Marshall Valvestate VS100 Combo Hiss

[JamesFilth]

Hi!
I recently bought used Marshall VS100 combo. I noticed that it hissess a little bit on clean channel. It's not that loud, but it's slightly annoying. I plugged the guitar in fx return and the hiss was gone, so I assumed it comes from the preamp. The hiss is there even when the volume is set to 0, only on clean channel. I replaced two 12V stabilizers, some electrolytic capacitors, IC4 TL072, IC1 switch and LM348. Also I put a new tube. The hissing is still there, but it goes away after a couple of minutes. Have you got any ideas what could it be? Maybe it has to be like that? I've read somewhere that most of new tubes have this issue (it's Electro Harmonix 12AX7).

[The Dude]

Do you have a scope? If so, you should be able to see the HF noise on a scope. Trace through the circuit and see where it starts.

[JamesFilth]

I do have a cheap scope, but I don't even know where to start measuring, I checked output of reg1 and reg2 and they are fine, also there is no dc on speaker output. I pulled out the tube and shorted pin 3 and 7, without the tube there was no hiss, so i suppose that it's coming from the tube.

[Jon Snell]

I would suspect a noisy valve. Do you have access to 12AX7WC or 12AY7 or even a 7025.
12AX7A can be noisy and the bog standard 12AXy is noisy.

[JamesFilth]

I have old Marshall ECC83, used Sovtek 12AX7WA and brand new Electro Harmonix 12AX7EH, they all hiss after startup for couple of minutes.

[Jon Snell]

In that case, in the absense of a good quality oscilloscope, I would replace the anode load resistors on pins 1 & 6 of the valve. They can be a cause of hiss and crackle.

[JamesFilth]

I replaced R22, R23, R17 and R18 and the problem still persists.

[g1]

hiss is there even when the volume is set to 0, only on clean channel.

If you can get rid of the noise with the dirt channel, I don't see how it could be the tube circuit?

[JamesFilth]

If you can get rid of the noise with the dirt channel, I don't see how it could be the tube circuit?

Neither do I, but without the tube there is no hiss. I had played on a VS65R for couple of minutes and clean channel seemed to be noisier than od too, but i don't remeber if it hissed like my VS100 after power on. With volume at 0 on clean channel there is the hiss, on od with volume and gain set to 0 the amp is dead silent. It seems to me that when the tube is warmed up the hiss is much quieter.

[Enzo]

Well both channels run through the tube, do they not? The clean channel is IC4a and IC2. The dirt channel is ICC4b and IC7, IC6 and IC2. They join at IC2 and proceed through the loop stuff and on through the tube and finally IC3. So pull the tube and everything before it is gone. If I disconnect the speaker there is no hiss either, but I can't blame the speaker. I don't see how the tube is involved.

You have hiss in the clean channel. OK, and turning th volume to zero still hisses, and the tube out kills it. SO the hiss is between the volume control and the tube. That sure looks like IC2 to me. Try swapping it for one of the other ones.

Hiss usually comes from a gain circuit - tube or op amp or transistor, whatever.

[JamesFilth]

Well both channels run through the tube, do they not? The clean channel is IC4a and IC2. The dirt channel is ICC4b and IC7, IC6 and IC2. They join at IC2 and proceed through the loop stuff and on through the tube and finally IC3. So pull the tube and everything before it is gone. If I disconnect the speaker there is no hiss either, but I can't blame the speaker. I don't see how the tube is involved.

You have hiss in the clean channel. OK, and turning th volume to zero still hisses, and the tube out kills it. SO the hiss is between the volume control and the tube. That sure looks like IC2 to me. Try swapping it for one of the other ones.

Hiss usually comes from a gain circuit - tube or op amp or transistor, whatever.

I've already swapped M5201 opamps with ones from different Marshall (VS30), the old ones worked fine in the VS30 and there was no difference in VS100.
I took the tube out and shorted pins 3 and 7, bypassing the tube. The signal from guitar came out of the amp without hissing.

[g1]

I took the tube out and shorted pins 3 and 7, bypassing the tube. The signal from guitar came out of the amp without hissing.

With the tube bypassed you lose all the gain of the 'A' side triode in the schematic. So any hiss coming in before the tube does not get the amplification the tube would give.
If you can kill the hiss with the gain channel master, that eliminates the tube. It is doing the exact same job as with the clean channel, yet it is quiet.

Ground pin2 of IC2 with clean channel engaged and see if the hiss goes away. If so, hiss is coming from before IC2. If not, maybe something in IC2 circuit that is only used in clean mode, like R11,R12,C5, or C24.
That being said, it could still be normal hiss as IC2 is set up in gain configuration in clean mode, but a unity gain buffer for gain channel.

[Helmholtz]

With nothing connected to the amp's input the input grid of the tube should be AC grounded by the switching jack.
So if there's still hiss its not coming from before the tube.

[JamesFilth]

With nothing connected to the amp's input the input grid of the tube should be AC grounded by the switching jack.
So if there's still hiss its not coming from before the tube.

With nothing connected to the input the amp is silent.

[JamesFilth]

With the tube bypassed you lose all the gain of the 'A' side triode in the schematic. So any hiss coming in before the tube does not get the amplification the tube would give.
If you can kill the hiss with the gain channel master, that eliminates the tube. It is doing the exact same job as with the clean channel, yet it is quiet.

Ground pin2 of IC2 with clean channel engaged and see if the hiss goes away. If so, hiss is coming from before IC2. If not, maybe something in IC2 circuit that is only used in clean mode, like R11,R12,C5, or C24.
That being said, it could still be normal hiss as IC2 is set up in gain configuration in clean mode, but a unity gain buffer for gain channel.

Ok, so I grounded pin2 of IC2 and it doesn't affect the hiss, also I noticed that in place of C5 there is a resistor