100W SL grounding

[angelodp]

Hi, I am detecting some hum on a previous 100w build. I found a post by Jana that is indicating that the speaker ground should be directed over to where the PI and presence pot are grounded. I followed the Metro layout which has the speaker common grounded at one of the power tubes. I just want to double check that the way Metro suggests grounding the speaker jack should be adjusted. I am including the layout of the final amp I built. You can see the speaker ground is at one of the power tube grounds where the 1ohm resistor is also located. I have the chassis out and will make the change..... make sense.

[LOUDthud]

Yes, move the black wire from the speaker jack to the ground at the Presence control.

[Roe]

Yes, move the black wire from the speaker jack to the ground at the Presence control.

yes these go together. better still, a heavy wire straight from the speaker jack to ground point 3.

Basic overview of multi-star grounding, larry/novosibir style:

1) only V1 prior to coupling caps
2) only V2 prior to coupling caps
3) V3/phase inverter (prior to coupling caps) pluss output jack if you use negative feedback
4) bias circuit, screen filter caps ground, and - arguably - grounding of the output tubes
5) mains filter cap
6) wall power ground

keep at least 2" distance between the grounding points.

For details see here: http://www.valvewizard.co.uk/Grounding.pdf

[angelodp]

Thanks of the replies. The quad of power tubes in this amp are not well matched. I suppose that the mismatch is causing some of the hum (more apparent at hi gain levels) in this amp. The quad (6CA7EH) began as a matched set and now drifted.
I have the power tube cathodes set up with 1ohm 1% R's to ground for bias readings.

I have five ground points (6 including the power tubes)

1. Input jacks, V1 & V2 & Preamp filter cap
2. Bias Circuit & Mains caps
3. Rectifier and Screen caps
4. PI filter & Presence and Output jack ( I used 20g from Output jack to PI filter? Ok? )
5. Mains

6. Power tubes to 1ohm R's to ground at cathode

So I have some juggling to do. I need a new node for V2 and flip the Bias and Rectifier setup.

[Roe]

Thanks of the replies. The quad of power tubes in this amp are not well matched. I suppose that the mismatch is causing some of the hum (more apparent at hi gain levels) in this amp. The quad (6CA7EH) began as a matched set and now drifted.
I have the power tube cathodes set up with 1ohm 1% R's to ground for bias readings.

I have five ground points (6 including the power tubes)

1. Input jacks, V1 & V2 & Preamp filter cap
2. Bias Circuit & Mains caps
3. Rectifier and Screen caps
4. PI filter & Presence and Output jack ( I used 20g from Output jack to PI filter? Ok? )
5. Mains

6. Power tubes to 1ohm R's to ground at cathode

So I have some juggling to do. I need a new node for V2 and flip the Bias and Rectifier setup.

before you go over the grounding, try to get at least roughly matched pairs of output tubes. e.g., one side reads 28+38, another 33+33 (both sides then read 66ma)

[Stevem]

Any 120 hz hum level that mismatched output tube might add will not change with the level of gain they get fed from the preamp section.

[angelodp]

Ok so the tubes are close except for one. From the left hand (looking at IEC / North)working towards the IEC. @ 120VAC.

31.3ma 24.1ma 37ma 25ma - so cannot get a great balance.

55.4 vs 62

This is a now rearrangement, which was further split and now a bit better on hum.

[angelodp]

Roe, what's your take on the Heater CT ground and the Bias ground? Metro has that at the PI power cap. I have seen that moved to a separate ground for just those two.

[Roe]

Ok so the tubes are close except for one. From the left hand (looking at IEC / North)working towards the IEC. @ 120VAC.

31.3ma 24.1ma 37ma 25ma - so cannot get a great balance.

55.4 vs 62

This is a now rearrangement, which was further split and now a bit better on hum.

change the order to

31.3ma 25ma - 37ma 24.1ma.


The heaters CT goes to v1 ground, according to larry. If you eleveate it it goes to v3/PI normally.

Bias ground goes to screens ground (4 above)

[angelodp]

Thanks Roe. So do you have a 100w Marshall style amp that is absent of white noise of buzz with the amp dimmed? Have you seen the Psionic (lyle) ground scheme?

https://www.youtube.com/watch?v=ZySW1bq ... cJ&index=8


Adding the Larry scheme as well.

[angelodp]

Larry scheme implemented and the Bright channel is very quiet as is the Normal. I do get an increase in hum when jumpered though??

[maxkracht]

Grounding the filament ct to input just looks wrong to me. I actually had a marshall kit in a few weeks ago with that ground scheme and didn't know it was a thing. Changed it to a bus, in the proper order, grounded to chassis at the input only and was dead quiet after I finished, even with some high gain mods. It had lots of problems, so I can't really say how well the original ground scheme worked...

Perhaps jumpering is inducing hum for another reason unrelated to ground loops?

[angelodp]

Thanks Max, so the amp here is pretty quiet with the Larry scheme and the Heater CT to the input as well. When jumpered is the issue now so, I am trying some stuff. I'll post what exactly I did soon to get any possible feedback on the actual build.