Marshall Valvestate VS100 Combo Hiss

[Helmholtz]

That schematic is a mess!

Just look at "C6", "R72", "C53".

[JamesFilth]

I've measured voltages on pin4 and 8 on opamps, in the schematic there is + and -15V, but in my amp its + and -12V. Is it correct? The stabilizers are 12V.

[Enzo]

Then 12v it is. They are even. The op amps will work fine on 12v rails.

I noticed that in place of C5 there is a resistor

ANy chance that resistor is a different color body from the other resistors? AND a yellow violet color bands? 47pf small ceramic cap may look like a resistor.

You swapped out the 5201, so did you look through the loop circuits, I see a TL071 there.

[JamesFilth]

Then 12v it is. They are even. The op amps will work fine on 12v rails.



ANy chance that resistor is a different color body from the other resistors? AND a yellow violet color bands? 47pf small ceramic cap may look like a resistor.

You swapped out the 5201, so did you look through the loop circuits, I see a TL071 there.

Ok, it actually is a capacitor, my bad. I've never seen one like that. I tried replacing TL071 with TL081 but it made no difference. Maybe I'll try replacing eletrolytic capacitors near the IC2, these old Philips caps don't look good.

[Enzo]

Did we determine you have a scope? DO you have a signal tracer?

A signal tracer is basically an amplifier you plug a probe into instead of a guitar. You can probe the circuit an hear what is going on at any point in it.

And then there is grounding the signal path. IN some spots you can actually ground the signal path. In other spots you can't, maybe due to DC offset or something. IN those cases we "ground" it through a cap. I use a 0.047uf because I have a ton of them. A -.1uf would work. The value isn't important. It shunts audio to ground. Your hiss is audio, unwanted, but audio nonetheless.

[g1]

Ok, so I grounded pin2 of IC2 and it doesn't affect the hiss,

I think it is the 5201 (IC2) setup for the clean channel that is making the hiss. You can check this by shorting out R11. This will make IC2 'A' side unity gain like the 'B' side of IC2 is.

[JamesFilth]

I think it is the 5201 (IC2) setup for the clean channel that is making the hiss. You can check this by shorting out R11. This will make IC2 'A' side unity gain like the 'B' side of IC2 is.

After shorting out R11 the hiss is gone.

[g1]

How does it sound with R11 shorted out? Clean channel too weak, or usable ?

[JamesFilth]

How does it sound with R11 shorted out? Clean channel too weak, or usable ?

Pretty weak, I can turn the volume to max and it's not really that loud.

[g1]

So you could play around with the R11 value and maybe find some compromise between noise and clean volume. Maybe try paralleling a 47K on top of what is there and see where that gets you.
I'm not sure how that would affect the frequency response as C5 will probably also be in play. Someone who has more experience with op amp design might have a better idea.

[JamesFilth]

So you could play around with the R11 value and maybe find some compromise between noise and clean volume. Maybe try paralleling a 47K on top of what is there and see where that gets you.
I'm not sure how that would affect the frequency response as C5 will probably also be in play. Someone who has more experience with op amp design might have a better idea.

I don't really need to change it, I just wanted to know if the hiss is there by design or there is something wrong in my amp, if it's normal then it's fine, I'll get used to it. Maybe someone had used this amp and could tell me if the hiss is normal?

[Helmholtz]

In your OP you're speaking of a little hiss that goes away after a couple of minutes.
I don't think that should be a real problem.

It seems as if there's a temperature dependency involved.
So if you're curious to find out what components are involved, you might use freeze spray on parts to bring back the hiss.
Just avoid to completely ice up components and board (showing as a white snow-like layer of ice crystals), as the melt water may cause all kinds of noises.

[JamesFilth]

Ok so that's rather strange, I put back old 12V stabilizers, powered the amp on and that annoying hiss was gone. There is still a little bit of background noise on clean channel but I think it's normal, VS65 that I've seen did that too. Previous hiss was more of a "whoosh", and it slowly dissappeared when the stabilizers were heating up (and they get quite hot, that's why I replaced them in the first place). Maybe the new stabilizers were bad.

[g1]

Ok so that's rather strange, I put back old 12V stabilizers, powered the amp on and that annoying hiss was gone. There is still a little bit of background noise on clean channel but I think it's normal, VS65 that I've seen did that too. Previous hiss was more of a "whoosh", and it slowly dissappeared when the stabilizers were heating up (and they get quite hot, that's why I replaced them in the first place). Maybe the new stabilizers were bad.

I guess you mean the 7812 and 7912 voltage regulators REG1 and REG2 ? Did you get them from a reputable electronics component supplier? There are a lot of fake/counterfeit parts on ebay and other online market places, usually shipping from China.

[JamesFilth]

I guess you mean the 7812 and 7912 voltage regulators REG1 and REG2 ? Did you get them from a reputable electronics component supplier? There are a lot of fake/counterfeit parts on ebay and other online market places, usually shipping from China.

Yes, Reg1 and Reg2, I'm pretty sure they were cheap knockoffs, at least one of them looked a bit sketchy, I got them at my local electronic store.