Amp whizzes: filter caps?

Post by **Doog** » Sat May 01, 2010 4:54 pm

Hey guuuuuuys, just after some thoughts from our resident amp gurus:

I picked up a Marshall VS100R 100w Valvestate combo a few weeks ago and it's got a minor but noticeable hum and hiss regardless of any dial settings- volume, distortion, EQ. It's as audible on any volume setting, although slightly quieter on the distortion channel vs the clean channel when both volumes are at zero. Follows is a quick vid showing the noise, it's a lot more apparent when you're in the room.

[youtube][/youtube]
At gig volumes, I'm sure it won't cause any problem at all, but at somewhat more restrained levels, it's slightly annoying and could be problematic when recording. It's definitely not just transformer hum, it's coming from the speaker. The humming increases with the 'Power Dimension' button (low mids and bass boost) so the humming is magnified.

I've done a lot of online research, and this forum post seems to be the closest to the nail: http://marshallampforum.com/forum/viewtopic.php?t=14966

Does this sound like filter caps or is it impossible to say? It's not just a noisy mains supply seeing as both my DSL50 and little Zoom practise amp are fine. Have I just been spoilt by a more pricey valve amp for too long? The amp is at most 14 years old, according to production dates.

Some schematics of the amp in question, in case they help..

► Show Spoiler

THAN-Q!

Post by **Doog** » Sat May 01, 2010 6:41 pm

Oh and just to rule out the preamp, I plugged the VS100's FX send output up to the DSL50; no nasty noises to be heard.

Soooooo, it's definitely the power amp in some shape or form.

Pens · Post by **Pens** » Sat May 01, 2010 8:49 pm

Normally these things are preamp tubes, based on what I've read, I've been searching for a solution to a similar problem with my Univox head. But you ruled that out...

Yeah, I'd check the filter caps, but first perhaps swap in some known good power tubes to rule those out as well.

Post by **Doog** » Sat May 01, 2010 10:33 pm

No powertubes as it's all solidstate, apart from one preamp tube which I've already tried the ol' switcharoo on with no change.

benecol · Post by **benecol** » Sat May 01, 2010 10:56 pm

There's only one man who can help, and his name is...

... NickS.

Are you man enough to go there? He'll mention quarks or something, make an ace off the cuff electronics pun, and leave you weeping into your physics book while he wanders off and builds a large hadron collider in his shed with bits of old Ford Probe and H/H amps.

Are you ready to go there Doog?

Post by **Doog** » Sun May 02, 2010 11:21 am

I.. I think I am.

Will · Post by **Will** » Sun May 02, 2010 5:53 pm

The hiss is normal SS background noise.

The hum comes from inadequate filtering in the power section. Basically, a transformer turns the wall voltage into the voltage/current needed for the amp. Then it's converted from AC to DC by a rectifier. Then the remaining 50hz "ripples" in the power are filtered out by large capacitors. This sounds like the filter capacitors either are going bad or were too small a uf value to begin with.

Post by **Doog** » Mon May 03, 2010 10:14 am

Excellent, thanks for that dude, I shall read up on replacing them.

Sloan · Post by **Sloan** » Fri May 07, 2010 1:54 am

just buy the same uf value, but equal or higher voltage rating.

to drain the old caps, you can short them out with a screwdriver across the two leads and the chassis, and it may spark, and don't do it with two hands. even after that treat the caps like they will kill you and only use one hand to touchy.

usually electrolytics will have a stripe on the side indicating the negative lead.

you can get either radial or axial lead caps, axial have leads coming out of either end, radial have leads coming from just one end. yours are most likely radial. if you can find a radial of the specified value, it's perfectly fine to use an axial.