Safely changing power tubes 4001/studio15

Discussion in 'Marshall Amps' started by jak, Jun 1, 2020.

  1. dragonvalve

    dragonvalve Well-Known Member

    Joined:
    Jan 16, 2010
    Messages:
    542
    Likes Received:
    433
    I get that but the OP is unsure of what he's doing. At least insulated gloves would give him some sense of safety.

    Can't hurt. [​IMG]
     
  2. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Bravely did it today. Gonna post pics of I can!
     
  3. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Yeah thanks for making sure. You never know. Dumber things have been done!
     
  4. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    You’re joking right?
     
  5. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    **********
    Edit
    **********
    (NEW) Something happed in addition to what i described in the earlier post below, which was last night, but I omitted it cause I attributed it to a poorly seated speaker cable. But This morning it happened again and worse. I’ll post this separately in a moment...
    ******************
    (OLD) Yeah you know something crazy happened. I happen to do just that without really thinking about it. I clean the pins and reseated the tubes and now I might be going nuts the amp sounds awesome. I seriously don’t know what the F is going on. I plugged straight in, with the speaker cable plugged into the headset jack for attenuated volume playing, with all the pots dimed (gain,treb,mid,bass,output), and it sounds beautiful. I’ve been put it on standby, plug the speaker into the eight on speaker jack for the full 15 watts, and played a like that with gain dimed and output on three, and then output dimes with gain on three, to alternate power and preamp crunch, and it sounds beautiful. The fading out/in and power crunch issues appear gone. Tomorrow I’m going to try this again but I’ll hook up my peddleboard and try cranking the gain and level on my OD pedal to see if I can push those power tubes hard enough to see if they start acting funny again, if that was them acting funny before.
    Crazy.
     
    Last edited: Jun 3, 2020
  6. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    I used the elbow length gauntlet yard gloves I use to yank yellow Himalayan raspberry devil bushes. Closest I could get to helicopter lineman Gloves I felt safe.
     
  7. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    F8834D4D-66BA-4E3B-8DF2-776D5E30773F.jpeg C4D74185-FBE6-4979-A06A-855110052703.jpeg BCE8B953-E4C9-403F-8693-8F4DE5C10C8B.jpeg Here are the tubes. There are no plates per se inside these tubes but the structure looks nothing like any of the 6v6’s I’ve seen. Look like dream catchers. They’re wide but very thin when turned looking along the thin access. I’m sure somebody will recognize these. Once again they have absolutely no markings whatsoever.
     
  8. Gene Ballzz

    Gene Ballzz Well-Known Member

    Joined:
    Feb 19, 2013
    Messages:
    2,123
    Likes Received:
    1,743
    Location:
    Las Vegas, NV
    @jak
    FWIW, I do believe that your 4001/Studio 15 is cathode bias and therefore requires no bias adjustment. Here are links to two schematics, printed in different years:
    https://drtube.com/schematics/marshall/4001m86.gif
    https://drtube.com/schematics/marshall/4001m88.gif
    Notice that the cathodes of the power tubes each go through their own resistor and capacitor in parallel to ground for the cathode bias. This is kind of unique, as many cathode bias designs share a common cathode risistor and cap.

    There are several other unique design features in this model, some that were ahead of their time:
    A) First production Marshall to use 6V6 power tubes. And cathode bias, at that!
    B) Master volume is post phase inverter.
    C) The heaters/filaments of V1 (first pre-amp tube) are DC, instead of AC, to reduce hum.​
    I have a friend who has proudly owned one of these since about 1988 and have played through it a bunch. VERY COOL little amp! I personally believe that if these amps had either an effects loop, reverb or even both, they could have given the Fender Princeton Reverb and maybe even the Deluxe Reverb a run for their money!

    Just My $.02 & Likely Worth Even Less!
    Gene

     
  9. dragonvalve

    dragonvalve Well-Known Member

    Joined:
    Jan 16, 2010
    Messages:
    542
    Likes Received:
    433
    Oxidation happens. Tubes need to be reseated or best cleaned pins and socket tangs. Now you may not have to buy new tubes, but you should have spares. It's SOP when owning a valve amp.

    Those tubes look vintage. As long as they work, don't change them.

    For power tube sockets get some women's mascara brushes.
    [​IMG]
    [​IMG]


    For the preamp tubes use a floss brush for the 9-pin stuff
    [​IMG]
    [​IMG]
    [​IMG]


    Anything 1/4" jack, get a 9MM gun barrel brush. I prefer nylon but they have them in brass. Thing about nylon if a whisker falls off in the electronics it won't short vs brass. If you get brass make sure you can see from all angles if a whisker was left behind to short something out. If you have jacks that have plastic be careful you don't use too much force. The all metal ones can take it but the better plastic housing jacks on better amps will handle the brush's expansion pressure.

    Pour some cleaner or denatured alcohol on the brush. Work it in and out about 10 times without gouging the jack by keeping the motion straight in and out so as not to bend the brush or crack plastic.

    The 9MM is an exact fit for guitar and amp jacks which get oxidated over time.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]

    Clean the jacks on your guitars and pedals!
    [​IMG]

    Guitar plugs can be cleaned with 1200 grit sandpaper and denatured alcohol.

    [​IMG]

    :agreed:
     
    Last edited: Jun 3, 2020
    South Park and wntbtw like this.
  10. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Thanks for that info. Fascinating and unique features I wasn’t aware of. And an effects loop and reverb would make this amp insane. The schematics of course jive with my limited practical knowledge. There is one YouTube vid of biasing this amp via the biasing pot on the board so I’m not sure if that was just academic or necessary for whatever service issue that particular amp had, compared to just dropping in new tubes on a well tuned and functioning amp.
    but I have a different problem for now which I’ll post below...
    Thanks for those links!
     
  11. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Damn man I would never have thought of all that. Thanks for explaining all that and for all the pics. And I have brass and nylon 9mm brushes but I think I’ll get a new plastic one cause I’d never be able to clean all the hoppes, clr, and fouling off them to even think of using them on the amp. thanks again!
     
    dragonvalve likes this.
  12. scozz

    scozz Well-Known Member

    Joined:
    Jul 20, 2008
    Messages:
    3,577
    Likes Received:
    8,110
    :lol:
     
  13. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Please everyone, bear with the detail below and read through so you can be sure not to miss anything in case you can help determine what might be the issue:
    -Last night, after pulling out and reseating the power tubes, and turning on the amp with a guitar plugged in, the amp was making a loud humming sound and loud pops from the speaker.
    -I put the amp in standby, switched guitar cables, took the speaker cable out of its 8 ohm jack, put it in the headphone jack for attenuated playing (Marshall directs this as an option for attenuated volume at full power in the owners manual, fyi), turned it back on, and everything sounded ok. I did this cause I wasn’t sure if it was the guitar cable or speaker connection that was the source of the sounds.
    -I then put the speaker cable back in it’s full power 8 ohm jack, and all was ok again.
    -So I concluded the speaker cable, which I unplugged to get it out of the way while working on the power tubes, was not fully seated at first—or the first guitar cable was bad. I played a while and all was good.

    Well, this morning, I powered up using the first, suspect, guitar cable to see if it was actually bad, and heard that same loud hum and pops I had last night. I then switched to the ‘good’ cable but the hum continued (without the loud pops).
    I tilted the amp forward to look in the back and smelled a slight electric smell. I then heard a loud pop that was not the speaker pops I heard earlier and last night, but an arc type pop coming from the chassis; and immediately smelled a strong electrical (burn?) type smell. I shut it down and unplugged it and I’m done.

    I’m guessing it’s a good idea to not power it up again and just take it in to service, but can anyone follow that chain of events and tell me what they think happened ok is going on?

    thanks :confused:
     
  14. Jethro Rocker

    Jethro Rocker Well-Known Member Premium Member

    Joined:
    Mar 11, 2011
    Messages:
    8,268
    Likes Received:
    9,805
    Location:
    Saskatoon, Canada
    It still could be a dead tube. They were reseated but still 40 yrs old. I say try 2 new ones first. It may have killed the HT fuse as well when a power tube kicks it. I have had a hot burnt electrics smell from dead tubes before. There shoild be fuse protections.
     
  15. Gene Ballzz

    Gene Ballzz Well-Known Member

    Joined:
    Feb 19, 2013
    Messages:
    2,123
    Likes Received:
    1,743
    Location:
    Las Vegas, NV
    My friend brought his over a few years back for a check-up/cleaning service. One of the captive (I think I recall "cage" style) nuts for mounting the chassis into the cabinet was floating around in the chassis. Luckily, no shorts had occurred and I rectified the issue!

    As mentioned by @Jethro Rocker , it still could be a bad tube, but I'm suspecting an errant bit of hardware and/or loose wire/bad solder connection.

    Please make sure that what ever tech you take this to, is well known with a good reputation. This amp is too cool and rare to allow a hack to even breathe on it and is certainly worth spending a few bucks on.

    If you are comfortable with it, I might suggest (with amp unplugged from power, of couse) carefully removing the chassis to inspect for anything loose and/or flopping around inside the chassis. Avoid putting your hands anywhere inside the chassis, but you could use a wooden/bamboo chopstick (NOT a wooden pencil with conductive graphite) to poke around looking for loose wires, hardware, etc.

    Best Of Luck & Please Let Us Know How It Goes,
    Gene
     
    Jethro Rocker likes this.
  16. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Ok. Thanks for the tip. I could turn it on tonight and look to see if the tubes light up. I was just afraid something shorted out and thus maybe the chassis or something else became hot and so it would be unsafe to touch. Admittedly here I go with the chassis danger thing again.
     
  17. coolidge56

    coolidge56 Well-Known Member

    Joined:
    Jan 30, 2019
    Messages:
    849
    Likes Received:
    1,835
    Location:
    WA state hinterlands
    Unplug that amp and take it to a qualified amp tech.
     
    Jethro Rocker likes this.
  18. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    Thanks for all the input. I was laying down the amp in order to play it while looking at the tubes, and when playing with the power tubes, and never heard anything loose, but of course that doesn't rule it out. I'll take a look tonight. Power it up first and see what I hear and smell. Anything like last night and I won't go any further.

    The service guy is an authorized Marshall service guy, at least I think that's what the music store said when they referred me to him last time I had him give the amp a once over a couple year a go for a scratchy gain pot.

    Before gave it to him he said he'd been working on marshalls since the late 60's and I thought he said he was authorized, but I'm not so sure I explicitly heard that. Was a few years ago.

    Unfortunately there is no one where I live, like those on this forum, who could approximate the expertise and years of experience needed to check it for me, and I'm not qualified. And also this guy is the solitary person with any expertise unless I'm going to ship the amp out of state, which is cost prohibitive.

    I'll see what happens and definitely let you all know.
    Thanks again.
     
  19. jak

    jak Active Member

    Joined:
    Feb 26, 2017
    Messages:
    147
    Likes Received:
    43
    ....the simplest solution !
     
  20. coolidge56

    coolidge56 Well-Known Member

    Joined:
    Jan 30, 2019
    Messages:
    849
    Likes Received:
    1,835
    Location:
    WA state hinterlands
    DO NOT do that! When I hear an amp is arching and popping and has an electrical smell yet FUSES ARE NOT BLOWING my electrical shock hazard warning buzzer goes off.
     
    Jethro Rocker likes this.

Share This Page