Hum issue on a 2204 build.

[warioblast]

Hello guys.
I built a JCM800 2204 amp a few weeks. I didn't notice it until a couple of days ago but I have a hum issue. I didn't hear it because at very low volume the amp works fine, on both channel, with max gain. All pots do what they supposed to do. My voltages are within range.

But when I start to raise the master volume, the hum appears.
With no guitar, and with the master volume cranked, the amp hums.
I was reading this thread. https://www.marshallforum.com/threads/jcm-800-hum.63197/page-2

Gain at 0, Master Cranked, I took out V1, it was still humming. I took V2 out too and the hum went away.
I had my heaters wired over the sockets, like the picture below. So I decided to raise them, Fender-syle. I used a the chopstick, moved the heaters, & the wires around V1 & V2.

I reflowed the preamp solderings. I tighten my grounds points; I'm using a Larry grounding style. I checked for continuity for the preamp circuit.

Nothing changed.

I noticed one thing. With V1 out, Gain at 0 & Master Cranked,no guitar, the amp is humming. If I touch the chassis, the hum drops down significantly. And if I hover my hand over V1 socket, while still touching the chassis with my other hand, the amp becomes dead quiet.

I'm starting to think this could be a ground problem ? I'm probably going to move to a more traditionnal grounding wiring. Maybe change the master volume pot. After that, I will be out of ideas. 

[Willabe]

Larry grounding style?

Here's Merlin's;

http://www.valvewizard.co.uk/Grounding.html

[pdf64]

The 2204 circuit is prone to hum from valves that have heater to cathode leakage, as none of the preamp cathodes are fully bypassed.
So the first step is to try a lot of different ECC83 in V1 and V2.

[Latole]

Did input jack 's shorting contacts to ground are working ?

[warioblast]

Larry grounding style?

Here's Merlin's;

http://www.valvewizard.co.uk/Grounding.html

It's a multiple stars grounding , popular among Marshall enthusiasts.

Did input jack 's shorting contacts to ground are working ?

Yes, inputs jack tested good.


I was doing more continuation tests and I found out my heaters are shorted to ground ?!

[pdf64]

…
I was doing more continuation tests and I found out my heaters are shorted to ground ?!

Not shorted, rather the winding CT will be connected to chassis 0V, and either end of the heater winding is just a few turns of thick wire away from the CT. So an immeasurably low resistance between the heaters and chassis 0V is to be expected.
Incredibly, via transformer magic, when powered up, VAC somehow appears across those apparent shorts 

[sluckey]

I was doing more continuation tests and I found out my heaters are shorted to ground ?!

That's true for any amp that has a PT heater center tap that is connected to ground.

[warioblast]

I was doing more continuation tests and I found out my heaters are shorted to ground ?!

That's true for any amp that has a PT heater center tap that is connected to ground.

Oh, thx sluckey. 
I have never noticed it before  :ashamed:

[warioblast]

…
I was doing more continuation tests and I found out my heaters are shorted to ground ?!

Not shorted, rather the winding CT will be connected to chassis 0V, and either end of the heater winding is just a few turns of thick wire away from the CT. So an immeasurably low resistance between the heaters and chassis 0V is to be expected.
Incredibly, via transformer magic, when powered up, VAC somehow appears across those apparent shorts 

I see, thx for the explanation 

[shaun]

When dealing with hum, I keep all grounds off the chassis - jacks, pots, everything - making only one star point. Don't know if you've done that already, but it has helped me in the past.

Also, these days I am taking the time to check the outside foil end of orange drop caps and similar; more shielding offered when installed with the outside foil end attached to lowest impedance point in circuit. So a coupling cap would have the foil end attached to the plate, with signal being shielded by the foil of cap. This practice can also help avoid oscillation and other diificult-to-find noise issues. There are youtube vids showing how to test caps for the foil end. I use an olde time oscilloscope - quick and simple.

[warioblast]

When dealing with hum, I keep all grounds off the chassis - jacks, pots, everything - making only one star point. Don't know if you've done that already, but it has helped me in the past.

One star point is my next move.

Today I tried different tubes, I changed the master volume pot.
I also changed my grounding following this model.

Hum is still here. 

[warioblast]

Daily update:
I tried the single star grounding.
I took off the low input jack.
I tried on open input jack instead of the cliff one.

Tomorrow, I want to redo the heater wiring.

Should I focus around V2 connections 
Giving the fact that the amp is quiet without V1 & V2, and humming with only V2.

[Latole]

Good pictures of your amp/ preamp may help .
JCM800 2204 ; V1,V2 circuit.

[sluckey]

There is another source of hum that has not been discussed... heater to cathode (HK) leakage. The only cure for this 60Hz hum is replace the tube with a good tube. Heater to cathode leakage can be observed by connecting a scope to the cathodes of the tubes, V1 and/or V2 in your case. Any visible 60Hz sine wave is a sign of HK leakage. The greater the voltage, the greater the leakage. You may even be able to measure this 60Hz sine wave with a good rms DMM.

HK leakage is more of a problem in low level, sensitive preamp circuits in high gain circuits such as the 2204, but can be a problem with any amp. Many high gain amps us DC voltage for the preamp tubes to overcome HK leakage hum.

Not saying this is your problem but it's easy to verify with a scope, or just get a handful of good tubes and start rolling. If you have some real 7025s use them for V1 and V2.

[warioblast]

I don't have a scope and my DMM is pretty basic.

I'll post pictures when I will have redone my heater wiring.

Today I tried to bypass V2. (see attached picture) With the master cranked, no guitar, the amp is silent. Way less gain I know, but still !?

While trying to rework V2 connections I broke a lug of the tube socket :(
Some wires i used are too thick.... I ordered the same wire; same brand, same reference, same AWG ... that i ordered a few years ago. And the wires I received are significantly thicker. A bit of pain to work with, with tiny sockets holes, twisting...

[sluckey]

Today I tried to bypass V2. (see attached picture) With the master cranked, no guitar, the amp is silent.

OK. Now put V2 back to normal and bypass V1 in a similar manor. And?

[warioblast]

Today I tried to bypass V2. (see attached picture) With the master cranked, no guitar, the amp is silent.

OK. Now put V2 back to normal and bypass V1 in a similar manor. And?

Input > 68k res > pin 2 of V2 =  I do have a bit of hum.

While changing V2 socket, I arranged the heater wiring.

Here are a few pictures. I started this project of gutting my DSL 50 with a plexi like 5 years ago. It went through numerous circuit... I'm a bit ashamed to post them. But here they are.

I used the 2204 circuit with 2 mods:
_ full wave bridge rectifier power supply from the JCM 900
_ 5k presence pot circuit.

[warioblast]

...

[warioblast]

...

[warioblast]

...

[warioblast]

...

[warioblast]

...

[pdf64]

What are the jacks / terminal strip / wiring by V1?

[sluckey]

You can possibly reduce/eliminate heater hum by using a simple elevated heater circuit. All it takes is two resistors and a filter cap. Worth doing even if it doesn't solve your hum issue. Read this, especially the section called "Heater Elevation".

     http://www.valvewizard.co.uk/heater.html

[warioblast]

What are the jacks / terminal strip / wiring by V1?

I used the layout of Triode Electronics for the inputs.

You can possibly reduce/eliminate heater hum by using a simple elevated heater circuit. All it takes is two resistors and a filter cap. Worth doing even if it doesn't solve your hum issue. Read this, especially the section called "Heater Elevation".

     http://www.valvewizard.co.uk/heater.html

I will give it a try . 

[sluckey]

You put the impedance selector and the speaker jacks right next to the V1 preamp tube! That's a terrible idea!!! Move that stuff down to the power section of the chassis.

Show us a pic of the top of the chassis so we can get an idea about tube and transformer layout.

[warioblast]

Ok, will do first.
I thought it would better to move the impedence selector & ouput jacks closer to the OT.

I used the DSL 50 as a reference for the layout of the tubes... but I omitted the fact that V1 & V2 have DC heaters on the DSL. I could have moved the preamp tubes a bit farther from the OT too. 

[pdf64]

What are the jacks / terminal strip / wiring by V1?

I used the layout of Triode Electronics for the inputs.

…

No, there are jack sockets and a terminal strip and associated wiring near V1 socket. They are not shown on your layout.

Edit ok it’s the OT output selector, wiring and jacks.
A screening plate to keep the input stages isolated from the OT stuff is necessary.
Otherwise you’ve got a massive transmitter next to a sensitive receiver - the 2 are bound to couple.
See plate behind the input jacks on an AC50, to screen them from the speaker terminal block on the lower left

[shaun]

You put the impedance selector and the speaker jacks right next to the V1 preamp tube! That's a terrible idea!!! Move that stuff down to the power section of the chassis.

Show us a pic of the top of the chassis so we can get an idea about tube and transformer layout.

Yes, indeed. Rule #1: keep the input signal as far from any/all power stage wiring as possible. Always. Took me a long time to accept that because I saw inside so many vintage radios that had stuff mixed up everywhere - I dunno how they got away with so much weird design back then. They must have known their stuff, that's for sure.  But I can't get away with it.

[sluckey]

I would name this amp DOOM!   

[Latole]

Bad wiring may be a issue ; heater, input jack......

[Leevi]

Have you already checked the negative bias voltage source that there is not ripple.
/Leevi

[warioblast]

Have you already checked the negative bias voltage source that there is not ripple.
/Leevi

I had not.
I googled it. I have just done it like this. https://sciencing.com/test-crystal-oscillators-8728348.html
It didnt read any AC voltage over the negative DC voltage !?

I moved back the output jacks and impedence selector. It did absolutely nothing. I have the same level of hum.

I read somewhere about testing DC heater for V1 & V2 by using a battery. Can I use a power adapter ? It has selectable values: 6V, 7.5V, 9 V & 12V.

[sluckey]

I moved back the output jacks and impedence selector. It did absolutely nothing. I have the same level of hum.

But the OT is probably still over there next to the preamp and you still have to run the OT wires right by the preamp tubes. Right?

Have you elevated the heaters yet?

[shaun]

I concur with Sluckey. I know it's a pita  to have to move that OT, but when I saw it, I thought oh sh*#. The OT should be down the other end, nearer the PT and output tubes. And yes, I made that mistake several times myself. I thought that everything needed to be away from the 120AC power stages, so I moved the OTs as far away as possible, and also because I'd heard that the OT and PT can couple if too close together.

But even more important is too keep them both away from the input signal stages. This is why in typical amps, you'll see the PT at one end with the rec tube nearby, then the OT accompanied by the output tubes, then the inverter stage, then the preamp stages.

When I look at your pic, I see how nicely it's all laid out, so it's unfortunate that you'll probably have to rearrange it all to move the OT. But that would also mean you're building according to best practices, and thus on your way to trouble-shooting successfully. Of course, you may move the OT and still have hum, but it will be much easier to diagnose.

As a final thought on my little meanderings, the placement of key components often creates various types of field within the chassis - magnetic and otherwise. Having the OT where you have it means you're extending electromagnetic activity all the way down the chassis because of the attached cables. This is only one caveman's summation - I ain't no expert - but I believe that's the general idea, and hard experience has supported the concept. Keep at it!

[warioblast]

I moved back the output jacks and impedence selector. It did absolutely nothing. I have the same level of hum.

But the OT is probably still over there next to the preamp and you still have to run the OT wires right by the preamp tubes. Right?

Have you elevated the heaters yet?

Up next ! ;)
I was thinking 100k & 10k for the voltage divider !?

OT wires are about 3 inches from the preamp tubes. I can easily take the OT out of the chassis and move the wires even further; 15 inches;  from the preamp tubes for a test !?

I concur with Sluckey. I know it's a pita  to have to move that OT, but when I saw it, I thought oh sh*#. The OT should be down the other end, nearer the PT and output tubes. And yes, I made that mistake several times myself. I thought that everything needed to be away from the 120AC power stages, so I moved the OTs as far away as possible, and also because I'd heard that the OT and PT can couple if too close together.

But even more important is too keep them both away from the input signal stages. This is why in typical amps, you'll see the PT at one end with the rec tube nearby, then the OT accompanied by the output tubes, then the inverter stage, then the preamp stages.

When I look at your pic, I see how nicely it's all laid out, so it's unfortunate that you'll probably have to rearrange it all to move the OT. But that would also mean you're building according to best practices, and thus on your way to trouble-shooting successfully. Of course, you may move the OT and still have hum, but it will be much easier to diagnose.

As a final thought on my little meanderings, the placement of key components often creates various types of field within the chassis - magnetic and otherwise. Having the OT where you have it means you're extending electromagnetic activity all the way down the chassis because of the attached cables. This is only one caveman's summation - I ain't no expert - but I believe that's the general idea, and hard experience has supported the concept. Keep at it!

"

No worries. It wasn't such a hassle. I had high expectation though. 
I used the chassis & the transformers of my DSL 50. I cut a metal plate to gut the amp.
I'm realizing now, I could have try to move the OT on the other side of the chassis, while I was making this plate. 
I'm so close to being done yet so far :(

[warioblast]

I have just tried elevating the heater. Got 42V from the voltage divider. It didn't change a thing. 

[Latole]

I have just tried elevating the heater. Got 42V from the voltage divider. It didn't change a thing. 

Are they far from all wires ?

Post pictures

[Williamblake]

40V heater elevation should reduce hum in any build in my experience, maybe in a well built amp only in situations where you have your guitar volume way down. Meaning you have something wicked going on, a bad ground connection or proximity to iron, powertransformer or choke. Or a weak connection in the signal path.

[warioblast]

I took more pictures.
https://drive.google.com/drive/folders/18rLhAfCpo1rWCWUIMlDVb9bPFSo4lvCf?usp=sharing

I tried to power V1 & V2 heaters with a power adapter. Hum got way louder. 

[Latole]

I took more pictures.
https://drive.google.com/drive/folders/18rLhAfCpo1rWCWUIMlDVb9bPFSo4lvCf?usp=sharing

 

We need access to these file 

[sluckey]

I tried to power V1 & V2 heaters with a power adapter. Hum got way louder. 

You need a 6V lantern battery!

[warioblast]

I tried to power V1 & V2 heaters with a power adapter. Hum got way louder. 

You need a 6V lantern battery!

Oh snap. 

I took more pictures.
https://drive.google.com/drive/folders/18rLhAfCpo1rWCWUIMlDVb9bPFSo4lvCf?usp=sharing

 

We need access to these file 

Oh sorry, it's the 1st time I'm sharing docs with google drive. It shoud be fixed now.
https://drive.google.com/drive/folders/18rLhAfCpo1rWCWUIMlDVb9bPFSo4lvCf

[Latole]

IMO so many bad wire dress may act like antenna. Picture arrow show only few
Solder to chassis ground look poor...

I don't know how you can silent this amp

[warioblast]

I tried to  move the wires around the preamp tubes. I tried to put them against the chassis, up in the air, crossing them ...

As for the solders, I don't really like the soldering tin I'm using right now actually. I could only find lead free tin. I don't know, even if I'm using 400°C with my soldering station, sometimes the soldering won't melt. I will try to clean them the best that I can. I dont know if it's because I soldered & unsoldered those parts many times through out the years, using different tins.

This is how my amp looked 5 years when I first try to build a plexi.

[Willabe]

Is that a different amp in reply 44?

[warioblast]

Same amp but different  circuit. This was my first attempt at building an amp. This was a JTM 50 circuit.

[pdf64]

IME, that type of layout can work, but the OT and associated wiring needs screening off from the preamp, eg with metal plates, channelling, conduit. Anything that can help to block the coupling between the massive transmitting antenna of the OT wiring and the sensitive receiving antenna of the preamp wiring and components.

[warioblast]

I don't know. I took the OT and the output jacks out of the chassis. OT was standing about 1.5 ft & and ouput jacks were about 1 ft from the preamp tubes. It made no difference.

I cleaned my grounding. I also changed the thin wire I used for jumpers, with copper bus wire.

I don't have a 6V battery for testing DC heaters, can I daisy chain 1.5V batteries ?

I was also thinking maybe I can try to implement the DC heater circuit of the DSL 50.

I will try to borrow a scope from a friend. Maybe I am delusional. But the fact that the amp is quiet at low volumes makes me want to believe.

[Willabe]

Same amp but different  circuit. This was my first attempt at building an amp. This was a JTM 50 circuit.

So your rebuilding a different circuit into that same chassis?

Did that old build hum too?