Marshall amp Tech Help...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Dickie Fredericks, Jan 26, 2006.

  1. Dickie Fredericks

    Dickie Fredericks Member

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    Hello all,
    Ive been a watcher and reader here for some time but have not had a reason to post until recently. The information stored here is amazing!!!

    Anyhoo, Ive got a Marshall 4102 (as the ID would show) 100 watts 900 series combo. Ive always loved this amp and have used it for gigs in the Disney area for 4 years daily from 98 until 00. BTW it is a 94 model.

    The amp hasnt been used much since 2000 just the occassional recording thing. Here is the dilema. I must admit that I have run this amp without the speakers being hooked up so I could record direct and silent. This didnt seem to hurt the amp (I dont think) however the volume had decreased and Im sure the tubes were getting old and used up.

    I recently replaced the tubes with YJ's. I only installed 2 of them so far. The reason I did this is because there are no techs where I live now to do the biasing and I dont want to do it myself.

    Now it seems I have lost alot of tone in the low end which Ive heard is due to the YJ's. Im having to run the bass on 10 and everything else on like 4 or even 3. I have also lost sustain. The thing wont give me the feedback thype sustain.

    Im wondering if Ive hurt the amp by not running the speakers. Maybe the tone controls are shot? Im not sure what Im asking here. It just doesnt have the same Marshall sound. Im sure the YJ's have something to do with it but am wondering if there isnt more to it.

    If Ive missed any info you may need for a simple diagnosis Ill be happy to fill in the blanks. Any help would be appreciated. ts not that the amp sounds bad, it just seems not right. The bass tone control seems to not affect the sound as much as the other controls.

    The amp is very bright...

    Looking forward to your replies and am very much glad to have joined the community here.

    Richard
     
  2. rockstah

    rockstah Member

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    no more running your amp without a speaker load... if you haven't already damaged the output transformer.
    Mark
     
  3. Dickie Fredericks

    Dickie Fredericks Member

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    hehe yeah Ive learned. Anyway to check the transformer? I mean the amp still gets pretty loud for whats supposed to be 20 watts with the YJ's

    Not sure what the power transformer would do to the volume or tone... OK Im ignorant on these things LOL:jo
     
  4. rockstah

    rockstah Member

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    head over to metroamp.com forums and do a search on how to check that tranny. many helpful peopel there

    good luck,
    Mark
     
  5. John Phillips

    John Phillips Member

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    It IS possible that you haven't damaged the transformer, especially if you didn't have the amp really cranked with no load. A quick way to tell if it's probably OK is to run just ONE power tube (use one of the EL34s, or 5881s if it came with those), first in one side (1 or 2) and then in the other (3 or 4). Don't crank the amp right up, but get a reasonable volume on a clean sound. Don't do it for very long. It will sound quite bad - rough, and not all that loud - but if both sides sound the same at the same control settings (don't change them while testing, just put the amp on standby to swap the tube) then the chances are that the OT is still OK, because if it was blown either one side wouldn't work at all, or the two sides would be badly unbalanced. I would take a small bet that you're probably going to find it's OK, because usually if an OT blows it will cause an arc which will take out the HT fuse, and you didn't say that had happened.

    These amps are pretty bright and thin-sounding anyway IMO - I wouldn't consider running the bass on 10 and everything else low at all unusual :). Try the EL34s/5881s and see if the bass comes back, that will at least eliminate that as a possible cause.

    There is a major source of confusion on the 900 series, which is that the impedance selector is marked confusingly with this 4(16) thing. On the combos, that is a 4 ohm setting; on the heads it's marked (4)16 and is a 16 ohm setting. Why they did this I do not know - it's really stupid and often leads to people running a 16-ohm cab from the 4-ohm setting, or the other way round, thinking they're doing it right... both very bad. (The OT in both models actually has all three taps but only two are used, differently on the two versions; why they didn't just mark the panel simply and correctly is a total mystery to me.)

    Anyhow, if you're running two tubes you need to set it to 4 ohms with the 8-ohm load (ie two 16-ohm speakers) hooked up.
     
  6. Dickie Fredericks

    Dickie Fredericks Member

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    John, thanks for the post. Ill let you know how I fare in a bit.

    Richard
     
  7. Dickie Fredericks

    Dickie Fredericks Member

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    John,
    Tried your suggestion. Yeah both sides sound the same. Although you said it would sound bad it just sounded like the amp is sounding on the clean side. Which I think is sounding bad. Not like it used to. BTW putting the 5881's in didnt help the bass issue but I took those out and put the YJ's in cause I figured the 5881's were bad.

    It definetly has a loss of power as when I first got the amp setting it on 4 in the master was pretty loud and these days at 4 she dont go that loud.

    I was just assuming it was the power tubes. Maybe I need to buy a set of 5881's and just put new ones in to eliminate the tubes from the equation?

    Im thinking the OT is fine as I never ran the thing hard at all with the speakers out of line. Maybe the master was at 2 or 3 tops.

    Thanks again for your reply. Any other widom you have to share is welcome by me:BEER
     
  8. Dickie Fredericks

    Dickie Fredericks Member

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    BTW, this amp has seen some beach time. Is it possible since its never been done that the amp just needs a good cleaning with some deoxit or something? Wouldnt be a problem for me to remove the tubes and drop the chassis out of the cab and spray it down...

    Richard
     

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