"Larry" grounding on 18 watt + noisy channel switc

[Lindz]

I started this new thread since my "anybody near LA want to help me?" thread was getting pretty off topic with all the hum killing suggestions for my 18 watt build.

First is a short MP3 of the hum I am trying to kill - a few seconds of the hum on the normal channel then I footswitch to the tmb and back again. Will give educated ears a chance to hear what is happening. Both channels are dimed, guitar volume is off. My voice is quiet because the mic is pointed at the speaker cone. http://www.snowboardmaterials.com/18%20watt%20hum.mp3

I yanked the bus bar and re-wired the grounding as suggested by M Fowler and others to a "Larry" style star grounding system. I used the layout I posted below again for any new viewers.

This helped somewhat - noticeably quieter but still hums too much in my opinion (way more than other amps I have).

The most noticeable improvement was on the normal channel when I have it dimed and open up the volume on my guitar there was literally NO additional buzz - previously it would buzz a fair bit more once the guitar volume was opened up. Its also a little quieter in general

TMB channel hums less than it did as well but still way more than I think it should - its really noisy - some 120hz noticeable in addition to the same the 60hz hum present on the normal channel.

In any event this grounding scheme was a worthwhile improvement vs what I started with I am going to rewire my 18 watt combo this way for sure.

Martin - If I pull v1 and v2 tubes its pretty quiet. put in V2 and the vast majority of the buzz I am trying to kill is there - the volume and master both reduce it to nothing if it turn them off. Put in v1 and its noisier still but mostly additional hiss which is not surprising as this amp is pretty high gain.

I'm starting to think that after the Larry grounding changes that the noise is now mostly my channel switching implementation - However before I yank it or do any major surgery, perhaps people can critique what I have done and suggest what might help quiet it down and help get rid of the small amount of relay pop I have

I have installed a footswitch relay to switch between the normal and tmb channels inserted in the circuit after V1. Footswitch unit is a Skipz circuits jack (2 opto-fet form c relays) on the back of the chassis. http://www.skipzcircuits.com/CHSW.html

relay configured as follows: 1NC taps the normal channel volume wiper, 1 common connects after the coupling cap on v1b and has 1 meg resistor to ground, 1 NO to ground, 2NC to ground, 2 Common inserted after coupling cap on V1a, 2 NC to volume pot on TMB channel also 1 meg resistor to ground

I used shielded cable for all the signal runs to and from the relay board with the shields grounded only at one end for each cable - with the Larry grounding setup I grounded the shields on the volume pot grounds for the 2 relay leads to the pots and at the #2 star on my sketch for the 2 other relay leads to the main board

This setup switches channels fine, but does have a small amount of relay pop.

picture of the revised Larry grounding and relay setup below as well

thanks for any suggestions!

[martin manning]

My bad... I thought that thing hanging off the jack on the back of the chassis was some kind of loop buffer. So, it's a SS relay powered from the filament circuit, and it's not supposed to pop, but it does. Any chance that there is something wrong with it? You now know that the hum is created at V2. Not sure I like that style of filament wiring around the V2 socket where the plate-to-grid jumper and plate resistor are there too. Do you have a link to the exact 18W schematic you are using? Even better if you could mark it up with your mods to save folks here from having to translate from your verbal description. Looks like in principle you've got something like the TRex-S from here http://mhuss.com/18watt/schematics/ going.

[ChrisM]

Where are you getting the voltage to supply the relay? Is there regulation and filtering?

I'd remove the channel switching or hard wire it and see what the noise is like.

[M Fowler]

thread was getting pretty off topic with all the hum killing suggestions for my 18 watt build.

What do you mean it was getting off topic or was it you weren't getting the answers you thought you should have?

Mark

[Lindz]

I only meant that in the original thread I had asked to see if a local here in LA was willing to work in person with me on my amp so the subject of the thread was kind of off topic now that that thread had gone in the direction of people trying to help remotely.

I am grateful that people are trying to help

I am tapping the heater filaments for power to the relay -

[ChrisM]

I only meant that in the original thread I had asked to see if a local here in LA was willing to work in person with me on my amp so the subject of the thread was kind of off topic now that that thread had gone in the direction of people trying to help remotely.

I am grateful that people are trying to help

I am tapping the heater filaments for power to the relay -

Bad idea, been there.

Add a tiny auxiliary transformer. Rectify, filter, regulate and filter again. You'll have no problems.

[Lindz]

Pulled the channel switching board and while its now quieter, there is still a lot of noise/buzz remaining that I have yet to chase down.

but for today I noticed 2 things today when mucking about

1. if I have the amp on with volume dialed right up, guitar plugged in but volume on guitar off that if I touch the chassis I get a pretty good pop through the speakers. is this normal? Or is there a filtering or other ground issue I should look at.

Maybe its just static because I have to take one step to make it pop (does not seem to pop as much if I hold perfectly still and do this) - I'm on a hardwood floor so I would not think moving 1 foot sideways would generate static so it just seems weird. Its a dumb problem that hardly merits mentioning but I thought it might relate to a filtering or grounding issue especially with the Larry style grounding and all the stars now on the chassis

Also has pretty loud pop going from on to standby - I've seen builds with resistors and caps on the standby switch - I assume you can kill the pop with something like that?

2. my upstairs power is obviously noisy - move the amp downstairs and plug it in it is much quieter in terms of buzz - still noisier than I want but better. But if I plug in my Marshall 1987 reissue to the same outlet upstairs it has 1/10 the buzz/hiss/other noise of my 18 watt build running on the same power. It is a bit noisier upstairs than downstairs too but you can hardly tell the difference and it is completely tolerable by comparison. The 18 watt is exponentially worse and the improvement downstairs vs upstairs is more noticeable too

Suggestions as to why? filter caps? coupling caps? ?? I realize the Marshall is a more modern PCB amp etc but it seems crazy that my amp is SO much worse, so I assume something is amiss...

Again I appreciate any help

[LOUDthud]

What is the yellow line from the preamp ground, 1, to the transformer?

Have you tried moving the zener string ground to the ground terminal of the can cap?

[ChrisM]

Put it back to the Trinity grounding scheme with the channel switching removed.
I'd go from there.

I have built many many amps on this platforms with very minimal noise. I have then added switching to many Trinity amps without the noise increasing so I can help you there.

[Lindz]

Loudthud - the lead from the PT to the 1st ground star is the Heater center tap - the "Larry" grounding scheme suggests running it to that point.

I have not tried moving the zener string as you suggest but that is an easy 5 min experiment to try. How different is the voltage drop if I put it there? I can measure easily enough but want to have a rough idea in case I am running really hot vs how I am now

ChrisM - I'm stoked you have done some switching on similar builds - the only thing I could find similar was the Huss T-rex and it is not exactly what I have built.

How and were in the circuit did you install the switching in your amp? Or a better question would be how would you install it in my build - 1 input jack, to switch between the normal and TMB? I'd appreciate as much detail as possible as I tried a few different things but am somewhat electronics challenged as you can tell by some of my posts - ha ha

I guess the one major question I have is regarding the amount of hum and hiss I have vs the Marshall - it was very noisy vs the Marshall before I added the channel switching and tried the Larry grounding. Some of the hiss would be carbon comp resistors in my build I am sure, some would be PCB vs my mediocre hand wiring - but it is 10x noisier not just somewhat noisier - and I was wondering if I perhaps have some bad filtering somewhere?

Why would the noisy AC in my upstairs be so much more noticeable in my build vs the Marshall? Once I move my amp downstairs its still very hissy, but the majority of the buzz, odd crackles and pops etc goes away. with the Marshall you can hear a bit more noise upstairs but it is still VERY quiet by comparison

[ChrisM]

See the Triwatt build docs. There is some info there on relay switching supplies and the PSU for it. It is similar to the Dumble ODS switching which again I have used a lot with success.

http://trinityamps.com/phpbb/viewtopic.php?f=12&t=1567

I have used a similar setup before with good results. I would try tapping the AC voltage off the heaters first then going from there. You may need a small auxiliary transformer though.

[Lindz]

If I follow the Triwatt correctly it is 2 channels where each can cascade into the OD gain stage, and the footswitch switches that in or out, vs a true channel switcher? correct?

So in theory you are either in the normal or bright jack with one volume active or the blend jack and both volumes would be active like jumpered channels on a plexi or 18 watt? correct?

In my case I would rather switch between the normal and TMB channels vs a boost.. or is that more difficult/noisy for some reason

[ChrisM]

If I follow the Triwatt correctly it is 2 channels where each can cascade into the OD gain stage, and the footswitch switches that in or out, vs a true channel switcher? correct?

So in theory you are either in the normal or bright jack with one volume active or the blend jack and both volumes would be active like jumpered channels on a plexi or 18 watt? correct?

In my case I would rather switch between the normal and TMB channels vs a boost.. or is that more difficult/noisy for some reason

Ignore what the Triwatt is switching (it's switching a triode)

Just follow the PSU and relay arrangement. Just wire the relay how you want the switching done.