2204 Question

HHwarner

Iv got the chance to pick up a nice 1981 Marshall JMP 2204 (one of the last ones) Iv asked for photos of the chassis, it all looks very nice and untouched, however I have noticed that there are no 1k 5w resistors over pins 4 & 6 of the EL34 valve bases, I have seen this before but would it have left Marshall factory like that? Is there risk of damage upstream if a valve goes bad without them. Also the bias caps look the wrong voltage to me. They are 10uf 100v but saying that they also look original. Other than that it looks a great amp and If it ticks the box when I try it out tomorrow, i will deffo buy it. Cheers in advance 

ICBM

The lack of screen resistors is stock and is a bad idea - it's very hard on the power valves. Old Mullards and RFTs seem to tolerate it but they really should be fitted.

The bias caps are correct, but make sure they haven't started to leak - if you can't get the bias voltage up high enough to go slightly too cold then they need replacing. This is more common than the main caps failing, in fact.

HHwarner

Ye I will be checking the bias voltage and will replace with new if required, Iv got a few 10uf 160v here. Ill look at fitting screen resistors too I think. Other than that it looks a very nice amp. Will try it out tomorrow.

HHwarner

Iv had another look at the photographs of the underside and as well as not having the Screen resistors there are no Grid Stoppers on pin 5. Would it be a good idea to fit those as well as the 1k's

ICBM

They aren't really necessary on the 50W amps, but it wouldn't do any harm.

HHwarner

On another note, my mate has a 2204 and when the treble control is set to zero, the volume drops right off, like no volume at all. This doesnt happen on my JCM800 2204, is that normal???? Thinking about it it may have been the entire tone stack at zero because he was trying each control on at a time. 

ICBM

No, not normal. The tone stack on a Marshall is very slightly different from a Fender and turning all the controls to zero doesn't completely kill the volume. Your mate's amp may have a short in the tone stack, possibly one of the pot terminals touching something it shouldn't be.

HHwarner

Ill have a look at it for him next time I see him

HHwarner

I didnt think it was normal as every 2203 or 2204's Iv ever had have never done that

MartinB

It's just about possible that someone may have intentionally changed the mid pot to the Fender wiring (shorting two terminals so that it operates as a variable resistor), otherwise something is amiss. 

ICBM

MartinB said:

It's just about possible that someone may have intentionally changed the mid pot to the Fender wiring (shorting two terminals so that it operates as a variable resistor), otherwise something is amiss. 

I've heard of that being done on a non-MV Marshall so it can be used as a sort of crude master volume, but on a 2204 it would be pointless because the real master volume comes straight after the tone stack anyway.

HHwarner

I had a look at the 2204 with dodgy tone stack this morning. Its a later 2204 so pots are board mounted. Some of the pots have already been replaced & not all that well I might add. Soldering was very poor and had lifted the tracks on one of the legs of the Presence and MV pots. Iv tidied that up but the problem persists. I think the Bass pot is bad. With all the tone stack set at zero, no sound at all. Turn the bass pot slightly and volume comes on. Also the bass pot doesn't seem to be having any effect on tone at all going through the full range of the pot. Could this be the culprit. Given the price of a new pot ill just order one anyway. The bass pot is original btw and one of 2 which have never been replaced

ICBM

That sounds like a possibility, yes. The bass control only works as a variable resistor, so it will mute the output when the rotor is touching the zero terminal end, but if the track has cracked just above there it will do that - go to an open circuit, so no mids will get through because they go via the bass pot to the bottom of the treble pot.

HHwarner

Just out of interest, what sort of tolerance would you expect from a 1M pot? Im getting a reading of 1.3M, pot removed from board. 30% seems a bit much to me

DJH83004

HHwarner said:

Just out of interest, what sort of tolerance would you expect from a 1M pot? Im getting a reading of 1.3M, pot removed from board. 30% seems a bit much to me

It varies with the quality of the pot you buy, for a non precision type, +/- 10% or +/- 20% is typical, suspect yours is a +/- 20% that is just getting old   

ICBM

I suspect it may have a crack or a poor contact at the CCW end of the track. That would explain the high reading, and if it wasn't making contact at all when it was in the amp, the tone stack behaviour.

Try squeezing the terminal rivet hard onto the track board with a pair of fine-nose pliers and see if the reading goes down to 1M.

If it does, try it back in the amp - and also bridge the rotor to the CW terminal, which will stop the pot going open circuit if it's actually a rotor contact problem - this is good practice with any pot used as a variable resistor.

HHwarner

The reading does drop back to just under 1M if i press the rivets but creeps back up again. Iv got a new one coming tomorrow

HHwarner

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