Cap job

Discharging the old Davy capacitors

Discharging the old Davy capacitors with my new tool. Note the horrible quality soldering, courtesy of Marshall.

This past weekend I was able to replace the larger 50-50 caps in my amp. It meant I got to use my capacitor discharge tool which I’d been toying with for a while. Soldered between two alligator clips, I have a 5W 5k Ohm wire-wound resistor. I left them clipped on each positive cap terminal for a few minutes. That seemed to do the trick; my test was to do “the screwdriver trick” and short them to ground. There was no “loud pop and sparks” so my tool was successful.

Original Davy 50-50 cap being removed

Original Davy 50-50 cap being removed

So I de-soldered the old wiring and removed the old clamps. It was easy to put the new ones in and re-attach the wiring. I had taken pictures along the way,so it was trivial to get the wiring back in the right place. History has shown that my memory is close to useless, so I didn’t even attempt to hold it all in my head.

You can see the underside of the main PCB through the hole in the chassis (see left). I left this cap, the one sitting next to the choke,  sitting quite high in its clamp (see the picture at the end of this post) as I was paranoid about shorting the wires against the PCB.

Two of the new caps in place

Two of the new caps in place.

You can also see in the above pictures that Marshall wasn’t too picky about employing people that could solder particularly well. Their preparation seemed to be the key…i.e. when they stripped insulation off wires they did not bother to spend half a second to twist the strands together before tinning it. I had to replace a few wires as the original wire was not going to hold up too well.

Once completed, I fired her up and checked the bias. All was normal, and it appeared to have greatly reduced the amount of hum…especially when cranked. Success!

Next step will be replacing the electrolytic caps in the grid bias circuit.

New filter caps installed

New filter caps installed

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2 thoughts on “Cap job

  1. Mick Brehaut says:

    Hi Simon,
    What a facinating interesting blog that was. I was drawn right into it!
    Good on you for posting your findings on what is to me, a most facinating subject.
    I just love the youtube posting of ‘this is what all tube amps should look like’. Great stuff!
    Im really not the technition you seem to be as i wouldn’t really trust myself to go as far as you have with your 2204 re-vamp i have to say.
    Tubes, (or valves), as i tend to call them are, as most of us know a vital part of the sound you get and i have tried a lot of them. The ones i now use after much trial and error are Groove Tubes EL34’R’s, which i believe are Russian made and are slightly more heavy duty than most. More importantly they sound great with excellent top and bottom end response. I had problems with the EL34Cs, (Chineese made),which although they have had good reviews, i found very ‘mid-rangey’and the bias kept changing for some reason until one of the quad glowed bright orange to yellow and took the next one out with it. Doug Chandler who co-owns Groove Tubes UK is a great guy to deal with and is very customer friendly. Rock on! Mick. Australia- NSW.

  2. simonallaway says:

    Mick! Thanks for your kind words.
    My next step is to try different tubes (valves, of course) in my 2204. Ironically I bought a pair of those Groove Tubes EL34M. We shall see if they make any difference. I hear lots of good things about JJ valves (from Eurotubes). They do a whole set of preamp and power amp tubes for a great price. I might try them shortly.

    And when you say Doug Chandler, do you mean the same one that owns Chandlers in Kew?

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