5E3 Amp Whistling - Bright 1 Jack

[paul-e-mann]

This has been happening on occasion since I built the amp, it starts by pulling the instrument cable out of the jack to switch jacks and the amp would make a god awful whistling sound (but didnt do it all the time but now does). I'd power the amp off to make it stop then turn the amp back on, this problem only happens with bright jack 1. So I pretty much power down before moving cable to a different jack.

So with no cable plugged in I can reproduce the whistle by spreading those two metal parts that hold the cable in on the jack, see video below. Then plugging a cable into the jack I can reproduce the whistle by turning up the volume or tone above half way, see other video, you'll also see the master knob doesnt affect it (its loud cuz the cable isnt connected into a guitar).

Also see the photo and drawing of the jack layout below. I need some help figuring out how to fix this so no whistle when pulling the cable out.

 

[Pete Farrington]

Exact schematic and layout of the amp you built, along with idle voltages (mains, heaters, HT nodes, valve pins), would be helpful.
Not generic 5E3 stuff.
And photos of the chassis. Not as zoomed in as the photo above.

 

[paul-e-mann]

Exact schematic and layout of the amp you built, along with idle voltages (mains, heaters, HT nodes, valve pins), would be helpful.
Not generic 5E3 stuff.
And photos of the chassis. Not as zoomed in as the photo above.

 

[paul-e-mann]

Exact schematic and layout of the amp you built, along with idle voltages (mains, heaters, HT nodes, valve pins), would be helpful.
Not generic 5E3 stuff.
And photos of the chassis. Not as zoomed in as the photo above.

A couple mods from when I first built the amp last year, removed the standby and added a master volume, replaced all .1uF caps with .022uF to tighten bass response.

 

[mickeydg5]

This has been happening on occasion since I built the amp, it starts by pulling the instrument cable out of the jack to switch jacks and the amp would make a god awful whistling sound (but didnt do it all the time but now does). I'd power the amp off to make it stop then turn the amp back on, this problem only happens with bright jack 1. So I pretty much power down before moving cable to a different jack.

So with no cable plugged in I can reproduce the whistle by spreading those two metal parts that hold the cable in on the jack, see video below. Then plugging a cable into the jack I can reproduce the whistle by turning up the volume or tone above half way, see other video, you'll also see the master knob doesnt affect it (its loud cuz the cable isnt connected into a guitar).

Also see the photo and drawing of the jack layout below. I need some help figuring out how to fix this so no whistle when pulling the cable out.

The shunted contacts which give connection to ground when nothing is plugged into the jack(s) may not have low continuity.
Scrap and clean the contacts.
Also use a good electronics lubricating contact cleaner.

 

[Pete Farrington]

Those V1 grid wires are far too long and loopy around the valve socket.
Much more physical space between the bright grid wire and the second stage anode wire is needed. Between those wires there’s 2 polarity flips and a heap of gain, even a tiny degree of coupling will cause this kinda crap, all the ingredients for oscillation are present.

 

[paul-e-mann]

So I was poking around with a chopstick and found the power cord ground wire solder joint was loose, resoldered it and the amp seems fine now. When I built the amp, I tugged on every solder joint to check them before moving on to the next, why would this joint come loose on its own?

 

[BlueX]

So I was poking around with a chopstick and found the power cord ground wire solder joint was loose, resoldered it and the amp seems fine now. When I built the amp, I tugged on every solder joint to check them before moving on to the next, why would this joint come loose on its own?

One advantage with IEC socket over fixed power cable.

Is it the green ground wire, grounded at one of the PT bolts, that came loose? I think it's better with separate ground bolts, that don't have to carry heavy load as trafos at the same time.

Edit: Over-heated solder can become brittle, and small movements can make it come loose.

 

[paul-e-mann]

One advantage with IEC socket over fixed power cable.

Is it the green ground wire, grounded at one of the PT bolts, that came loose? I think it's better with separate ground bolts, that don't have to carry heavy load as trafos at the same time.

Yup it was that wire. I'll definitely do it differently next time.

 

[BlueX]

Yup it was that wire. I'll definitely do it differently next time.

I have the same design in my 5E3 kit. Been thinking to redo that.

 

[Pete Farrington]

A mains transformer mounting screw, especially if it passes through the lamination stack, may be about the most inappropriate fastener for the safety earth chassis connection that it’s possible to conceive
The weber layout doesn’t show that arrangement.

 

[playloud]

Good find.

I would still sort out those grid wires while you're in there. Oscillation is a real pain.

 

[paul-e-mann]

Good find.

I would still sort out those grid wires while you're in there. Oscillation is a real pain.

Yeah, it's real quiet though otherwise. I did order a new pilot light assembly, the old one is really crappy and barely holds the bulb in, it's always flickering. I think I might put in a couple grounding bolts and move all the grounds off the transformer.

 

[paul-e-mann]

So I figure while I'm in here I'll tackle another peculiarity thats going on. The amp makes some noise now and then while I'm playing and I tap the amp to make it stop, I thought maybe a dodgy tube so I tried isolating one at a time but that wasnt it. I dont see any more loose connections anywhere either. I'm able to duplicate the problem by tapping on the chassis, see below video. Any thoughts?

 

[Pete Farrington]

I suggest to undertake voltage surveys for the normal and the fault modes.

 

[paul-e-mann]

I suggest to undertake voltage surveys for the normal and the fault modes.

See the column on the right for measurements this week compared to last April, theyre not that far off.

 

[Dblgun]

I believe Pete was referring to voltages when the amp is displaying the issue rather than when operating to your liking. Taking the measurements while the amp is acting up may lead you to where the issue resides.

 

[Pete Farrington]

Exactly, the point is to compare current flows when the amp is idling normally to when it’s acting up.
To that end, output valve cathode voltage is missing

 

[paul-e-mann]

I believe Pete was referring to voltages when the amp is displaying the issue rather than when operating to your liking. Taking the measurements while the amp is acting up may lead you to where the issue resides.

Exactly, the point is to compare current flows when the amp is idling normally to when it’s acting up.
To that end, output valve cathode voltage is missing

OK understood. I'm just following the Robinette startup procedures, how/where do I measure output valve (I assume that means power tube) cathode voltage?

 

[Pete Farrington]

how/where do I measure output valve (I assume that means power tube) cathode voltage?

Set your digital multimeter DMM to measure V DC 200V range, clip the black probe to the chassis and the red probe at the V3&4 6V6 pin 8 node.
Switch the amp on, allow it a few minutes to warm up and stabilise, then take the reading.

FYI a circuit node is an equivoltage point where 2 or more circuit elements meet aka connect.
Usually represented on a schematic by a dots on the lines linking stuff together.
The above node also includes one end of the cathode resistor and its bypass cap.
So you can clip on to whichever lug / lead etc of any of those 4 nodal points is most convenient