Marshall Valvestate noisy input stage

[saint gillis]

Hello,
I've fixed for a friend an old Marshall Valvestate, 8240 I think. It had bad solders on PSU caps and sratchy pots (cleaned the pots and replaced series electrolitics).

It still has a problem I can't fix (for now) : input stage seems to pick-up ground noise.
- When I touch the strings of the guitar or the chassis, this noise dissappears.
- When the guitar volume pot is at mid position, noise is really worse.
- When I remove Earth from the outlet, noise is again really worse.

Any ideas ?

schematic : https://drtube.com/schematics/marshall/8240-61.gif?fbclid=IwAR1jURPf7ZrGfoud0Zn5Wy9TxvT7erlu4LbgkJ_V7xPeAoF8jrmmXbDpHGs

 

[abbey road d enfer]

It still has a problem I can't fix (for now) : input stage seems to pick-up ground noise.
- When I touch the strings of the guitar or the chassis, this noise dissappears.
- When the guitar volume pot is at mid position, noise is really worse.
- When I remove Earth from the outlet, noise is again really worse.

Any ideas ?

All this is the normal symptoms of electrostatic interference, probably because your earth is not very good. There's probably nothing to fix in the amp. Check the connection between earth and chassis, though.

 

[12afael]

it is noisy when the guitar level is at zero? maybe you are just too near the amplifier. pick ups/transformers... I happen me yesterday with a gallien krueger 250RL.

on old amps it is a good idea to clean the connectors and be sure they are making good contact with the chassis. if they are isolated then no worries, it depend on how it was designed.

 

[saint gillis]

Thank you.
Yes the earth is tight to the chassis.  Adding a 68K resistor in series with the input (Fender style) seemed to improve the situation.

Also on this amp the reverb return buffer picks up a LOT of radio ! Even when I disconnect the reverb cable...

 

[Tubetec]

I think Abbey was suggesting a possible problem with your electrical connections ,not the amp .
I seem to remember certain era's of valvestate enthusiastically oscillate with reverb tanks disconnected.

 

[abbey road d enfer]

Thank you.
Yes the earth is tight to the chassis.  Adding a 68K resistor in series with the input (Fender style) seemed to improve the situation.

that seems to indicate that it's the guitar that's picking up noise.

 

[Whoops]

It still has a problem I can't fix (for now) : input stage seems to pick-up ground noise.
- When I touch the strings of the guitar or the chassis, this noise dissappears.

It seems to me that you have a grounding problem in the guitar itself and not the amp.
Measure continuity between the Guitar bridge and the sleeve of a cable connected to the guitar, there should be continuity with almost zero resistance.

Try it with another guitar

I owned a couple of Valvestate amps in the 90s, I also serviced a lot of them over the years and although these amps sound pretty bad they don't have any problems with noise in input stages. They're not noisy amps.
When the distortion channel is selected you will have some noise if the Gain is high but that is just normal in High Gain situations, you will have the same thing in any amp and any distortion pedal specially if you use a guitar with single coil pickups.
The clean channel shouldn't have any noise

 

[saint gillis]

Using 2-coils positions with several fender strats, and volume at mid position, brings quite a lot of noise. It is not the case with the same guitars and other amps here, and the ground is very good here.

 

[saint gillis]

Also this behavior is really worse in the owner of this amp's house, where the ground seems to be bad. And in this house, this happens with guitars that don't do this on other amps in the same location.

 

[saint gillis]

Also I don't understand why there is such a huge hum when I remove earth from the outlet.. never seen that

 

[saint gillis]

I remove earth from outlet :
- big hum
- if I get my finger close to the + pin of the 1st op amp or to the input jack, hum gets huge.
- if I touch the chassis : hum disappears.

 

[saint gillis]

Something else :
- Gain and volumes of both channels at zero. Master volume full position.
- Reverb volume full : a bit of parasitic noise (crrk prrkk tss rcckk) - When I get my finger close to the zone where the reverb return is buffered (without touching anything) the parasitic noise disappears.
- Same settings (channels volume at zero, reverb volume full) : when I simply plug a jack cable at the input of the amp, I clearly listen to the radio (quite loudly), I remove the jack cable and the radio disappears.

 

[walter]

It does sound like a ground issue. I would connect an ohm meter to a known ground, and test all other ground points. I would think this issue was caused by something, broken connection or proximity to a radio tower, and was not happening when the amp was new? The last Marshall I worked on was a newer model and there was a metal retainer over the two pre-amp tubes. If I touched this retainer, it would induce hum in the output. I know no one would ever touch this, but I added a ground wire between the retainer and chassis. I didn't like the idea of an rf antenna on the pre-amp tubes! Another thought just occurred to me, are you testing this amp dis-assembled on the bench? It may need to have a shielded panel on to quiet down, some amps have shielding in the cabinet that contacts the chassis.

 

[saint gillis]

The cab is not shielded but I tried to put aluminium plates all around, it didn't change much the behavior...

 

[abbey road d enfer]

Using 2-coils positions with several fender strats, and volume at mid position, brings quite a lot of noise. It is not the case with the same guitars and other amps here, and the ground is very good here.

have you checked continuity between the earth pin and the chassis and 0v?

 

[saint gillis]

Yes I did, it was good.

For the reverb getting some radio, I've change the op amp and re-soldered all the components around its zone, it solved the problem.

For the input stage picking up parasitic hum, the guy says it's much much better now, maybe the 68K I added at the input...