Quieten the Express

[Littlewyan]

Since building my JTM50 I've realised just how noisy my Express is. I know they will hiss A LOT but mine sort of hums or has a low end rumble as well when the volume is up high. Also when the volume is turned down I can still hear a lot of hiss through the speakers, whereas with my JTM50 its pretty much silent with the volume on 0.

Few things come to mind when first looking at it.

1. The Heater CT is grounded at the same place as the HT CT and the Cap Cans. Might be worth moving it to the input ground.

2. The right hand phase inverter wire goes underneath the V4 grid wire and is obviously kinda close to the OT secondaries and primaries.

3. The bus bar is still on the back of the pots. Might be worth removing it and grounding the volume and tone controls on the chassis ground just behind them.

Any other ideas? And yes I've tried different valves which help bring the hum/rumble right down but its still there.

[JazzGuitarGimp]

I don't know much about Marshall (or Trainwreck) circuits, but doesn't the Express have significantly more gain than the JTM45? If so, then I think a lot more hiss is to be expected.

[Littlewyan]

Yeah but its not just hiss. Its like a low end rumble.

I just tried lifting the phase inverter wire above the V4 grid wire and all it did was increase feedback and microphonics. Putting it back now.

[Littlewyan]

I just realised that all of the valves I tried in this amp when it came to reducing hum were either russian or slovakian valves. I tried some Mullards in it tonight and the hiss increased a bit but the hum definitely decreased.

I also tried moving the Heater CT to the input, but I did it using a jumper cable so had to leave the bottom shield off. It sounded like the hum went down a bit but was hard to tell with all the extra noise! I'll test it better another night.

[Bob S]

You could try moving the presence pot ground and chopstick the wire routing.
Also the bright switch caps can be a pain. If you take the ground bus wire off the pots, try turning the vol pot 90 degrees to shorten the distance to the bright switch.
Good luck !

[oj]

Nice build !
But your grounding is all over the map
It even looks like u have earth ground and chassi ground connected (?) if so then that is a big no no.
Earth ground and chassi ground should be 5-10 cm apart .

More on grounding see : http://www.valvewizard.co.uk/Grounding.pdf

hope this help !

Owe "OJ" Johnson

[Littlewyan]

Nah the earth ground has it's own lug next to the IEC socket.

The Mains CT, Heater CT, Bias and Cap Cans all ground at the same lug next to the Cap Cans.

The Volume, Tone Controls and Presence pot ground to themselves on the old bus wire. Cathodes ground to lugs on the chassis.

I was thinking of moving the volume and tone control grounds to the cathode grounds. Then possibly moving the presence ground to it's own lug and connecting the speaker output ground there as well.

Bob - I will look at doing that as I remember someone else doing that with good results. I've tried moving all over wires around but where they are now is the quietest position for them.

I know the Express is a noisy bugger but I have heard other people say they have got their's to be quiet.

[M Fowler]

I like your grounding except the buss bar grounding all pots. Your grounding is similar to what Komet used one ground tab at input, another at mid board and the rest by the PT and safety ground by itself.

Some like to ground the speaker jack with the center taps.

Nice looking build.

Mark

[rooster]

This won't help your hiss, but the Presence control ground should be separated from the other pots. Logic here is that it's function affects the PI, unlike the other controls so keep the two separate.

My hiss is small but it is there. Better tubes can't hurt. I don't have any low end hum so this something else. Your built is nice looking, but try a shielded cable from V1 to the Volume pot.

[Littlewyan]

I was looking at the Komet grounding actually Mark. I can only find a layout for the Asteroid though. What I can't understand is they ground the Volume pot to the V1 Cathodes and input but ground the tone controls to the V3 Cathode. Surely the tone controls would ground at the same place as the volume?

I tried using a shielded cable Rooster but oddly it made the noise worse. So solid cable from volume pot to socket suspended in the air seems to work best. In this amp anyway.

I will give the Presence Control it's own ground lug I think and try better valves. I noticed with a Mullard in V1 the hum decreased but the hiss got louder. Or maybe the hiss was more pronounced without the hum.

[Littlewyan]

Ok plan of action.

1. Drill hole and add ground lug for Presence Pot and possibly speaker jacks (they already have a ground lug so might leave them as they are)

2. Remove the ground wire for the 150K resistor between V1B and V2, then add a jumper wire from that resistor to the turret for the 10K cathode resistor for V2.

3. Connect Volume and Mid Pot grounds to the now free lug.

This will fall into place with Merlin's grounding scheme and will hopefully make the amp a tad quieter.

Also turning the volume pot round in my case won't help that much as my pots are the opposite way round to the way Ken had his. I'm using a Marshall layout whereas Ken's was the opposite. I think I looked at this a while ago but forgot my reason for not doing it.

[guitarmike2107]

You know I am not a fan of grounding to multiple places on the chassis, but defo get rid of the bus bar on the pots.

have you scoped the noise to know that it is heater noise and not 100hz hash

To solve noise issues you have to divide and conquer, Start at the power tubes, ground the grids and check the noise levels then work you way to the front end of the amp, you should be able to find where the noise is getting into the circuit pretty fast.

If you ground the power tube grids and still have hum you have coupling between your transformers and will need to move them apart, I still can't believe most of these amps have some hum with the PT and OT so close

[Littlewyan]

I haven't scoped it yet no. I'll try that at the weekend.

What if the noise goes when I remove V1? Would that not tell me the noise is getting in via that part of the circuit?

[guitarmike2107]

If you remove V1 and all the noise goes that mean those two triodes are picking up the noise and amplifying it. it could be getting in at your tone stack grounding as it sits between the two triodes, ground the output of the tone stack to see if it get rid of the noise.

[Littlewyan]

I was about to take the Express out of the cab but then I realised that turning the volume down to 0 does the same thing as grounding the tonestack output. Volume at 0 is fairly quiet, you just get hiss.

In fact with the controls set to 5 across the board its not very noisy. Its only if the volume gets turned right up.