DIY JCM 800 2204 on JTM45 chassis

[Haze13]

Hello every one. I just fineshed my first tube amp - Marshall JCM 800 2204. Every thing seems to work fine, but I have some osrt of a problem with preamp 1Mohm potentiometer. When I go from 1 to 9, it's souns as it should be, but when I go to "10" I hear strange crackling, fuzzy noise and only when I play. No humm or buzz, just that strange noise, and only at "Hi" input.
The 1nf bypass cap after treble picking circuit (470k + 470pf) is not soldered. The amp sounds grate. I think... Better than carvin XT112 that I had before this.

[demonufo]

I suggest you start chopsticking components to see if any make the noise when tapped. Make sure to use something NON-conductive, and keep one hand behind your band.

Also, that capacitor on V1 socket is excess to requirements since you have shielded cable on both grids.

[Haze13]

Removed the 100pf cap on V1...
When guitar is unplugged there is two sounds that I hear when cranking the amp... Small amount of hissing of a working amp, and some kind of a humm. When preamp pot is turned all the way up, there is only the humm. Chopsticking revealed only that when I tap the second leg of a 1Mohm pot, I hear that 'tap' very clearly. Some amount of the same thing on 1 leg of a V1 that goes to 22nf and plate 100k resistor, and even less on 1 leg of a v2.
When I moove my hand with plastick chopstick close to the components, there is a sound of a magnetic feild. Like when a monitor of a computer that stands close to the cabinet. That's it.
I can add some more photos if need be...

[demonufo]

Sounds like a ground issue somewhere...

[Haze13]

I will check once again.
I replaced this pot with another one, and it didn't help. Then I measured a resistence on the potentiometer in that area where my problems are. It's less then 3Kohms. So when resistance between 470k + 500pf and second leg of V1a, becomes less than 3Kohms or so, I start to hear that strange noise... May be it's better to use normal wire and 100pf bypass cap on V1?

Thanks for a reply...

[demonufo]

or better to take the grid resistor off the board, solder it direct to the tube socket, and wire the shielded cable to that. No point having it on the board, and then taking a normal flylead from the resistor.

[Haze13]

You are correct on 100%, but I started to think about it only after drilling holes on the board.
May be it will help...

I was looking alot to Joe Popp builds and he has that kind of a wiring of this resistor, and he had no problem with this.
If it will help, it will be not logical!

[demonufo]

To be fair though, whilst Joe Popp has done incredible work, he was working to instructions, and did not necessarily understand all the 'whys' at the time...

[Haze13]

His work helped me more than a Drem Tone site/project, but some photos are small and it's hard to see usefull information:)
Any way many thaks to him and as I understand I'm not the only one who had completed an amp using his work as instractions.
I've been thinking about bias... To adjust the current, we use the trim pot, but than the voltage between two grid resistors (220k for El34) is changing too. The whole prosses is depends on plate voltage. Isn't it will be mutch easer to calculate the wright voltage between those to grid resistors knowing the plate voltage. It's much faster and there is no need for the probes. Cheap and easy. Do I miss some thing?

[demonufo]

We bias by plate current, so we need to know the plate voltage directly at pin 3. Even if you are doing a ball-park bias by measuring the negative bias voltage you still need to know the plate voltage and base your decision upon this.

If measuring current at the cathode of the power tubes, the inaccuracy here is that you are also measuring the screen current combined with the plate current. In EL34's the screen current can be around 3-5mA, so you should allow a little extra for this when biasing.

Don't sweat it too much though, use 70% as a GUIDE for a sensible limit, but most of all, make sure the point you've chosen to bias your amp at SOUNDS GOOD! It may be beneficial to bias colder, based on the tone you're after. Many people just bias to a rule, and forget about it, rather than testing what actually sounds good with their amp.

[Haze13]

Connected the 68k just like you told me and it WORKED!!!
But why? I just don't get it!

[Haze13]

That's how it looks now.

[demonufo]

Connected the 68k just like you told me and it WORKED!!!
But why? I just don't get it!

Given the gain on that lead (that is then amplified through the entire circuit) any small portion can act as an antenna. It's always best on higher gain amps to shield as much as is possible on the grids. Not much gain created after v1 on this particular circuit though, so doing it on v2 is a waste of time and effort.

[Haze13]

Thank you for your help and info.

[Joe Popp]

To be fair though, whilst Joe Popp has done incredible work, he was working to instructions, and did not necessarily understand all the 'whys' at the time...

This is correct demonufo! I am doing a new build and glad I found this here. I am getting back to building and digging in on some of the 'whys" this time. I am glad my work has helped people but sorry if it has misled anybody. I am still going to board mount the 68K resistors but I will put them right near the tubes. Good to be back here and learning more stuff. I have not built and amp since 2008!