Marshall Valvestate Hum

Tube or solid-state, this section goes to eleven!
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deltafred
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Post by deltafred »

FuzzTony wrote:I discovered the trick was to rotate them by 90 degrees. The hole in the chassis is sort of an oval shape and they have to be at just the right orientation, and then they will pop out.
I never thought of that. :applause:
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Post by FuzzTony »

Lt Right 8240 wrote:
...

In my case the 8240 makes hum when the master pot is completely closed/left(!) and without a connected guitar(-cable). In fact it is nearly not a problem while playing but (for example) while sitting in front of the running amp without playing "to tweak some things".
And: The hum I'm talking about gets more and more quiet the more the master pot is tweaked to 12:00.

I also downloaded the circuit schematic to look for a solution.
After a while I think found the reason:

When the master-pot wiper is at zero (connected to ground) then the input of the power-stage (TDA 1514A pin 1) is not directly connected to ground:
There is a voltage divider in between (R125/R126 and R130/R131).
So I think my hum-problem is by design:
When the master pot is on the left then the input pin 1 of the TDA 1514A is connected with 6k8||33k to ground. And that's enough "to catch the hum out of the air" isn't it?
What do you think? Are you agree in this point?
Do you have an idea to get rid of this problem?

Maybe the 200V AC and 12 V AC power cables for the tube-heater and the anode working as 50Hz "antennas"?
Does it make sense to move the rectifiers from the main PCB near to the transformer to get only DC on the long white and red cables?
(ups: is the heater-voltage also rectified? Can't find it in the schematic/not sure at the moment...)

...
I have the same issue as described here. Has anyone found a solution? I have just replaced all the scratchy pots on my 8240. There is a spot approx halfway (12:00) on the master volume knob which has the least amount of hum. The hum is on both channels and is not affected by any other knob, even with everything all the way down, guitar plugged in, or not plugged in, the hum is there. The lower the master volume pot, the more hum.

Based on the suggestion above, I actually clipped off the red & white wires right at the transformer to test (I only really need the clean channel) but this did not seem to have any change in the hum. So I've resoldered those wires back onto the transformer.

I can't see any caps that look problematic. I do note that C34 in my amp is 33uF , not 47uF as the schematic indicates, but it doesn't look like it was replaced. I've reflowed the solder on those caps and C94/95 also.

I am wondering if the hum I've got is indeed "by design". My desire is also to use this amp at lower volume levels, and that is where the hum becomes annoying and noticeable. At stage volume it would not be a big concern.

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Post by Intripped »

I have an 8040 and a few years ago I had to change the first opamp (tl072 if I'm not wrong) because it was working badly: Crackling noise and also hum.
I don't know if the 8240 has a similar circuit but I guess so.

I've read somewhere that there is a problem with the input of the valvestate amps, and also there is a very simple solution to it. Have someone here ever heard of this? I can't recall the site neither the solution.

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Post by Intripped »

Well I've tried and searched for that input-issue with the valvestates but I've found nothing
...I don't know, maybe I dreamt about it. Possible.

Anyway I've read that there is another well known issue with these amps: oxidated contacts on the LINE OUT and POWER AMP IN jacks. With no plug inserted the signal passes through these jacks (switched jacks), but if contacts get oxidated some volume-drop issues may happen.
An easy and quick check would be to put a jack/jack cable from LINE OUT back to POWER AMP IN
Yes, probably it's not a solution for the humming issue, but I think it is worth sharing

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Post by Intripped »

Sorry for multiple posting, but maybe I've found something on that input issue and solution I was talking about:

http://www.diyaudio.com/forums/instrume ... owing.html

Clamping diodes for IC1 protection, that is.

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Post by deltafred »

Intripped wrote:Anyway I've read that there is another well known issue with these amps: oxidated contacts on the LINE OUT and POWER AMP IN jacks. With no plug inserted the signal passes through these jacks (switched jacks), but if contacts get oxidated some volume-drop issues may happen.
An easy and quick check would be to put a jack/jack cable from LINE OUT back to POWER AMP IN
Yes, probably it's not a solution for the humming issue, but I think it is worth sharing
Happens with a lot of amps where the signal passes through the switching contacts of a jack socket.

Far better to to clean them once than futz around with extra cables forever.

I usually run a piece of card soaked in switch cleaner (with lubricant) between them a few times. The lubricant helps stop them oxidizing again.
Politics is the art of so plucking the goose as to obtain the most feathers with the least squawking. - R.G. 2011
Jeez, she's an ugly bastard, she makes my socks hurt. I hope it's no ones missus here. - Ice-9 2012

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