!

As far as 1, now that the amp is better grounded the issue reported is much less audible, previously it would hurt my ears so loud it was, but now it is much quieter although still noticeable

Thanks for the suggestion Richard, I replaced V1 and V2 tubes but made no difference. My single amp input is FET style but currently set at FET bypass. I just tried with the FET on and the issue is not present. The problem must lie on how I wired this..

EDIT: this is how the FET jack and the FET toggle are wired. The jack has white and blue which go to each side of the middle terminal of the toggle, and also has two lugs grounded and one floating.

The toggle has a 1M to ground with the blue wire on the exit side (ie after the FET if the circuit is engaged, or equivalent to being on the jack if FET bypassed).

White cable is tip spring, blue is ring shunt, the terminal between both which is grounded is ring spring and the one to the right of the blue is the sleeve

[Richard1001]

I don't understand the purpose of the blues wire.

As i understand:
The white wire is your guitar signal comming in. It can be switched between the fet input and V1 grid with the switch. The 1meg grid resistor of V1 is wired on the input of V1 on the switch. When the fet is engaged the output of the fet is switched to the grid of V1. When the fet is not engaged it seems both the input and output of the fet are floating. If this is thrue, this is a problem.

In any case, you only need ground and signal from the input, so what does the blue wire do?

I should wire it like this:
The Jack input switches between the fet and tube. The fet output between the tube input and ground.

[Bombacaototal]

Many thanks Richard, all your comments and remarks do make sense. I will review all thoroughly and come back to you.

Just for reference this is the spec sheet of my jack. And here it is the part listing
https://www.mouser.co.uk/ProductDetail/ ... gIvDPD_BwE

I remember being a bit thrown off with the ring shunt and the fact that it would usually feeds the normal input jack on HAD amps with two different inputs.

[norburybrook]

wouldn't you be better to just use a regular jack and the FET switch in the files section but use your switch instead of the relay?

FET_Relay_Schematic_r2 (1).pdf

m

[Richard1001]

I see the relay switches the same as the drawing i made.
It doesn't matter if a stereo jack is used here. You can use it as a mono jack without problems.

[Bombacaototal]

Great suggestions Marcus and Richard. I will swap the jack to a mono one and rewire the switch, hopefully this weekend. I will report back once done

[Bombacaototal]

I see the relay switches the same as the drawing i made.
It doesn't matter if a stereo jack is used here. You can use it as a mono jack without problems.

Hi Richard, I managed to find some time today. Wired the toggle exactly as per your diagram. Many thanks for the help again

I could not find my spare jacks, they are probably in a different box and we haven’t fully unpacked since moving houses. I did find some stereo jacks tho, but decided to keep the same as I had for now, just removed the blue wire.

I still have the same issue. At startup, with FET engaged no issues, but with the FET bypassed I have very loud crackles and pop from the guitar volume control, and a bit less loud from the 5 way blade.

After the amp is operating I can switch without problem between FET on and bypassed but if I stat the amp with the FET ON, once I turn it to bypass it does a loud pop (only for the first time I switch).

[Richard1001]

For the guitar crackling: what is the value of the gridstopper resistor? And is the 1m resistor ok? Is there a input capacity used on the grid of V1.
What kind of guitar are you using?

Popping occurs when there is a small DC voltage diffrence present.

On a passive guitar the DC voltage on the sleeve and tip of the output should be 0V. The pickup coil had a resistance of 5 to 15 k (depending on the pickup) and is between the tip and sleeve. There is no DC voltage, only AC.

The output of the fetboard i also 0V. There is only AC on the 10K output trimmer since one side is connected to ground and no DC is present.

The input of the tube (grid) is also 0V. No current should flow in the grid (theoreticly) and the voltage is kept to 0V with the 1M grid resistor.

Crackling in the switch and turning the guitar pot means there is a small DC voltage present. Since the problem is only present using the tube input, the grid had to have a small DC voltage present. This Will cause crackling in pots and popping when switching.

Van you draw the schematic of your input stage?

[Richard1001]

Another question: is there a negative feedbackloop on the first triode (V1) from anode to the grid?
If so, loading this cap by the high value feedback resistors and high 1M input resistance of the grid would explain dc voltage on the grid on startup untill the cap is fully loaded.