Bogner Helios Help: Killing Radio/RF

[FourT6and2]

I decided to just install a grid stopper. 27K. That did the trick. Dead quiet.
But yeah... it did affect the sound/feel of the amp. Kinda surprised me 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.

[dai h.]

Cool! Glad it's resolved.

re: the redundant connections, I don't think ground loops merely existing is necessarily a problem. For example, (if you're old enough) take those "component stereo systems", where there is a ground loop from the left/right RCA cables going from one unit to another. If you added another wire between the two ground points, would it do anything? I doubt it. (Not with the extra wire but in the cable's normal state) if you separated the cables making a big hoop and maybe moved it around, maybe you could induce some noise into the loop. So even though there is a ground loop, the other ground isn't introducing stuff from somewhere you don't want, and (in their normal state cables/wires hugging each other strict side to side) loop area is kept small (signal send(hot) & return(signal return/shield(ground) routed right next to each other all the way and grounds also resulting (I think) in less induction and tiny bit of common mode rejection (not much compared to a balanced line IIRC). Also, like I mentioned before with the "brute force" approach, more connections = less DCR, less L = less voltage drop = closer to the ideal zero ohms. So (as long as you're not introducing something that shouldn't be there) could be better not worse.

Another example (sorry if this is boring, lol). As somebody advised (in my research on grounding) I sort of "traced" or "marked out" the grounds on a picture of the PCB of the mixer section in an old Tascam 488mkII (cassette multi-track) that I have in order to try to make sense of the grounding. I found a bunch of loops, so again I don't think a ground loop automatically means a humming, buzzing mess.

[FourT6and2]

Cool! Glad it's resolved.

re: the redundant connections, I don't think ground loops merely existing is necessarily a problem. For example, (if you're old enough) take those "component stereo systems", where there is a ground loop from the left/right RCA cables going from one unit to another. If you added another wire between the two ground points, would it do anything? I doubt it. (Not with the extra wire but in the cable's normal state) if you separated the cables making a big hoop and maybe moved it around, maybe you could induce some noise into the loop. So even though there is a ground loop, the other ground isn't introducing stuff from somewhere you don't want, and (in their normal state cables/wires hugging each other strict side to side) loop area is kept small (signal send(hot) & return(signal return/shield(ground) routed right next to each other all the way and grounds also resulting (I think) in less induction and tiny bit of common mode rejection (not much compared to a balanced line IIRC). Also, like I mentioned before with the "brute force" approach, more connections = less DCR, less L = less voltage drop = closer to the ideal zero ohms. So (as long as you're not introducing something that shouldn't be there) could be better not worse.

Another example (sorry if this is boring, lol). As somebody advised (in my research on grounding) I sort of "traced" or "marked out" the grounds on a picture of the PCB of the mixer section in an old Tascam 488mkII (cassette multi-track) that I have in order to try to make sense of the grounding. I found a bunch of loops, so again I don't think a ground loop automatically means a humming, buzzing mess.

Ground loops and redundant connections are certainly a hurdle when trying to trouble shoot an amp and track down gremlins/noises, like I just did.

[FourT6and2]

As far as the grid stopper goes, I tried 15K, 10K, 5k6, and 2k7. And yep... as I go lower, the original sound/feel of the amp is coming back. But still no more noise. There's an audible difference in tone between 15k/10K and 5k6. But not as much of a difference between 5k6 and 2k7. Or at least, it's hard to say because it takes 5 minutes or so to swap out the resistors and your ears can't remember.

I might try clipping in a pot and sweep until I hear radio. Measure pot value and go with smallest resistor that kills RF. I'm guessing it will be around 1K. Just a hunch.

And now I want to lower the value of my other amps' grid stoppers lol. It really does change the feel/response of the amp in a good way.

[dai h.]

That's interesting how it apparently affects the "feel". My (vague) understanding about a grid stopper at the input is none is best for noise, but more chance of RF. I just wonder if you couldn't just implement the input jack ground direct DC connection to chassis right at the input (as you seemed to have done in your experiment) and have it work without the grid stopper? (There is a Cliff part for this: FCR14422 - EMI / RFI SCREEN FOR 6.35mm JACK SOCKETS)

Other ideas (inductor plus cap) :

https://gearspace.com/board/geekzone...ml#post9462424

also ferrite bead over input wire (I have no idea how to properly choose one unfortunately). Only (vaguely understand that) different types can effect different target frequency ranges, and there are "high Q" and "low Q" types (I think this is sort of like a parametric EQ where you can adjust the sharpness of the peak, and the low Q types would be used in general and the high Q (sharper, higher peak) for when there is a very particular problem range (provides lots of impedance in that range). Also I think they are supposed to be insulated and not touch the conductor directly.

re: the 0.022uF RF shunt cap, it bothers me a bit how it looks because (according to my understanding) the cap doesn't look like a low impedance (low inductance) type for high (as in RF and not audio) frequencies (such as ceramic, multi-layer ceramic, and stacked film) where the pieces inside the cap are stacked on top of each other in parallel (as opposed to wound) which should be a lower inductance and result in a lower impedance path. I guess the connection is reasonably
short (?--unsure) to chassis via the lug to metal standoff to chassis, but the chassis is in this case being used for circuit ground returns (and I thought this was not desirable--that is--I thought the RF should be shunted away to either chassis or a large ground plane area before it can keep going inside and mingle with the circuit ground). (Correct me if I'm wrong anyone, but) I thought the basic idea (for the more modern one connection only from circuit ground to chassis sort of configuration) was that the enclosure is just a shield (which the safety ground has to connect to), and things are set up so it's sort of a garbage dump for RF and leakage currents from the power transformer and these stay out (or enough stays out) of the circuit ground to not be a problem.

Also, I wonder where the RF is getting in. Would it be the guitar end? Are the cavities shielded? Or they are but the RF is getting in there anyway on the "hot" side?

[FourT6and2]

That's interesting how it apparently affects the "feel". My (vague) understanding about a grid stopper at the input is none is best for noise, but more chance of RF.

Yes, the larger the value of the grid stopper, the more hiss it causes. No grid stopper is best for eliminating that kind of noise. But then you get the other kind: hum, buzzing, radio, etc. So ideally, you want to use the smallest grid stopper value possible. And you can then put a small cap right on the tube pin, from grid to ground to help make up the difference. Valve Wizard describes this process.

I just wonder if you couldn't just implement the input jack ground direct DC connection to chassis right at the input (as you seemed to have done in your experiment) and have it work without the grid stopper?

The grid stopper seems to work better. Simply putting a wire from jack to ground like I did was a temporary thing to help track down where the noise was coming from, but it didn't eliminate it completely. Grid stopper does.

also ferrite bead over input wire (I have no idea how to properly choose one unfortunately). Only (vaguely understand that) different types can effect different target frequency ranges, and there are "high Q" and "low Q" types (I think this is sort of like a parametric EQ where you can adjust the sharpness of the peak, and the low Q types would be used in general and the high Q (sharper, higher peak) for when there is a very particular problem range (provides lots of impedance in that range). Also I think they are supposed to be insulated and not touch the conductor directly.

I've tried ferrite beads in the past on other amps and I've never found them to actually work.

re: the 0.022uF RF shunt cap, it bothers me a bit how it looks because (according to my understanding) the cap doesn't look like a low impedance (low inductance) type for high (as in RF and not audio) frequencies (such as ceramic, multi-layer ceramic, and stacked film) where the pieces inside the cap are stacked on top of each other in parallel (as opposed to wound) which should be a lower inductance and result in a lower impedance path. I guess the connection is reasonably
short (?--unsure) to chassis via the lug to metal standoff to chassis, but the chassis is in this case being used for circuit ground returns (and I thought this was not desirable--that is--I thought the RF should be shunted away to either chassis or a large ground plane area before it can keep going inside and mingle with the circuit ground). (Correct me if I'm wrong anyone, but) I thought the basic idea (for the more modern one connection only from circuit ground to chassis sort of configuration) was that the enclosure is just a shield (which the safety ground has to connect to), and things are set up so it's sort of a garbage dump for RF and leakage currents from the power transformer and these stay out (or enough stays out) of the circuit ground to not be a problem.

Also, I wonder where the RF is getting in. Would it be the guitar end? Are the cavities shielded? Or they are but the RF is getting in there anyway on the "hot" side?

The RF seems to be getting in via the guitar cable. It's acting as an antennae. I've tried multiple cables and they all do it. Possibly the guitar too. But the guitar is shielded well.

[dai h.]

ah, thanks for making that clear. So I guess the DC connection was not a complete solution.

My understanding is that (I guess all?) conductors can behave as antennas. Any wire, trace, plane, etc.

re: the cavity shielding: that looks quite good. I like how the copper foil is very neatly over the edges (I'm presuming the cover also has foil). I tend to think it's better than paint but for mass production paint probably makes more sense (IIRC Rick Turner (RIP) said paint is fine as long as the resistance is low enough). (Looking at the picture) I see a seam but no solder, so I guess conductive adhesive-backed? The wires going to the output jack maybe should be a shielded wire (or at run tightly together).

My general understanding (for the guitar end) : keep loop areas small (to not inadvertently make a good receiving antenna esp. for crucial points like input loops), have the wires hug the shield, more (or more complete--as in make a sort of box but with apertures for the pickups) is better, and the output after the pots is more crucial (when you turn down and make the signal smaller and put more impedance in the way making it more vulnerable to noise). And maybe even keep going with the protection at the guitar amp input end (to keep accounting for the turned down pot state) with a shield over the jack (I've seen stuff like a metal compartment(steel?) on a THD amp and an alu one in a Dan Gower modded Marshall), or possibly even go as far as kg (ken gilbert), moving the V1 extremely close to the jack (IIRC he said one inch between jack hot and grid), so the vulnerable run is minimized as much as possible (but looking at amps like this and ones modded by Soldano, etc., maybe just a shield and grid stopper is okay?). (Dunno how necessary in practice, but my understanding is (ideally)) you want 360 degree low impedance termination for the guitar cable shield to connector shell (so maybe in practice (since I've never seen that for guitar cables) just try to keep the (ground) pigtail as short as you possibly can?), make sure the contacts are clean.

(I have some nits to pick about the amp, lol...) I don't understand why both side of the heater for V1 seem to be hooked up when (it looks like) only one side (of the dual triode) is being used. Seems a waste of (150mA?). Also, I thought the unused grid, plate, and cathode were supposed to be tied together (and grounded?) but they are open. The little toggles for bright cap(?), ch. switching, etc. seem nice (low profile so would seem less likely to be caught on something and be damaged, look cool), but maybe just hook the other (open) side for redundancy ( = more reliability)? Power resistor looks kind of close to that alu electro. (Not really a nit, but) the parts selection is interesting...Lots of mica, one (non-temp compensating?) ceramic, some "boutique" caps, polypropylene and some alu electros for bypass, and carbon comp resistors.

[FourT6and2]

(I have some nits to pick about the amp, lol...) I don't understand why both side of the heater for V1 seem to be hooked up when (it looks like) only one side (of the dual triode) is being used. Seems a waste of (150mA?). Also, I thought the unused grid, plate, and cathode were supposed to be tied together (and grounded?) but they are open. The little toggles for bright cap(?), ch. switching, etc. seem nice (low profile so would seem less likely to be caught on something and be damaged, look cool), but maybe just hook the other (open) side for redundancy ( = more reliability)? Power resistor looks kind of close to that alu electro. (Not really a nit, but) the parts selection is interesting...Lots of mica, one (non-temp compensating?) ceramic, some "boutique" caps, polypropylene and some alu electros for bypass, and carbon comp resistors.

I hadn't thought about the unused side of V1. I'll look into that more and see what info I can find, thanks.

The mish-mash of parts selection is... interesting lol. I'm not a huge fan of silver mica caps. A lot of them tend to leak DC and ceramic disk sound better to me for bright caps. And I prefer modern metal/thin film resistors. They're definitely better at lowering the noise floor of an amp. PRP and Beyschlag are great options. For signal caps I like ERO MKT1813. I've thought about rebuilding this amp. Keep the same circuit, but correct some of the grounding issues I see and replace some of the parts with the ones I just mentioned.

[dai h.]

Here are some suggested measures from R.G. Keen (the "47 to 100pF" across input jack sounds easy to try (maybe even try going bit smaller than 47p? (Very generally speaking (as a non-expert) I think you are supposed to do appropriate bandwidth limiting to devices and not allow (upper or lower frequencies) that could create problems such as to an op amp or A/D converter input or whatever the signal is going to. Also very basic filter theory: make it easy for stuff you want coming in to do so, make it hard for stuff you don't want coming in to do so--layout, shielding, grounding, filtering, etc.) :

https://www.diystompboxes.com/smffor...topic=109021.0

Also, (also not a tube input) here's what looks like an amp with a modern-looking implemenation to keeping out RF with a confusing-looking "3 terminal feedthrough capacitor" (in the Blackstar amp thread below). So if one was used for that 0.022uF, it should provide a much lower impedance shunt path for any RF coming in on the ground line (pretty sure it would need to be implemented correctly to get the full benefit (shunt path not going through a narrow trace, etc.)).

https://music-electronics-forum.com/...-chassis-query