Help, please, with Lead 12 5005 - hum and metallic sound

[kysrsoze]

Hi all,

I could use some help with my 5005. Its got two main problems.

1. There is a fairly loud hum when plugged in or not. It is loudest at zero volume, and the steadily drops in volume as you’re the amp up to halfway. Multiple people have reported this, but i’ve never seen a solution to it. Ive seen indications it is probably a grounding problem, but I can’t find anything unusual.

2. As I pick harder, the amp produces a very thin, harsh metallic sound on all strings… almost like a metallic slap-back. It is more pronounced on the higher strings, and it sounds a lot like fretting out due to a high fret. I tested the DI output to another amp, and the sound is still the same, so no odd speaker issue.

I have re-capped the amp, and checked all the resistor values. There are a few which seem to be out of spec, like R25 and R27, which are measuring 0.3 ohms and 0.4 ohms. All others are fairly close to what i’ve seen as spec.

Pics of the board are attached. One of the mosfets appears to have overheated somewhat. I’m wondering if it’s shot.

Any ideas as to what could be the issues? Thanks.

 

[Jon Snell]

The hum when volume is either up or down but reduced when in the centre position, is normally a ground loop issue and in this case maybe an incorrect ground wiring issue.
As you pick harder, the pre amp goes into overload and gives a harsh distorted sound. They do that as they only produce 12Watts output. The 5005 is a practice amp.
The annomolies with R25 & R27 are probably meter error. They are fine.

 

[neikeel]

Most likely a dirty contact on the front switching jacks.
You could always do what I did to my 5010:

Marshall 5010 combo completed

Just waiting on the bias pot and then I'll add the presence pot cap (going with 5k/0.1 for now). The OT has 16 ohm and 8ohm taps so I am going to try 100k on the 8ohm tap (vs 100k/4). I have used x3 output jacks like the modern Marshalls so I can use the 16ohm T65 I've installed in the cab or a...

www.marshallforum.com

 

[Jon Snell]

Most likely a dirty contact on the front switching jacks.
You could always do what I did to my 5010:

Marshall 5010 combo completed

Just waiting on the bias pot and then I'll add the presence pot cap (going with 5k/0.1 for now). The OT has 16 ohm and 8ohm taps so I am going to try 100k on the 8ohm tap (vs 100k/4). I have used x3 output jacks like the modern Marshalls so I can use the 16ohm T65 I've installed in the cab or a...

www.marshallforum.com

Wrong amplifier model!
The 5005 doesn't have an output transformer, it uses a complimentary pair of Darlington output transistors from a low voltage power supply. Solid state, not valve.

 

[neikeel]

I did not refer to a transformer and very well aware of the design - the original boards of 5005 and 5010 are almost identical. Dirty contacts on the hi/lo switching jacks cause failed earth and hum that varies with the signal input from the pots.
You presumably did not read that in my tread I removed the old ss board and installed all the new transformers, valve bases and pcb for my conversion. 😎
My first suggestion was what I think is wrong and the second part in jest to the OP because the transformation with valves and proper iron is night and day.

 

[kysrsoze]

Thanks for initial replies. I’ll look into ground loop issues, including Jack contacts.

On the sound… it is REALLY harsh. I’ve head two Lead 12 amps prior, and neither had this level of harshness. Even when the gain is way down, the harshness is there unless I pick very lightly. It’s just a buzzy mess at higher gain.

 

[Jon Snell]

Have you checked the large value main smoothing capacitors ... . lack of a stable supply may be your cause.
It is the solid state amplifier? There seems to be some confusion.

 

[kysrsoze]

Have you checked the large value main smoothing capacitors ... . lack of a stable supply may be your cause.
It is the solid state amplifier? There seems to be some confusion.

Yes, confirming this is the solid-state Lead 12 combo.

I already replaced all the electrolytic capacitors, and confirmed they’re in the correct way. I used a matched kit with the correct values. I was hoping that would fix the issue, but unfortunately it didn’t.

 

[Jon Snell]

In that case did you check the quiescent current across one of those wire wound resistors? (R25/27). Should be just drawing current at idle with no signal, around 5mA ish.

 

[kysrsoze]

Hey, thanks for all the suggestions. Before I got a chance to read the last one, I figured out the issue. The bias control was turned all the way down, completely cold. I’m assuming the seller did that for some reason… what that reason was, i have no idea. I played with the bias a bit, and both the harshness and hum went away completely. Issues fixed.

I still find it bizarre that there actually is a bias adjustment on a solid state amp. I wouldn’t have expected a cold bias to make the amp hum like that. Does that make sense? Is it effectively creating a ground loop?

 

[neikeel]

That is odd as the 5010 (and as far as I can see neither does the 5005) is self biasing (I see no pot and can't see on the schem. of the versions I can see, but I do see RV1 on your board!!!). Interesting that the 5010 does not have R25 and R27 0.3ohm emitter resistors....... The reason mine was so bad was that it oscillated badly at higher volumes (and someone had added 100nF cap on the ps pin (4 and 8) under the board of the 458 OPAMP to try to reduce that.

 

[junk notes]

The bias control

bizarre that there actually is a bias adjustment on a solid state amp.

That is odd

That is interesting!
very!

 

[Jon Snell]

That is interesting!
very!

RV1 on the PCBoard, as shown.

 

[PelliX]

It's always confused me a little why more solid state guitar amps don't have a 'bias' control. It can really change the character of the amp and is cheap to implement. Perhaps the feature is abused too much by those who don't understand what they're doing and thus causing more issues and replacements/repairs?

 

[neikeel]

I guess this is how it is implemented?

I've been advised that this is probably simplest way of implementing it (the Master 30 is missing the emitter resistors seen on the Lead 12 so it would not be ideal).

 

[chris84dc]

I thinl that I have the same problem in my Lead 20...With gain at 8 or 8,5 I listen a ugly note decay...like a freet problem, you know...but my lead 20 not have a trimpot for bias