New Marshall jcm800 2204 clone build.

[TonyM]

Hi,
I've just completed my Marshall jcm800 2204 clone build. It puts out a great sound but what I've found is that the high gain input is picking up scratchy sounds from all of my guitars that being two fender strats, a les paul and a new PRS. And yet in the low channel it's a clean as a whistle.

I can only figure is that the high gain coupled with a too large a filter cap is allowing too many other frequencies through.

Any thoughts on this dilemma?

[acheld]

For a new build based on a known design, far more likely that there is a poor solder joint in the high gain circuit. I suppose a coupling cap (or another component) could be bad, but I'd focus on the basics first.

Please post your schematic and hi-rez pictures of your build. 

And welcome to the forum!

[pdf64]

Maybe it’s unstable, oscillating, ‘scratchy’ can be a symptom of that.
Good photos showing details of the build’s layout and lead dress may be beneficial.

[TonyM]

What I left out of my comment was that, it's only when the volume pot on all these guitars when being  turned is that the scratchy noise comes through otherwise it's a clean sound with very little hum or other noises.
Below is attached are the schematics and the layouts for this build.

Fundamentally all the amp layouts are just about identical except for a couple of variations on layout position and the schematics supports the layout.

The one main difference I have found is that in one layout the first 12AX7, the shielded wires to the input jack and the Volume control, are swapped from pin 2 and pin 7 but the schematics support the way I have attached them.

[tubeswell]

Could be a dodgy input jack tip-switch contact (on the 'low gain' input jack - which connects the Hi-gain coupling cap to the Lo-gain stage). Try tensioning the tip switch contact slightly

[PRR]

> when the volume pot on all these guitars when being  turned is that the scratchy noise

V1b grid is leaky. Replace the first tube.

"too large a filter cap" is not right.

[TonyM]

Yes the scratchy noise is only when I turn the guitar volume up or down on all my guitars otherwise is sound really great.

I've replaced the the first and second 12AX7 and resoldiered anything the looked dodgy. I did a tap test with a chop stick and nothing is noisy or clunky.
I do get a loud pop sometimes when turning the power on and off.

[TonyM]

attached are the layouts I was working from as well as the schematics posted earlier.

[pdf64]

Slight but potentially significant differences between them all.
Info regarding which you actually used, and detailed photos of your actual build, would be beneficial.
With a lead plugged into the high gain input, all controls turned down, what DC voltage do you measure at the jack of the other, unconnected end?

[TonyM]

I used the Triode electronics version but used the other two versions to draw correlations and to determine the consistencies and inconsistencies between them. I also did a mod which is to put in a dual bias so as to balance both EL34 power tubes and this works a treat considering that for a matched pair of valves they had 10ma difference between them. They're biased as 36ma to plate 452v.

[TonyM]

JCM 800 2204 Clone

[TonyM]

more photos

[sluckey]

With a lead plugged into the high gain input, all controls turned down, what DC voltage do you measure at the jack of the other, unconnected end?

Do this. Looking for small DC voltage.

[TonyM]

While the amp was at full operating capacity with just the guitar lead plugged in. I ran a multimeter across the open end set on 200ma DC and the reading was -1. I then set the multimeter to 2DC and its initial reading was -.398v and then settled on -.410v. The Low input gave a Nil Reading.

So does this suggest there is DC current leaking through to the input? and there for a leaky capacitor.

[sluckey]

That -.4V is causing the guitar volume pots to be scratchy. While monitoring that -.4V, pull V1. Does the voltage drop to zero?

Now turn the amp off and switch the meter to read resistance. Meter should still be connected to the end of the cable. Should be about 1MΩ. What have you?

With the cable still inserted into the high gain input measure resistance from the tube grid pin to ground. Should be about the same as the above resistance reading.

Finally, remove the cable from the high input jack. Repeat the resistance measurement from the tube grid pin to ground. Should be about 68K.

[TonyM]

It's nearly midnight here in Oz. So I'll have to get back to you tomorrow with the info. With thanks.

[TonyM]

Good morning
So I've taken out the V1 and the open lead reads zero. I did a resistance test across pin 2 and it gives me .990ohms @2M setting and pin 7 gives me .068.

[WiderGates]

.990ohms @2M setting and pin 7 gives me .068.

Hello,

with the 2M setting that is 0.990 megaohm (=990 kiloohm), not .990 ohm.
.068 is then 68 kiloohm.

[tubeswell]

So, as the others have already said, leaky* 0.022uF coupling cap


*not leaking liquid. Leaking 0.4VDC

[TonyM]

This system is running .022 coupling caps. I've just replace one I'll try the other one.

[pdf64]

I don’t see how a bad cap would put DC on to the high gain input stage grid?
Typical causes are a bad valve, bad operating point.
What’s the anode and cathode VDC for that stage?

[TonyM]

I've decided to go back to square one and redo the board and tidy it up and put the 5.6k resistors back at the tubes. I poking around with a chopstick where I had mounted the 5.6k resistor on the board and the first EL34 started to red plate. So perhaps part of my issue is a bad joint around the 5.6k connection.

Anyway I'll be back with my revised board, albeit the board came pre-turretted.

[tubeswell]

I don’t see how a bad cap would put DC on to the high gain input stage grid?
Typical causes are a bad valve, bad operating point.
What’s the anode and cathode VDC for that stage?

AFAICT, the OP said there was DC on the low gain input grid switch contact (the contact with the 0.022uF cap from the high gain input) (but I might have missed something?)


However, I suppose it could instead be a cold solder joint on one of those socket pins. (Or a loose pin clamp). Other solder joints in the amp are now apparently bad. Perhaps the soldering just needs a decent amount of reflowing everywhere.

[pdf64]

Post 1 mentions “the high gain input is picking up scratchy sounds from all of my guitars”, so the input that was tested should have been that one 

[TonyM]

I stripped the board down and redid the whole again board giving particular attention to the soldiering. The amp red plating must have killed half low gain triode of the first 12AX7. I replaced the valve and the low gain is working again.
So I'm back to square one with scratchy guitar pots.

[TonyM]

Thanks to all the people on this site who gave me some guidance. A friend suggested I place a .022 cap between the high gain input connection and the junction of the 1M resistor and the 68k grid resistor.
This has resolved the problem of the scratchy guitar pots and there's no hiss (not that there was a lot of noise anyway) and hardly any hum at all.

[tubeswell]

Thanks to all the people on this site who gave me some guidance. A friend suggested I place a .022 cap between the high gain input connection and the junction of the 1M resistor and the 68k grid resistor.
This has resolved the problem of the scratchy guitar pots and there's no hiss (not that there was a lot of noise anyway) and hardly any hum at all.

Must’ve been some voltage on the grid.

[TonyM]

Yes it would seem that way. I read a comment from another group where someone had a similar issue with scratchy pots and the problem was solved by replacing the first valve socket even tbough there was no apparent damage to the pins.

[pdf64]

It might be better to resolve the root cause of the issue, rather than bodge it with a mod.
As the grid voltage of that stage isn’t 0, its bias will be wonky.

[TonyM]

Yes I agree with you on that point but as I can't find the underlying issue I'm going to with run with this. I'll have take some grid readings and see what they are. Potentially anything may or may not happen to the amp so maybe if another issue arises then this may lead to the root cause so it can be identified.