Bogner Helios: RF Radio Noise

[FourT6and2]

I recently got a Bogner Helios 50. I like it. But there's a good bit of RF/radio noise. The amp is really two different amps in one. "Plex" input and "Hot" input are two different "amps". Not the traditional hi/lo jacks. Plex input bypasses V1. Plex input = no noise. Hot input = tons of noise: hum, buzz, radio stations. With no instrument cable plugged in = no noise. With cable plugged into Hot = noise. If I temporarily ground the instrument cable's barrel to the chassis at the jack = noise disappears. Perfectly quiet.

The jacks are Cliff-style. So they are isolated from the chassis. There is a 0.022uF cap from the Hot jack's ground, to chassis ground. So this should kill RF, theoretically. But the noise is there. The jack is also grounded via a separate wire to chassis ground. V1a input grid has no grid stopper. I was about to install one to attempt to kill the noise, when I tried grounding the cable and discovered that worked.

So... what's going on here? How can I solve this noise problem? The jack seems to be grounded already. And there's already a cap installed. Somewhere, there's a ground problem and I can't seem to track it down.

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[Phil_S]

in the first picture, the one where the black wire on the terminal strip is labeled "not grounded," where does that black wire go at the other end? Where does the other wire (pin 2 of the socket) go? Maybe you said, but I can't get it from what you wrote.

[FourT6and2]

in the first picture, the one where the black wire on the terminal strip is labeled "not grounded," where does that black wire go at the other end? Where does the other wire (pin 2 of the socket) go? Maybe you said, but I can't get it from what you wrote.

That's the input grid of the first stage. That wire comes from the input jack. Best-practice is to ground the shield of that coax at the jack and the end at the socket is simply cut short and heatshrinked. You only ground one end (at the jack) to help buck noise. I don't know why Bogner attached the socket-end to a terminal strip that isn't connected to anything. Maybe it's to help keep the wire from moving. I dunno. I've never seen anybody do it this way before. And usually there's a grid stopper on pin 2. Bogner didn't use one. I was going to install one to help kill the RF/radio interference. But then I noticed grounding the guitar cable's ground/barrel to the chassis right at the jack killed the noise completely.

So I wonder if there's a bad ground or a missing ground somewhere. I can't tell.

[Phil_S]

That's what I was getting at, but I didn't want to make an assumption on the basis of something I can't see. I think I'd want to ground the shield, too. As for the lack of a grid stopper, I think you know that it is typically the 68K or 33K mounted at the jack. Assuming it's not there, I'd want to add that, too, at whichever end you think it best.

Before I did anything, I'd contact Bogner and see what they say. Maybe there is something about the design that these are intentionally omitted? I know this seems unlikely. Who knows, they might actually be helpful. After all, I'd expect they'd want the amp to function well and it's obviously not. If you don't hear back, you've only lost a little time and then you can do those things. Or if you already done them, ask anyway!

Good luck.

[FourT6and2]

That's what I was getting at, but I didn't want to make an assumption on the basis of something I can't see. I think I'd want to ground the shield, too. As for the lack of a grid stopper, I think you know that it is typically the 68K or 33K mounted at the jack. Assuming it's not there, I'd want to add that, too, at whichever end you think it best.

Before I did anything, I'd contact Bogner and see what they say. Maybe there is something about the design that these are intentionally omitted? I know this seems unlikely. Who knows, they might actually be helpful. After all, I'd expect they'd want the amp to function well and it's obviously not. If you don't hear back, you've only lost a little time and then you can do those things. Or if you already done them, ask anyway!

Good luck.

A bunch of people have run into this issue with the Helios from what I've found on the web. And they have contacted Bogner and the response from Bogner was to dismiss the reports of noise. Eventually, Jorg from Bogner told someone to install a 68K grid stopper on the socket. But... this is a band-aide fix because there seems to be an issue regarding circuit ground and chassis ground somewhere. The grid stopper is helping to block outside RF, but there's also a problem somewhere else. Maybe a missing ground connection. Maybe a missing circuit ground. Maybe a redundant ground/loop. I'm trying to track it down. Maybe a bad solder joint somewhere.

I'm tempted to just rewire the input completely. The amp sounds good otherwise. I did send an email to Bogner, but it's a holiday so will wait and see what they say. My guess is they'll tell me to install grid stopper.

But really... the guitar cable's shield is not connected to the circuit/chassis ground in this amp. And that's the problem. I'm trying to figure out if it's intentional or a mistake I need to correct.

[FourT6and2]

I decided to just install a grid stopper and pretend like there's no grounding issue lol. 27K. That did the trick. Dead quiet. But yeah... it did affect the sound/feel of the amp. Kinda surprised because the math says with a 27K grid stopper, low pass filter has a cut-off of -3dB at about 25kHz, which shouldn't be audible. But it is.

I also just got an email from Bogner and they said same thing. They intentionally left the grid stopper out because it improves the feel/tone of the amp, even though it makes the amp susceptible to noise.

I might try a lower value like 10K or 15K and see if that brings back some of the lost mojo while still blocking RF.

[Stevem]

You need to shield the input jack and tie that shield to ground.
All the metal on the hot side of the jack acts like one big ass antenna.
I make a thin cardboard form and then contact cement.005” thick brass around it.

I get this brass sheet from hobby shops or on line.

I mainly use this mod on the Orange Terror amps due to interaction between there speaker jacks and input jacks being only 4 inches apart. But it works for shielding the input jack 100%.

Implementation of this simple cap and resistor mod from the hot of the jack to ground may do the trick all on its own, so you might try that first.

[FourT6and2]

I tried various grid stopper values and settled on 5K7. The lower I went, the more the original sound/feel of the amp returned (good thing). 33K affected the amp's sound too much. RF and radio noise came back at 1K2. 2K7 was better, but still a slight bit of radio when amp is cranked. So I went up to 5K7 and called it good for now. Dead quiet. No RF.

[bmx]

You need to shield the input jack and tie that shield to ground.
All the metal on the hot side of the jack acts like one big ass antenna.
I make a thin cardboard form and then contact cement.005” thick brass around it.

I get this brass sheet from hobby shops or on line.

I mainly use this mod on the Orange Terror amps due to interaction between there speaker jacks and input jacks being only 4 inches apart. But it works for shielding the input jack 100%.

Implementation of this simple cap and resistor mod from the hot of the jack to ground may do the trick all on its own, so you might try that first.

I like the shielding idea, but a 5nf + 47k to ground is going to have a serious effect on passive pickups.