Fender Musicmaster Bass amp buzz - help!

[electrafrench]

Hello, this is my first post i'm new!

I recently bought a Fender Musicmaster Bass amp second hand. I was told it had a buzz, the guy who sold it to me knew very little about the amp and just wanted rid of it.

I've read a couple of things that said it may be the tubes or the filter cap (or it could be something worse). I've replaced all the tubes (6v6 x2, 12ax7) but don't know where the filter cap(s) are. I've uploaded some gut shots and a video of the hum (hum starts at about 25 seconds in). It gets louder when the volume is turned up and the tone pot affects it too.

any advice would be great! I'm handy with a soldering iron so if it's just replacing capacitors I should be ok as long as I know which ones it is and what would be best to replace them with!

Photos via flickr
https://www.flickr.com/photos/117414543 ... 4213108840

Video via Vimeo
https://vimeo.com/94867812

or Video via YouTube
https://www.youtube.com/watch?v=RHMoTHt3we8

thanks! - electrafrench

[xtian]

Welcome!

Start here for lots of general info: https://tubeamparchive.com/viewtopic.php?t=9023

Make sure you understand that the electrolytic caps can hold onto a charge that can actually kill you long after the amp is unplugged. Learn how to safely discharge the caps before touching anything inside.

I have highlighted the caps you'll need to replace on the attached schematic.

However, without further investigation, I'm not convinced this is what's needed. I noticed in your video that the hum is almost gone when you turn the volume control down. Guys? What do you think?

[electrafrench]

cheers! I was reading/watching videos about draining caps last night, and also checking they are drained. I was thinking of checking all the caps are working or not before fully replacing them.

Any tips on quality caps i can replace them with?

i really hope it's not the transformer!

thanks for the help

[Stevem]

Unless you have a Cap ESR meter to use in regards to checking the power supply filters / caps, there is no surefire way to know if they are good or not!
In fact I have filters out of 70s Marshalls, Hiwatts, Fenders and other amps that are leaking out crap and or swelling up at the pressure release plug AND THEY STILL TEST GOOD FOR LOW ESR!!!! so nothing is cut in stone without the meter test!

I would replace without question all the filters at this point with Ruby brand, F&T ,or blue cased Atom caps and know that at least that's out of the way and the amp will sound better for it!
Its not either of the Transformers, so do not sweat that.
One definite circuit change I would make is to un ground the filament string and then solder on a 100 ohm 1/2 watt resistor to each of the 6.3 volt feed wires ( likely to be green colored) and the ground the other end of each resistor.
You can do this at any of the tube sockets where its easy to get near a point to solder them to ground.
A good spot would be at one of the output tubes as they already have there pin 8 going to ground right near by, all you need to do is unground that one wire and solder a resistor from a 6v6s pin 7 to ground at pin 8, and the do the same thing at pin 2.
This mod will lower the 60 hz hum that the amp makes also.

[ampedup]

watch out some amps have a doghouse on top of the chassis with caps. I got caught with that one, it was the shock of my life.

[ampgeek]

Welcome to TAG!

Is is buzz the same/similar with an instrument plugged into one or the other inputs?

Does the buzz change character between one or the other input?

Good luck,
Dave O.

[gui_tarzan]

What is that transformer between V1 & V2/3?

[Smokebreak]

What is that transformer between V1 & V2/3?

Interstage transformer used as the PI. I heard someone playing some Stones' country stuff with one of these the other day, and it sounded fantastic.

[gui_tarzan]

Oh, ok. Thanks.

[sixstringer]

Stevem said it right, change the electro caps and add an artificial center tap to the hearter (6.3v) circuit. There is a great section on how to deal with the heater circuit at valvewizard.com