Monthly Archives: February 2010

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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Bias adjustment prior to replacing capacitors

This last weekend I was able to begin the process of restoring the innards of my 2204. The very first thing I did (after discharging the filter caps properly) was solder a 1 Ohm resistor between the cathode and ground of the EL34 output tubes. Once complete I was able to check the bias of the existing setup by measuring voltage across that resistor. Ohms law provides that he voltage I measured was equivalent to amps. It was way down at 19 mA.

So I referred to this Eurotubes biasing page which told me:

The formula for calculating the bias is the dissipation of the tube divided by the plate voltage which gives you 100% and then you can bias anywhere between 65% up to 85% of this.

The bias components on the 2204

The bias components on the 2204 board. They’ll need replacing soon.

My plate voltage is 400 so using 25 watts as a basis, 25 / 400 = 0.062. 70% of that is 0.062 x 0.7 = 0.043 = 43 mA. Tweaking the bias trimmer gave me 39mA on one tube and 41mA on the other. A reasonable match.

This didn’t seem to affect the tone using the original tubes and filter caps. I recorded the resulting ‘tone’ at a known setting that gives me the Marshall sweet-spot tone.

Two things I noticed that will need investigation:

  • The bias adjustment trimmer was almost at the end of its travel to give me the value I wanted. This may mean the trimmer is close to death?
  • Why was it set at 19mA in the first place? Is this also an indication that my plan to replace the bias components really must be done soon?

UPDATE from the future! March 2015

I replaced the output tubes recently and biased it the same way as above. When playing the amp in anger at a rehearsal, I found that the amp breaks up into quite pleasing distortion much earlier than I expected. It is indeed a great sound, but I’ve lost the strident clean tone I would normally get. There’s one song we do that needs that…for my Telecaster and a tremelo effect. I now suspect that the new tubes were simply born this way, and that I need to change the bias so the amp is running a little colder.

Look at this post: http://www.mylespaul.com/forums/squawk-box/31210-getting-jmp-2204-bias-within-range.html

FWIW my rule of thumb for JMPs is 28-32 mA for EL34s, 34-36 mA for 6550s. I sometimes go a little hotter, but to my ears there’s generally no sound improvement for the shorter tube life. I generally shoot for something like 15 watt plate dissipation.

Most tube dealers recommend you bias really hot. I wonder why?

This implies I biased it REALLY hot at 41mA. We are going into the studio tomorrow, so I’m going to rebias first thing, and see if I get my ultra clear ‘barking’ clean tone back.

UPDATE again: Dec 2015

It is worth stating that I was indeed able to get my loud and clear tone back. My amp was indeed biased way too hot. I can still get good 2004 grinding tones by using lots of pre-amp (I might put it on halfway) and dial the master volume to suit the loudness I need. But mostly I run mine with the master all the way, and adjust the pre-amp knob to suit loudness. There’s a sweetspot where I get a great strident clean tone that takes pedals very well.

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