Marshall 2205 very bright- drifted R9 resister?

[luzianna]

I bought a 89 model 2205 (used to own 85 model) I have some limited knowledge of electronics and good soldering skills, and a complete paranoid freak about high voltage safety and awareness. I have a DVM fluke model and have worked on my own amps years ago in limited capacity.

This amp has a very bright treble tone but is weak on the lower freq tone. I replaced 3 of the crappy but functional preamp tubes with mullard reissues but not the driver or the output tubes (still bright). The el34s in it are not that old but don't know much about them except they are labeled "the tube amp store" "perfect pair" and number "24".

Based on my previous experience with my old amp, this thing is weak on lower frequencies and too painfully bright. I do not know how to evaluate capacitors but measured the resister values by comparing with the schematic with my fluke meter. R9 is supposed to have a value of 10K but the one in the amp only reads about 200 ohms. All other values are close or dead on. I do not know the function of this resister or if it could be the cause of my problem.

[oc disorder]

Try removing C4 the 100pF thats across VR1 the volume control.
At low volume settings it just may be too bright.
A lot of players like Marshalls with this removed - a more vintage sound.
A resistor in series with it will decrease its effect.
You only have to lift one leg of the cap to try it.
R9 will have no effect on the sound.
It goes from supply to the ring of the footswitch jack (for reverb switching) and
via the 1k to the BC104 which mutes the reverb or not.
If you lifted one end of it I'm sure it would measure 10K .
As the supply comes from the heaters which are referenced to ground I presume
with the footswitch socket normally shorting to ground you are reading a low ohms resistance in parallel with the 10k. If you plug a TRS (stereo) jack into the footswitch
lifting the ring from its normally grounded contact I guess you will find R9 measures much higher.

[Enzo]

Or you have an open in the signal path, and the resulting crosstalk you hear is tinny.

[luzianna]

O.k. after further evaluation, it is really weird. I tried turning the treble down. from 10-5 it has almost no effect, really bright. Then from 5 on down to 0 it seems to get almost normal (playable-acceptable even with some bite when I dig in almost like it should be around 6-7) and the amp even gets a little stronger in volume as the high knob gets from 4 down. Then, bass although never gets deep and boomy, does function somewhat normal, just not what I would expect. The mid is non functional if I have the bass all the way down, is functional if the bass is up, don't know if this is normal.

[jrfrond]

I agree with removing the Bright Cap (C4). In some Marshalls, it is just too much. I had to do this recently on the same model.

[luzianna]

o.k. I'll try that. Thanks

[Chuck H]

How are you running the amp? Cranked? The tone controls will seem less effective if this is how your running it. R9 may be giving you a false reading because of the circuit. You may have to lift one end to get an accurate read on that resistor.

The 2205 is a notoriously thin, bright, harsh, whatever, amp. Some examples seem to be OK but most do get the complaint that they lack bass and sound thin. I've read where a lot of guys post here with this complaint after trying the presence and trble set to 0.

If the mid doesn't seem active with the amp running clean then I would suspect a problem there. Otherwise I think a revoicing of the circuit may be what you need to make this amp to your liking.

Chuck

[Enzo]

Why don't you pull that treble control and see if the element is open? Your description of its operation suggests exactly that.

[luzianna]

what about caps

Just to be sure, I evaluated the amp again. There is a definite problem with the amps sound when the treble knob is turned up. It gets way too thin and tinny sounding from 6 up and excludes bass/mid freqs and gets fuller/thicker when the treble knob is turned down, and although not tinny anymore, can still hear some highs. I read another thread about a similar problem but that turned out to be speakers out of phase, these are not. I checked and no dc voltage on tone stack pot legs from leaky capacitors. The big caps not the original capacitors (JJ) and I can't tell if the blue ones on the board are..
From the previous suggestion, the pots are all soldered into the pc board so it will be a chore to test the pot. I am hoping that the caps could all be bad and I could test everything when replacing if you guys think that should be done. There is not any big hum but some low level hiss and crackling sounds (probably not abnormal hiss while high gain channel is on).

[Enzo]

ALL the caps bad?

COnsider, how would the caps know when the treble control is turned from 5 to 6? Yet your symptom sems to be sensitive to which side of that point your treble control setting sits on, if I understand correctly.

SOmetimes you have to break eggs to make an omelet. Yes, you have to dismount the circuit board to extract the controls. That's life in the amp repair game.